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2006 - 2008 Literature Review Archives - Climate Trends
Abeysirigunawardena, D.S. and I.J. Walker. 2008. Sea Level responses to Climatic Variability and Change in Northern British Columbia, Atmosphere-Ocean, 46 (3): 277-296; Mazzotti, S., C. Jones, R.E. Thomson. 2008. Relative and absolute sea level rise in Western Canada and northwestern United States from a combined tide gauge-GPS analysis, Journal of Geophysical Research, vol. 113, C11019, doi: 10.1029/2008JC004835, 19 pages.
Short term climate variability, superimposed on the longer term sea level rise, can lead to hazardous conditions for some coastal systems of British Columbia. Variations in coastal uplift or subsidence can also have a strong effect on the resulting regional sea level rise.
Sea level rise caused by climate change is considered a major threat to coastal environments, particularly due to erosion and flooding of low-lying areas. Two recent papers looked at the historic sea level response to climatic variability and climate change along the coast of British Columbia, using tide gauge data. In addition, one of the studies (Mazzotti et al.) also used GPS stations co-located with the tidal gauges, to measure vertical land motion. In both studies, the tide gauge records were corrected for vertical land motions resulting from coastal uplift due to tectonics and postglacial rebound. The results from the two studies are similar, in terms of sea level rise during the 20th century. The study by Abeysirigunawardena and Walker, which focussed on Prince Rupert, on the north coast of British Columbia, found that the rate of mean sea level rise was 1 ± 0.4 mm per year for the period 1909-2003, increasing to +1.4 ± 0.6 mm per year for the shorter period of 1939-2003. The acceleration of sea level rise can be partially attributed to global warming, but cyclic climate variability had a larger influence. Stronger ENSO and Pacific Decadal Oscillation phases over the period 1939-2003 impacted winter and summer sea level fluctuations respectively. The authors note that previous work has shown a negligible influence of crustal motions on relative sea level at Prince-Rupert. Mazzotti et al. used a larger set of stations covering all of the western coast of British Columbia and the northwestern United States coast. Their results show a regional sea level rise of 1.8 ± 0.2 mm/year through the 20th century. For the shorter and most recent period of 1993-2003, they found a rate of 1.5 ± 0.4 mm per year, which they noted was puzzling and worthy of further investigation. Spatial variability along the coast was very large. Mazzotti et al. also estimated regional sea level change by the end of the 21st century, based on the projected global mean sea level rise and on their assessment of vertical coastal motion. Sea level change projections varied from about -10 to + 40 cm, with strong spatial variations. According to the authors, future sea level rise along the West coast will be strongly affected by variations in coastal uplift or subsidence.
Anderson, R.K., G.H. Miller, J.P. Briner, et al. 2008. A millennial perspective on Arctic warming from 14C in quartz and plants emerging from beneath ice caps. GRL 35, LO1502, doi:10.1029/2007GL032057, 2008.
Analysis of vegetation emerging beneath receding ice margins on Baffin Island indicates that this ice has existed since at least 350 AD. The rapid decline of these ice caps over the past century lends further evidence to the unusual character of the 20th Century warmth.
On Baffin Island, satellite and aerial photography show that since 1958, the area of ice caps has been reduced by more than 50%. In the journal Geophysical Research Letters, a team of scientists report on the use of multiple dating techniques for estimating the age of the vegetation emerging from beneath these receding ice caps and the use of this approach for documenting glaciation trends. Studying the 14C content of vegetation, sediment cores and in quartz from rock surfaces now exposed, the authors found that the current warming exceeds any sustained warm period in at least the past 1600 years in this region and that permanent ice has been sustained since that time. Furthermore, using a linear extrapolation based on the observational record, the authors project average melt rates for the coming decades and find that most ice will disappear on the plateau before the year 2035, with all the ice gone by 2070.
Chapman, W.L., and J.E. Walsh. 2007. A synthesis of Antarctic temperatures. J. Climate. 20, 4096-4117. doi:10.1175/JCLI4236.1
Researchers conclude that temperatures over the Antarctic region as a whole have increased over the last 45 years by about 0.37°C.
Over the last few years, changes in Antarctic temperatures have been examined to determine regional trends. Most studies have concluded that the Antarctic Peninsula has experienced the greatest warming, and some areas of the interior continent have cooled. This study combines the available data for the region into a gridded data set to provide spatial and temporal data that are comparable to the resolution provided by GCMs. With the understanding that any calculation over the data sparse areas of the continent have their inherent assumptions, the results confirm not only the warmer Peninsula and cooler interior, but also show that the 50°-90° S region as a whole has warmed by 0.371°C over the 1958-2002 period. Winter temperatures have warmed the most (+0.172°C/decade) and summer the least (+0.045°C/decade). These patterns are simulated well by a composite global climate model, which also projects a shift from the majority of the warming in the peninsula towards continental warming over the rest of this century.
Christy, J.R., Norris, W.B., Spencer, R.W. and Hnilo, J.J. 2007. Tropospheric temperature change since 1979 from tropical radioasonde and satellite measurements. JGR 112, D06102, doi:10.1029/2005JD006881, 2007.
A new study published in the Journal of Geophysical Research may revive the controversy about differences in the rates of recent warming of the Earth's lower troposphere and its surface. While model studies suggest the tropical lower troposphere should be warming at 1.3 times the rate of surface temperatures, this new analysis suggests that the lower troposphere in the tropics may actually be warming at less than half the rate of surface temperatures.
A team of researchers led by John Christy (University of Alabama) have re-examined possible errors in the radiosonde temperature data collected in the lower troposphere in tropical regions (20S to 20N) and adjusted the data for these errors through intercomparison of day and night time data, and of radiosonde versus satellite MSU data. Once adjusted, the researchers find good agreement between tropical lower troposphere temperature trends derived from the radiosonde data and the University of Alabama analysis of the MSU data. These show trends of 0.05°C warming per decade (since 1979), much less than the observed surface trends of 0.13°C per decade. They also argue that the analysis of satellite data undertaken by Mears and Wentz, which indicates a lower tropospheric warming of 0.146°C/decade (1.2 times that at the surface, and hence close to model projections) is faulty. They contend that the inclusion of NOAA 12 satellite data in 1991 appears to have introduced a discontinuity that does not appear to be real.
Comiso, J.C., C.L. Parkinson, R. Gersten, and L. Stock. 2008. Accelerated decline in the Arctic sea ice cover. GRL 35, L01703, doi:10.1029/2007GL031972.
The summer of 2007: accelerated decline in the Arctic sea ice cover.
The decline of Arctic sea ice has been quantified since 1979 using satellite passive-microwave imagery with increasing improvements in resolution. This paper reports on ice retreat rates for both perennial sea ice and full ice cover (perennial plus seasonal ice) with analysis of the March through September period and long term trends. By late spring 2007, negative ice anomalies were apparent around almost the entire ice edge and in August had broken through north of 80°. By September, sea ice extent and area of the ice pack had reached record minima (4.1 million km2 and 3.6 million km2 respectively) running at 37% and 38% below climatological averages. Key factors that can cause changes in sea ice are air temperatures, winds, waves and currents. In all months except May over the period March-September, 2007, there were anomalously high air temperatures. The southerly wind direction in the Beaufort and Chukchi seas may have contributed to the northerly retreat of the ice. In much of the Arctic basin, the open water area has increased by 23% per decade and the sea surface temperature by 0.7°C per decade. Updated trends for 1996-2007 now show a 10.1 % per decade decline for ice extent compared to the 1979-1996 trend of 2.2 % per decade. The recent low ice extent and area suggest a departure from the expected effects of the modes of the Arctic Oscillation proposed by earlier research. Perennial sea ice cover has been consistently low since 2002 and given the extended time taken to regrow perennial ice thick enough to survive the summer melt, presumably a few years of abnormally cold temperatures would be required for the ice to recover to 1980 levels. Seasonal loss of sea ice cover will undoubtedly have ecological implications. Continued loss of ice in the seas peripheral to the Arctic, where the ocean productivity is already high, could also have consequences for arctic and sub-arctic ecosystems.
Curry, J.A., P.J Webster and G.J. Holland, August, 2006. Mixing Politics and Science in Testing the Hypothesis That Greenhouse Warming Is Causing a Global Increase in Hurricane Intensity. Bulletin of the American Meteorological Society 87(8): 1025-1037 DOI:10.1175/BAMS-87-8-1025
This article is a summary of the arguments on both sides of the debate over whether or not climate change is causing an increase in the frequency and intensity of hurricanes around the world. As authors of a previous paper consistent with this hypothesis (Webster et al., 2005), they are also able to briefly describe in a case study the furor caused by the release of their previous paper. The authors describe and critique the logic of 14 challenges to the hypothesis. The critiques, drawn from mass media sources but including content originating in peer-reviewed scientific papers, are divided into those having obvious logical fallacies, those which have logical fallacies but nevertheless raise important related issues, and those based on logically valid arguments. The authors conclude that the central hypothesis cannot be invalidated by the 14 arguments considered, although considerable uncertainty and need for further research exists. The authors call for better involvement of scientists with the media, and for multidisciplinary collaboration among various groups of scientists studying the issue.
Della-Marta, P.M., M.R. Haylock, J. Luterbacher, and H. Wanner, 2007. Doubled length of western European summer heat waves since 1880. J. Geophys. Res., 112, D15103, doi:10.1029/2007JD008510.
Western European heat waves have doubled in length and the frequency of hot days has tripled over 126 years of records.
Examining homogenized daily summer maximum temperatures from 1880 to 2005, this study finds that the length of summer heat waves has doubled and the frequency of hot days has tripled over Western Europe. The study adjusts for a bias towards warmer temperature readings in the early (late nineteenth, early twentieth century) instrument records in most of the 54 stations used. A hot day in this study is defined as a day that exceeds the 95 percentile of the long-term maximum daily temperature. A heat wave is defined as maximum number of consecutive days which exceed the 95 percentile of the long-term maximum daily temperature. These findings are in keeping with other research, but the use of the homogenized data set has pushed the numbers higher, and the authors feel this could still be a conservative estimate, as not all the stations used in this study were homogenized. They find that most of the changes in the trends have occurred since 1950, but recommend further study to formally attribute these trends to specific causes.
Douglass, D.H., Christy, J.R., Pearson, B.D. and Singer, S.F. 2007. A comparison of tropical temperature trends with model predictions. Int. J. Climatology DOI: 10.1002/joc.
Study claims models are unable to accurately simulate tropospheric temperature trends over the tropics since 1979.
In 2005, a team of research scientists led by Ben Santer reported that trends in tropospheric temperature for the past few decades, as simulated by climate models, were in good agreement with those observed, and that differences were within reported error margins. However, in a new study published recently in the International Journal of Climatology, David Douglass (University of Rochester) and several other American researchers present contrary results. In their study, they compare simulation results from 22 state-of-the-art climate models with those from updated observed surface, radiosonde and satellite climate data sets. Based on their statistical analyses of these data, they conclude that the observed data fails to show the amplification of tropospheric warming relative to surface trends that the models all predict. They conclude that, given this disagreement with observations, projections of future climates generated by current climate models should be used with caution. Other experts, however, have noted that Douglass et al. have not used the latest updates in radiosonde data and have used incorrect methods for calculating model uncertainties (see www.realclimate.org). They argue that, when these are taken into consideration, the conclusions of Douglass et al. appear questionable.
England, J.H., T.R. Lakeman, D.S. Lemmen, J.M. Bednarski, T.G. Stewart and D.J.A. Evans. 2008. A Millennial-scale record of Arctic Ocean sea ice variability and the demise of the Ellesmere Island ice shelves. GRL, Vol 35, LI9502, doi:10.1029/2008GL034470.
The Ward Hunt Ice Shelf, which contains some of the oldest multi-year sea ice, may have been stable for the last 5500 years or more. This is some 2000 years older than previously thought.
The discovery in 2002 that the Ward Hunt Ice Shelf on the northern coast of Ellesmere Island had started to break apart caught many by surprise. Further investigations revealed that the Epishelf Lake, a body of freshwater floating on denser ocean water trapped by the ice shelf, had drained out. Trapped behind the ice shelf on the shores of the former Epishelf Lake were pieces of driftwood that were radiocarbon dated to be 3000 years or older. Since the ice shelf trapped the driftwood, it was thought that the ice shelf must have formed 3000 or more years ago, making this ice the oldest in the Northern Hemisphere. This new study tries to clarify the age of the Ward Hunt Ice Shelf using samples of driftwood transported across the Arctic Ocean and deposited on the northern shores of Ellesmere Island over the past 10,000 years. The radiocarbon dating from a total of 69 samples revealed a minimum age of 5500 years indicating that the driftwood had been deposited on the landward side of the modern Ward Hunt Ice Shelf some time before 5500 years ago. This is about 2000 years older than what was found in previous studies and serves as a proxy for the adjacent Ward Hunt Ice Shelf formation. If the ice shelves continue to disintegrate to open historically ice-filled fiords nearby, this will be an event not seen in the last 5500 years. This underscores the magnitude of current and projected changes in Arctic Ocean sea ice in that region due to the 20/21st century warming.
EPICA Community members (2006), One-to-one coupling of glacial climate variability in Greenland and Antarctica, Nature, vol.444, pp. 195-198.
Within the European Project for Ice Coring in Antarctica (EPICA), a new ice core covering 150,000 years was drilled in Antarctica. This ice core is special for two reasons. For one, it is the first deep ice core in the Atlantic sector of the Southern Ocean region, and as so represents the first direct Antarctic counterpart of the Greenland ice core records also taken in sites facing the Atlantic. Two, it was taken in a region where snow accumulation is much more abundant than at other drill sites on the east Antarctic plateau, thus allowing more detailed analysis on a much finer time scale. In comparing synchronized climate records from this new Antarctic ice core and ice cores in Greenland for the last glacial cycle, the research team found that temperature changes in the South had a strong connection with changes in the North that can be explained by the process of heat transfer of the meridional overturning circulation (MOC). This was not only for large millennial-scale events, but also for shorter and smaller temperature change events. Thus, their analysis showed that warming in the South is linearly related to cold periods in the North, a process they called "bipolar seesaw". Moreover, they found that the magnitude of warming events in Antarctica is linearly dependent on the duration of cold stages in the North, which suggest that for all events, the strength of the MOC is similar. However, the question of what triggers the switch from cold (stadial) to warm (interstadial) conditions remains.
Fedorov, A.V. et al. 2006. The Pliocene Paradox (Mechanisms for a Permanent El Niño). Science VOL 312 (5779): 1485-1489.
This review paper attempts to explore answers to a number of questions related to the "Pliocene Paradox" (the fact that the early Pliocene period (3-5 million years ago) had solar forcing, continental positioning and GHG concentrations similar to today's earth, but significantly warmer polar regions, leading to a Northern Hemisphere free of continental glaciers and a sea level ~25 m higher than today's). Recent paleodata seem to confirm that the Pliocene was characterized by permanent El Nino type conditions rather than the oscillating El Nino/La Nina conditions that characterize present climate. Therefore one question of interest is what triggered the resurgence of La Nina conditions and a return to a period of global cooling 3 million years ago. Conversely, it is important to understand what maintained the permanent El Nino conditions in order to assess whether continued global warming through the 21st century could trigger such a state. The authors indeed propose the existence of a trigger, flipping the climate from one state to the other, even as external forcings remain similar. The authors suggest that such a trigger is related to processes in the tropical oceans. During (and probably before) the early Pliocene, the thermocline (the region of relatively warm water sitting on top of the cold deep ocean water) was deep, due to the persistent El Nino like conditions. High latitude cooling and drying associated with the longterm cooling trend (beginning about 50 million years ago) would have gradually increased high-latitude deep-water formation. The authors posit a threshold around three million years ago when the expansion of cold deep water meant the thermocline became sufficiently shallow to allow winds to bring the cold waters to the surface in upwelling zones (much like today's conditions in between El Niño years). The cold tropical surface waters are thought to have amplified the climate response to Milankovitch forcing by reducing atmospheric water vapour and increasing the amount of reflective stratus cloud. The authors point out that the persistence of high carbon dioxide values in today's atmosphere could result in a return to Pliocene-like conditions if temperature and precipitation increases in high latitudes caused a deepening of the tropical thermocline.
Gedney, N., P.M. Cox, R.A. Betts, O. Boucher, C. Huntingford and P.A. Stott. 2006. Detection of a direct carbon dioxide effect in continental river runoff records. Nature 439: 835-838.
This study used the well defined methodology of optimal fingerprinting methodology (widely used in climate change attribution and detection studies) to determine the cause of the observed 20th Century increase in continental river runoff. Four possible contributors were examined: climate change; deforestation; solar dimming (reduced solar radiation reaching the atmosphere); and the direct effects of CO2 on plant transpiration. They found that the 20th Century climate alone was unable to explain the observed runoff trends. However, the trends were found to be consistent with a suppression of plant transpiration due to CO2-induced stomatal closure. This study is important as, although laboratory experiments have shown that enhanced CO2 would decrease stomatal openings of many plant species and thus reduce evapotranspiration, it was unclear whether this would be the case in the field. Furthermore, this study is the first to suggest that raised CO2 levels are already having a direct influence on the water balance at the land surface.
Gerdes, R. and Koberle, C. 2007. Comparison of Arctic sea ice thickness variability in IPCC climate of the 20th century experiments and in ocean-sea ice hindcasts. JGT 112, C04S13, doi:10.1029/2006JC003616, 2007.
In a recent article in JGR, two German scientists report that ocean-sea ice model reconstructions of Arctic sea ice thickness over the past century show virtually no trend in ice thickness between 1948 and 2000. In contrast, climate models forced with past changes in atmospheric radiative forcing show a decreasing trend that accelerates towards the end of the century. They conclude that climate models inadequately capture multi-decadal sea ice variability.
In the study, the authors use a number of ocean-sea ice models to develop proxy sea ice thickness scenarios for use as a reference against which to assess climate model simulations. One of these is then selected as the benchmark for testing climate model performance. They acknowledge that this 'hindcast' proxy scenario is 'model' derived, not observed, and hence has some inherent uncertainty due to model limitations. However, they defend using it as a benchmark for testing climate models on the basis of validation of at least some of the sea ice model results against a limited amount of observed ice thickness data available. The benchmark scenario does not show the trend towards reduced ice thickness simulated in the seven different coupled climate models that they assess. The key difference is that the hindcast shows an increase in thickness trends in the 1960s that offsets the decline in subsequent decades, hence resulting in little long term trend. This multi-decadal variability is not captured in the climate model simulations.
Grebmeier, J.M., J.E. Overland, S.E. Moore, E.V. Farley et al. 2006. A Major Ecosystem Shift in the Northern Bering Sea. Science 311: 1461-1464.
This international team of researchers show that in the northern Bering Sea, the historic ice-dominated, shallow ecosystem favouring benthic communities and bottom-feeding species such as sea ducks, walruses and gray whales is being replaced by one dominated by fish species which feed in the water column rather than on the sea bottom, typically found further south. This comprehensive study used satellite records, field studies and local ecological knowledge of the Yupkit hunters of St. Lawrence Island to document the change from arctic to subarctic conditions in this region. Indicators included: bottom water temperatures; air temperature; sea ice cover and concentrations; sediment oxygen uptake; and mammal surveys. Authors suggest that the shift in climate in this region is due to the decrease in the strong positive phase of the AO beginning in 1996. While most studies discussed in this article are not of a long enough time period to make definitive links to global climate change, the results do clearly illustrate the effects of a warmer climate on this type of ecosystem. If the warming trend continues, these shifts will continue to affect both subsidence harvests and commercial fisheries.
Hansen, J., M. Sato, R. Ruedy, K. Lo, D.W. Lea, and M. Medina-Elizade, 2006: Global temperature change. PNAS, 103(39), 14288-14293.
An interesting article by American scientists, including James Hansen, speculates that we are living in the warmest of times, and that a further 1°C change will bring about dangerous climate change. Historic records show that global temperatures in the late 1800s were about a degree cooler than today. When compared to temperatures over the last million years, using paleoclimate data, they show that this period is among the warmest. However, global temperatures in the early 21stcentury have been only slightly cooler that the warmest period in the last million years. If temperatures increase another degree, temperatures would be warmer than they have ever been, over the last million years, which the authors constitute as dangerous. As the authors point out themselves, the comparison of the historic data to the paleoclimate data is a bit questionable. The temperature records that are being compared are on differing scales and resolutions, so it would be impossible to say that over the last million years that there wasn't a 5 year period that wasn't as warm or warmer that the present period, but it is masked by the 1000-year resolution of the reconstruction.
Hitch, A.T., P.L. Leberg. 2007. Breeding distributions of North American bird species moving north as a result of climate change. Conservation Biology 21(2): 534-539.
Adding to the evidence of climate change impacts on natural ecosystems, a new study in North America found bird specifies have moved significantly northward over the past few decades.
Changes in the breeding distribution of North American bird species were examined using a large data set covering over 50 species in central and eastern United States. The authors found that species have moved northward by an average 2.35km/year over the 26 year study and that there was not a corresponding southward shift. This led authors to conclude that warming temperatures, and not other factors such as land-use change, were the cause of the northward movement. Furthermore, the results of this study are consistent with results of a similar large study done recently in Great Britain. The authors conclude that when considered together, results from these two independent, multi-species studies, on two continents, support the contention that the northward expansion in breeding range is due to climate warming.
Hoerling, M., J. Eischeid, X. Quan, and T. Xu, 2007. Explaining the record US warmth of 2006. Geophys. Res. Lett., 34, L17704, doi:101029/2007GL030643
More than half of the warmth during record 2006 temperature in the U.S. can be attributed to greenhouse gas forcing.
2006 was the warmest year on record for the US - 1.1°C above normal - and four NOAA scientists decided to find out why. In particular, the authors were interested in how much warming could be attributed to the El Niño which developed during the year, and how much warming was attributable to greenhouse gas forcing. By averaging the national temperature over ten of the last El Niño events, they determined that if anything, El Niños caused a net cooling over the whole country. They also ran unforced integrations of two atmospheric climate models with 1971-2000 sea surface climatology. These simulations were then compared to a composite simulation using historical El Niño forcings. The conclusion from this comparison was that there was no effect of El Niños on average annual temperatures. The authors then focused on the potential role of greenhouse gas and aerosol forcing. Using 18 models (42 simulations), forced with estimations of GHG and aerosol changes from the late 19th century through 1999, then driven with the SRES A1B emission scenario until 2006, they found the mean signal was about 0.62°C - more than half of the 1.1°C anomaly of 2006.
Holgate, S.J. 2007. On the decadal rate of sea level change during the twenthieth century. GRL Vol 34, L01602, doi:10.1029/2006GL028492, 2007.
Researcher suggests highly variable rate of sea level rise during the past century.
In a recently published paper in Geophysical Research Letters, British oceanographer Simon Holgate argues that global sea level rise during the past two decades has not been particularly unusual within the context of longer term sea level variability. He bases his conclusion on the analysis of nine lengthy high quality sea level records (three at coastal sites around Europe, four along US and Central American shores and two in the central and south Pacific) that he argues are representative of global sea level behaviour. His analysis indicates a mean sea level rise over the past century of about 17.4 cm (which is consistent with other results). However, it also suggests that the rate of rise during the first half of the century (1904-1953) is an estimated 2.0 cm per decade, some 40% higher than the 1.45 cm/decade rate during the second half (1954-2003). One of the key factors is the 1.5 cm decline in levels during the 1960s (also a period of slight global cooling). The highest rate of decadal change (5.3 cm) occurred during the 1980s.
Holland, G.J. and P.J. Webster. 2007. Heightened tropical cyclone activity in the North Atlantic: natural variability or climate trend? Phil. Trans. R. Soc. A. Published online. doi:10.1098/rsta.2007.2083.
In a recent study on hurricane trends, researchers come to the conclusion that the number of Atlantic hurricanes in an average season has doubled in the last century, due in part to warmer seas and changing wind patterns caused by global warming.
In recent years, there has been a debate about whether climate change has altered the frequency and/or intensity of tropical storms and hurricanes. In this new study, two researchers looked at sea surface temperatures (SSTs) and tropical cyclone data for the period 1855-2005, to assess the longterm variability of tropical cyclone frequency in the North Atlantic Ocean. In addition, the period 1945-2005 was used for the determination of changes in hurricane intensity. Their results show that for the 1900-2005 period, there is a marked increasing trend in North Atlantic tropical cyclones and hurricanes that has arisen from sharp transitions between three distinct relatively stable climatic regimes (1905-1930, 1931-1994, 1995-2005). From one regime to the next, the number of tropical cyclones and hurricanes increased by 50%. That trend is consistent with a marked warming trend close to 1°C in eastern Atlantic SSTs (the main hurricane development region of the North Atlantic), which also occurred in two similarly sharp transitions of about 0.4°C. Superimposed on these regime changes, the authors find a completely independent oscillation that affects the proportion of tropical cyclones that become major and minor hurricanes. That oscillation has no net trend over the time period but is closely related to SSTs in the Gulf of Mexico and with meridional changes in hurricane genesis locations between mid-latitude (north of 25°N = minor hurricanes) and equatorial (south of 25°N = major hurricanes) regions. They also find that although the proportion of major hurricanes appears to be dominated by this cyclic behaviour, the number of major hurricanes has a distinct and statistically significant trend that arises from the increasing numbers of tropical cyclones. The authors note that the elevated level of hurricane activity since 1995 has arisen from the coincidence of the increasing trend in the number of tropical cyclones and a peak in the oscillation in proportion of major hurricanes to all troipical cyclones.
Kimball, J., Zhao, M., McGuire, A. et al. 2007. Recent Climate-Driven Increases in Vegetation Productivity for the Western Arctic: Evidence of an Acceleration of the Northern Terrestrial Carbon Cycle. Earth Interactions 11(Paper #4): 1-30.
A new study provides further evidence that regional climate change is causing a greening of tundra and boreal forest ecosystems of Alaska and northwest Canada. The 'greening' is accompanied by an increase in carbon storage.
A study by several American researchers examined the effects of recent climate change on productivity of Western Arctic ecosystems, and the resulting impact on the regional terrestrial carbon cycle. The study utilized satellite data, a biome-specific Production Efficiency Model (PEM), and two regional ecosystem process models to simulate changes in productivity in the boreal forest and tundra biomes of Alaska and northwest Canada between 1982 and 2000. Simulations indicate an increase in photosynthetic leaf area and vegetation productivity of 4% and 18%, respectively, during this period, and an average rate of increase in Net Primary Productivity of 53.7g c/m²decade. The rate of carbon sequestration for vegetation in these biomes increased more than that of respiration, leading to an increase in the fraction of total carbon storage in vegetation biomass. However, the study also suggests an increase in the rate of ecosystem carbon turnover. These results are consistent with those of past studies.
Kinnard, C., Zdanowicz, C.M., Koerner, R.M. and Fisher, D.A. 200. A changing Arctic seasonal ice zone: Observations from 1870 - 2003 and possible oceanographic consequences. GRL 35, L02507, doi: 10.1029/2007GL0320507.
Canadian scientists have undertaken analysis of more than a century of Arctic sea ice data to show that the area undergoing seasonal changes has been expanding and moving northward, particularly in recent decades. This appears to be affecting Arctic Ocean salinity, and may have implications for global ocean circulation.
A team of Canadian scientists from the University of Ottawa and the Geological Survey of Canada report in an article in Geophysical Research Letters that the Arctic Ocean's seasonal ice zone (the region along the margins of the Arctic ice pack that undergoes large seasonal changes from ice cover in winter to little ice in summer) appears to have been expanding gradually over the past century. Their results, based on an analysis of historical records and satellite observations collected between 1870 and 2003, also indicates that the rate of expansion has accelerated in recent decades, increasing from the long term average of 900,000 km2/decade to more than 2 million km2/decade. The seasonal ice zone has also migrated poleward. Observations indicate that this expansion may already have contributed to increased salinity of Arctic Ocean surface waters. It could also be enhancing the rate of formation and salinity of Arctic Ocean deep waters. This is because the freezing process that makes first year ice - the dominant ice form found in the seasonal ice zone - creates brine pockets within the ice that drain out of the ice into the ocean waters below. By comparison, much less salt solution is released into the ocean from the perennial ice that the first ear ice is slowly replacing. The advection of more saline Arctic Ocean deep waters into the North Atlantic may in turn have implications for global thermohaline circulation processes.
Landsea, C.W., Harper, B.A., Hoarau, K. and Knaff, J.A. 2006. Can we Detect Trends in Extreme Tropical Cyclones? Science 313, 452-545).
A recent article in Nature has emerged in the debate regarding the increasing trend of extreme tropical cyclones as a result of climate change. Despite several studies in recent years identifying and qualifying the link between increased temperatures and extreme tropical cyclones, Landsea et al. question the accuracy of the Dvorak technique, the most widely used global tropical cyclone database, specifically regarding the frequency of these events. According to this article, the fact that Dvorak technique does not directly measure maximum sustained surface winds and the operational changes made at many of these cyclone warning centers since 1990, have rendered it's results inaccurate and unreliable. The study found no significant trends in tropical cyclone activity since the 1960's using aircraft reconnaissance data of the Atlantic and Northwest Pacific, and thus assumes that the current trends observed come as a result of unwarranted bias corrections and inhomogeneous tropical databases dependent on satellite imagery. Finally, the study attributes the increase in hurricane frequency and intensity to better technology, allowing for more efficient monitoring and locating of extreme tropical cyclones.
Latifovic, R., and D. Pouliot, 2007. Analysis of climate change impacts on lake ice phenology in Canada using the historical satellite data record. Remote Sensing of Environment 106, 492-507.
Satellite data demonstrate a two week reduction in ice cover on Canadian lakes over the past 50 years.
Two Natural Resources Canada scientists have developed a technique to extract lake ice freeze-up and break-up timing from AVHRR satellite data. This method has extended existing in-situ data for 36 lakes, and created a new data set for 6 lakes in the far north. Changes in lake ice behaviour are a direct reflection of the changes in the local climate. The results show that for the 1950-2004 period, freeze up has occurred later by 0.12 days/year (or about 6 days over the 50 years), and break-up has occurred earlier by 0.18 days/year (or about 9 days over the 50 years). Interestingly, the shorter 20 year period for the newly constructed far north lakes show a very strong trend for both break-up (average of 0.99 days/year or 20 days in 20 years) and freeze-up (average of 0.76 days/year, or 15 days over the last 20 years). As this technique is not restricted by the sensor used on the AVHRR satellite, a potentially large database of lake ice phenology could be created.
Luthcke, S.B., H.J. Zwally, W. Abdalati, D.D. Rowlands, R.D. Ray, R.S. Nerem, F.G. Lemoine, J.J. McCarthy and D.S. Chinn. 2006. Recent Greenland ice mass loss by drainage system from satellite gravity observations. Science November 24, 2006, Vol 314 pp 1286-1289.
Since the early 1990s, remote sensing methods have provided the first observations of ice sheet mass balance. These data have indicated recent net mass loss of the Greenland Ice Sheet (GIS), where accelerated melting in coastal regions is dominating over mass gains in the central high-elevation region. This new data set of mass concentrations by Luthcke et al., derived from data from the Gravity Recovery and Climate Experiment (GRACE) satellite mission, provide a direct measure of mass changes on the GIS with improved spatial and temporal resolution. Mass concentrations were derived for each of six drainage systems, subdivided into regions above and below 2000-m surface elevation, every 10 days over the period July 2003-July 2005. From 2003-2005, the ice sheet loss 101 16 gigaton/year, with a gain of 54 gigaton/year above 2000m and a loss of 155 gigaton/year at lower elevations. There is a small annual cycle at high-elevations, consistent with low temperatures, negligible melting and small seasonal variation in precipitation. The low elevation coastal regions show a large annual cycle, with an extensive summer melt period, followed by mass gains from fall to spring. Although difficult to draw meaningful conclusions from a two-year time series, the authors note that their data confirm those of other studies and suggest that the processes of significant ice depletion at the margins of the GIS are beginning to dominate the interior growth as climate warming has continued.
See also the Perspective by A. Cazenave, 2006. How fast are the ice sheets melting? Science November 24, 2006, Vol 314 pp 1250-1252.
Lyman, J., Willis, J. and Johnson, G. 2006. Recent cooling of the upper ocean. Geophysical Research Letters 33, L18604, doi: 10.1029/2006GL027033.
A recent paper by American researchers estimates the overall global ocean heat content anomaly for the upper 750m of the ocean, from 1993 to 2005, using a series of buoy and ship-based temperature sensors. Despite a clear warming trend in ocean temperatures during the last 13 years, this study shows a net heat loss from the upper ocean between 2003 and 2005. Previous studies have mostly observed a warming trend and tend to attribute this interannual variability of upper ocean heat content to a lack of data or sampling errors. However, Lyman and colleagues argue that the 3.2 x 1022 J loss in the upper 750 m of the ocean - which corresponds to one fifth of the long-term upper-ocean heat gain between 1955 and 2003 - is significant and not a simple artefact of inadequate sampling. The study attributes the cooling to various causes - such as a net loss of heat from the earth to space or heat transport to the deep ocean - arguing that the amount of heat loss is too large to be stored in any single component of the Earth's climate system.
McKitrick, R.R. and Michaels, P.J. 2007. Quantifying the influence of anthropogenic surface processes and inhomogeneities on gridded global climate data. JGR Vol 112, D24S09, doi:10.1029/2007JD008465.
Some of the recent warming observed around the world may still be due to local effects of human activities.
Climate data analysts have long recognized that human activities have a local affect on temperatures that go well beyond that of rising greenhouse gas concentrations. In particular, urban heat island effects and changes in land surfaces are known to influence local temperatures. In addition, there are inevitable errors in temperature records due to inconsistent observing methods or other human error. Experts have tried to address these non-climatic factors by correcting the data records for such errors and compiling adjusted global records that also consider areal biases in climate station densities. Various past studies have argued that these corrections have been largely successful in removing such biases from global data sets. A new study by McKitrick (U of Guelph) and Michaels (U of Virginia) contend that there are still significant residual biases in the data that result in an overestimation of the global temperature trend over the past several decades. They first undertake a statistical comparison of trends in socio-economic factors that can affect local temperatures with temperature trends and show that these do not appear to be independent. They then adjust the data to correct for these residual influences. While there is little change in some regions, such as North America, Australia and Eastern Europe, they find significant reductions in warming trends in other regions, especially Western Europe and southeast Asia. They argue that the net global warming over land may be significantly less than that reported in past studies.
Mann, M.E., Z. Zhang, M.K. Hughes, R.S. Bradley, S.K. Miller, S. Rutherford and F. Ni. Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. PNAS September 9, 2008 Vol 105 No 36 pp 13252-13257.
New reconstruction of Northern Hemisphere climate over the past two millennia confirms the anomalous warmth of the past decade.
Michael Mann and co-authors present new reconstructions for global and hemispheric surface temperature extending back two thousand years. They respond to recommendations in the 2006 report of the U.S. National Research Council on surface temperature reconstructions by using a greatly expanded global set of proxy data and recently updated instrumental data, and by comparing two distinct methods of temperature reconstruction (referred to as CPS and EIV methods). The proxy data used consisted of over 1200 data series from tree-ring, marine sediment, speleothem, lacustrine, ice core, coral and historical documentary sources. Since some questions have been raised about the reliability of tree ring data component, they also develop reconstructions without these data. Instrumental data were from the University of East Anglia Climate Research Unit. Both reconstruction methods produced skillful NH land reconstructions back to A.D. 400, although extracting the tree-ring data from the proxy data meant reconstructions were only skillful back to A.D. 1500 with the CPS approach and to A.D. 1000 with the EIV approach. A skillful EIV reconstruction was possible going back to A.D. 700 even without the tree-ring data when NH land and ocean temperatures were combined. The authors note this is a significant advancement relative to earlier work. Overall, the authors conclude, that the EIV reconstruction appears more robust and reliable, particularly in the time period prior to A.D. 1000. Conclusions confirm the findings of the NRC and others that the warmth of the past decade appears anomalous for at least the past 1,300 years. If tree ring data are excluded, this conclusion holds going back at least 1,700 years. Conclusions are less definitive for the SH and the globe, largely due to the sparseness of SH proxy data.
Maslanik, J.A., C. Fowler, J. Stroeve, S. Drobot, J. Zwally and W. Emery. 2007. A younger, thinner Arctic ice cover: Increased potential for rapid, extensive sea-ice loss. GRL Vol 34, L24501, doi:10.1029/2007GL032043.
A new study that combines both ice age and ice thickness data shows that older, thicker sea ice in the Artic is being replaced by younger, thinner ice. These changes, in light of projected ongoing Arctic warming, indicate an increasing susceptibility to large abrupt changes in ice cover such as that which occurred this past summer.
Research published earlier in 2007 has shown that the composition of perennial Arctic ice has changed with a decline in multi-year ice and an increase in first-year ice. This paper by Maslanik et al. provides further evidence of such a shift in the nature of Arctic perennial ice. The authors use data from both satellites and drifting buoys to create gridded vector fields of ice movement over the period 1979 through to summer 2007 from which they calculate the age of ice in each grid cell. These data show that the most of the perennial ice pack now consists of ice that is 2 or 3 years old (58% in March 2006) and that older ice types have virtually disappeared (5% in 2007 for ice of 7+ years). The authors also use a 4 year (2003-2006) ice thickness data set estimated from ICESat data, combined with the ice-age data set, to develop a proxy time series for ice thickness for the month of March for the period 1982-2007. These data showed that the bulk of the ice loss associated with depletion of older ice types has occurred in the eastern Arctic. Ice thickness increased slightly in a few areas adjacent to the Canadian Archipelago and in the eastern Beaufort Sea. Thickness averaged over the Arctic recovered slightly in the late 1990s, but since then, thinning, associated with a transition to a younger ice cover, has continued. The authors note that the confinement of older thicker ice to a small portion of the Arctic Ocean is setting the stage for an Arctic ice cover that is increasingly susceptible to large, rapid reductions in ice extent, such as that experienced in September of this year.
Menzel, A., Sparks, T.H., Strella, N. et al. 2006. European phenological response to climate change matches the warming pattern. Global Change Biology: 12:1-8.
The results of a Europe-wide study of phenological responses to climate change between 1971 and 2000 appeared in the most recent issue of Global Change Biology. Phenological trends for 542 plant species in 21 countries and 19 animal species in 3 countries were obtained through the COST action 725 network while annual, monthly and seasonal temperature means for all European countries were found in the Tyndall Centre database. Results showed a distinct phenological response to temperature change during the spring while the autumn signal was significantly weaker. Although the mean advance of spring was 2.5 days/decade, yearly spring phenological onset advanced by as much as 4.6 days/°C, while the autumn phenological response was delayed by as much as 2.4 days/°C. A correlation study between the mean monthly temperatures and the month of onset showed the strongest correlation was with the preceding month (63%). There is clear evidence that spring and summer phenology across Europe is changing with 78% of leaf unfolding and flowering records advancing, 75% for fruit ripening records and 57% for records of farmers' activities. In contrast, phenological trends in autumn were more ambiguous but overall, showed negative trends regarding onset of various stages, with, for example, 52% of leaf colouring records showing delayed onset.
Milewska, E.J. 2008. Cloud type observations and trends in Canada, 1953-2003. Atmosphere-Ocean 46:297-313.
Recent studies show that high level cirrus and stratocumulus-type convective cloud covers over Canada are increasing while low level stratus type clouds are decreasing.
In a new study published in the Canadian scientific journal Atmosphere-Ocean, Ewa Milewska (an Environment Canada research meteorologist) analyzes cloud data across Canada for trends in different types of cloud, and considers some of the surface climate implications of these trends. The analysis uses carefully screened data from 84 weather stations across Canada over the period 1953-2003. Milewska breaks the cloud type data into three broad height categories -high (bases at 6 km and higher), middle (2-6 km base height) and low (bases below 2 km). Results for the summation of these categories confirm those from earlier study. The majority of the stations analyzed show that the annual number of hours when more than 50% of the skies are covered by all cloud types increased by between 10 and 17% over the 51 years of the study. However, there are significant downward trends in the western provinces and in northern Quebec in the annual number of hours when more than 50% of the skies are covered by low and middle cloud types - as much as 5 to 11% over the study period. Offsetting this is a strong positive trend in the amount of high level cirrus clouds across the entire country. Most regions show increases of between 10 and 23%, with those over Alberta reaching an astonishing 23-34%. Undoubtedly, some of this apparent increase may simply be due to more of the upper cloud becoming visible to the observer at the surface as the amount of low and middle cloud cover decreases. However, both the magnitude and the cross country extent of the rise in the reported cirrus amounts suggest most of the trend is real. Many of the stations in Canada also show upwards trends in convective cloud types of between 23 and 34%, which can be attributed to the increase in stratoculumus - a cloud type of weak and shallow convection. However, strong convective clouds associated with severe thunderstorms decreased significantly in Alberta and B.C. Milewska notes that the cloud trends could possibly be linked to rising maximum and minimum temperatures and decreasing mean daily rainfall there.
Moran, K., Backman, J., Brinkhuis, H. et al. 2006. The Cenozoic palaeoenvironment of the Arctic Ocean. Nature 441:601-605.
Stoll,H.M. 2006. The Arctic tells its story. Nature 441:579-580.
Two years ago, a large team of scientists extracted a long sediment core from a mid-Arctic Ocean ridge for use in reconstructing Arctic Ocean environmental conditions over the past 65 million years. In the June 1, 2006 issue of Nature, these scientists report on some of the first exciting results of the sediment core analysis. They report, for example, that Arctic Ocean surface temperatures peaked some 55 million years ago at temperatures of 23-24°C, much higher than previously thought. There are also indications of a much more intense hydrological cycle at the time. This intensely warm, wet period is coincident with high CO2 concentrations, and is therefore likely linked to a super-greenhouse effect period. Ten million years later, temperatures cooled significantly to about 10°C and sea ice cover first began to form. That was also when Antarctica began to glaciate, and coincides with a decline in CO2 concentrations. However, climate models that have tried to simulate the climate of 55 million years ago based on high CO2 concentrations alone fail to properly capture the full magnitude of the observed warming. This suggests positive feedbacks in the climate system such as, perhaps, stratospheric clouds of frozen ice crystals, that are as yet not adequately included in model processes. Hence, while these results support the concept of a super greenhouse gas period some 55 million years ago, many questions remain about what other processes were involved.
Mote, T.L. 2007. Greenland surface melt trends 1973-2007: evidence of a large increase in 2007. GRL Vol 34, L22507, doi:10.1029/2007GL031976.
Melting on the Greenland Ice Sheet over the 2007 summer melt season was 60% higher than the previous record, providing additional evidence of a very unusual Arctic summer.
The record breaking minimum Arctic sea ice extent this past September received a lot of attention, both among the media, and scientists. This paper adds to the evidence that Arctic ice conditions this past summer were highly unusual by providing evidence of record breaking melt on the Greenland Ice Sheet over the 2007 melt season. The study builds on previous analyses of passive microwave time series of surface melt on the Greenland Ice Sheet by extending the time series back a few years to 1973 and updating it to 2007. In addition, a new metric for melt is introduced - the seasonal melt departure (SMD) - the sum of the departures from average melt extent each day over the period June 1 to August 31. The SMD data show a series of years of above average melt beginning in 1997 and a clear upward trend since the 1970s. Results show that the duration of melt and the onset of melt both indicated more melt than average in 2007 for large parts of the Ice Sheet, with some locations experiencing as many as 50 more days of melt than average and with melt occurring 30 days earlier than average in some areas. Overall, the seasonal melt departure for 2007 was 60% greater than the next highest year (1998). The melt trend agrees with recorded increases in both surface temperature of the ice sheet and with increases in average daily maximum temperature at three coastal stations. However, more melt was experienced in 2007 than would be expected based on the summer temperature record alone. The authors postulate that consecutive years of warmth may have had a cumulative effect on ice melt in 2007 and note a number of possible mechanisms (e.g. a decrease in surface albedo or in the cold content of the snow). As yet, there is not enough evidence to state conclusively why 2007 was so anomalous.
Narisma, G.T., J.A. Foley, R. Licker, and N. Ramankutty, 2007. Abrupt changes in rainfall during the twentieth century, Geophy. Res. Lett., 34, L06710, doi:10.1029/2006GL0282628.
Abrupt changes in precipitation were a regular feature of global twentieth century climate.
Prolonged droughts like the U.S. "Dust Bowl" of the 1930s, and the Sahel drought in the 1970s, are held up as examples of modern abrupt shifts in precipitation. This American study examined the twentieth century records to see just how isolated these examples were. As it turns out, there have been about 30 significant regional drought events over many areas of the world that persisted for at least ten years and encompassed a drop in rainfall of about 10%. Most have occurred in arid or semi-arid areas that have a tendency to show large historical fluctuations in rainfall totals. This larger sample of events will provide researchers with a greater opportunity to both study the climate system and the impacts of drought on societies, and will provide more data for climate model evaluations.
Nghiem, S.V., I.G. Rigor, D.K. Perovich, P. Clemente-Colon, J.W. Weatherly, G. Neumann. 2007. Rapid reduction of Arctic perennial sea ice. GRL 34, L19504, doi:10.1029/2007GL031138.
Record losses of winter multi-year (perennial) sea ice in recent years mean that winter sea ice has become thinner, with proportionally more first year ice. These observations, together with other reports of record summer sea ice minima, point to dramatic changes in the character and extent of Arctic sea ice.
The declining trend in summer sea ice extent over the past couple of decades, with a record minimum in 2007, has been well reported in the scientific literature and by agencies that monitor sea ice. This does not tell the whole story, however, of changing Arctic ice conditions. Nghiem et al. report in GRL on how the composition of winter sea ice is also changing. Satellite data (QSCAT) show a reduction in perennial ice extent in March from 4.69 x 106 km2 in March 2005 to 3.61 x 106 km2 in March 2007, a reduction of 23% over just 2 years. While the perennial ice declined, total ice coverage was fairly stable because most of the loss from perennial ice was replaced with seasonal (first year) ice. The authors report that much of the loss of perennial ice was due to anomalous wind patterns favouring transport of ice out of the Arctic, although ice movement was likely enhanced by the fact that the ice was thinner. Therefore both dynamic and thermodynamic effects appear to be combining to expedite the loss of Arctic sea ice. Furthermore, the changes in winter ice conditions are suggested to be preconditioning the sea ice for more efficient melting and further ice reduction in summer. These changes in the distribution and proportion of perennial (multi-year) and seasonal (first year) ice in March impact both the sea ice mass balance and the ice-albedo feedback mechanism.
Noris, J.R. and Wild, M. 2007. Trends in aerosol radiaitve effects over Europe inferred from observed cloud cover, solar 'dimming' and solar 'brightening'. JGR Vol 112, D08214, doi:10.1029/2006JD007794.
European observations of incoming sunlight at the surface show that the region underwent a period of "solar dimming" (decreasing incoming shortwave radiation) between 1971 and 1986 followed by a period of "solar brightening" between 1987 and 2003. Analysis of these trends suggest that rising and falling rates of human emissions of aerosols were the likely significant factors.
Authors of the study first assessed the role of cloud cover in the trends, and found this to be a dominant factor on monthly to decadal time scales. However, after this effect was subtracted from the data, there remained a residual multi-decadal decrease in incoming solar radiation across Europe of about 3 W/m2/decade during the first half of the record, and a subsequent rate of increase of slightly more than 2 W/m2/decade.
(1) Osborn, T.J. and Briffa, K.R. 2006. The spatial extent of the 20th-century warmth in the context of the past 1200 years. Science 311:841-844.
(2) D'Arrigo, R., Wilson, R. and Jacoby, G. 2006. On the long-term context for the late twentieth century warming. JGR 111, D03103, doi:10.1029/2005JD006352.
In recent years, a number of research studies have provided evidence that the 20th century is likely the warmest century of the past two millennia, and that the 1990s has been the warmest decade. However, there are also indications that these studies may have underestimated uncertainties because of the limitations of both the data and the analysis methodologies used, and hence that their results still need to be used with caution.
Two new papers now attempt to address these limitations and hence improve the confidence in conclusions about the significance of recent climate anomalies. The study by D'Arrigo and colleagues departs from traditional approaches to hemispheric proxy analysis by first normalizing proxy records of climate at the regional level before integrating them into a hemispheric land temperature data base. That is, they determine departures from long-term means, then convert the amplitude of variations to a common standard deviation before integrating them into a hemispheric temperature record. D'Arrigo et al. use only tree-ring records (which are a proxy for climate of the extra tropical land regions of the Northern Hemisphere), extending back to almost 700 A.D. Comparison with traditional global approaches to standardizing data suggests that the regional approach better captures the amplitude of low frequency variability of climates on multi-century time scales. The study results suggest that, for extra-tropical land areas, the late 20th century temperatures are about 0.7°C warmer than the Medieval Warm Period, and that the range of variation between the end of the proxy record (mid-20th century) and the coldest period during the Little Ice Age is about 1.14°C. However, the authors note that, because of continuing unavoidable limitations in the small number of proxy records involved and other factors contributing to uncertainty, results must still be used with caution.
The second study by Osborn and Briffa also attempts to address some of the limitations of past studies by, among other things, first normalizing proxy data at the regional scale. They screen available multi-proxy records for strength of temperature signal, length of record and other factors to end up with 14 high quality data records to use in the hemispheric proxy. To test for random noise, autocorrelation and internal variability, they randomly shift the records in time to develop 10,000 variants and develop related probability statistics. They further assess how representative the 14 records are of hemispheric climate. Finally, they examine how often the records exceed 1 and 2 standard deviations and calculate from one time period to the next what percentage of such deviations are positive or negative. They conclude that, since 800 A.D., there is a significant temperature signal in the record that cannot be explained by random or internal variability and therefore must be due to external forcing, that the 14 proxy record data base adequately represents the hemispheric temperature variability, and that the strongest evidence for a large warm signal occurs during the latter half of the 20th century, when greenhouse gas forcing is at its highest.
Overpeck, J.T., B.L. Otto-Bliesner, G.H. Miller, D.R. Muhs, R.B Alley, and J.T. Kiehl, 2006: Paleoclimate evidence for future ice-sheet instability and rapid sea-level rise. Science, 311, 1747-1750.
Using paleoclimate data, the authors of this study theorize that the threat of sea level rise due to climate change could be greater that previously thought. By examining ancient storm beaches and barrier islands in the Arctic, the sea level was estimated for the Last Interglaciation period (LIG), 129,000-118,000 years ago, at 4-6m above present sea levels. Comparing the estimated volume loss from the Greenland Ice Sheet (GIS) during the same period, the authors have estimated that 2.2-3.4 m of that rise was due to contributions from the GIS and other arctic ice fields. They postulate that the remainder of the sea level rise is due to losses from thinning of the Antarctic ice sheet. However, the cause of the LIG is thought to be due to a change in Earth's orbit, which warmed the northern hemisphere disproportionately more than the southern hemisphere. They conclude that because anthropogenic climate change will affect the whole globe, not just one hemisphere, the contribution from Antarctic ice sheets will be greater for the same amount of warming than was estimated to occur in the LIG period. The contribution from Antarctica could force sea levels higher and at a faster rate than most models project.
Parker, D.E. A Demonstration That Large-Scale Warming is Not Urban, Journal of Climate, 19 (14), 2006, 2882-2895.
A recent study out of the U.K compared global and regional temperatures trends under windy and calm conditions to assess the overall contribution of urban development and other local or instrumental influences to the observed 20th century warming trends. The Global Climate Observing System (GCOS) was used to obtain daily maximum and minimum temperature data from 1950 to 200 for 26 stations while near surface wind components were obtained for the same period from NOAA. Results showed that the warming effects of urban climate on minimum temperatures on calm nights, when the impact is expected to be at a maximum, were non-existent as windy, calm and all conditions showed similar warming trends of 0.20° ± 0.06°C per decade. Further analysis showed that there was no statistical significance between windy and calm and between minimum and maximum temperatures trends. Finally, a positive annual trend was obtained when calculating annual maximum for windy minus calm conditions, contrary to a typical urbanization signal since the urban warming impact is expected to be greatest under calm conditions. This study, which supports previously obtained results presented in Jones and Moberg (2003), supports the IPCC TAR conclusions that the impact of urban warming on global and large regional trends is small and responsible for an "uncertainty" of 0.06° in the global warming of the 20th century.
Parmesan, C. 2006. Ecological and evolutionary responses to recent climate change. Annual Reviews of Ecology, Evolution and Systematics 37:637-669.
A major review paper on how plants and animals have responded to recent climate warming was recently published. Results demonstrate that the direct impacts of anthropogenic climate change have now been documented on every continent, in every ocean and in most major taxonomic groups. The review covers studies of changes in both distribution (animal studies mainly) and phenology (plant studies mainly). There is a terrestrial bias in the literature, although studies in marine and freshwater environments are increasing. The major data gaps are geographic rather than taxonomic, with few studies available from South America, Africa and Asia. Range-restricted species, particularly polar and mountain-top species show severe range contractions, while tropical coral reefs and amphibians are identified as groups most negatively affected to date. Phenological studies show that for many species, the primary impact of climate change may be the potential disruption of coordination in timing between the life cycles of predators and their prey, herbivorous insects and their host plants, and other such inter-dependent relationships. The evidence amassed to date provides little support for genetic evolutionary shifts that might mitigate the negative effects at the species level. However, such shifts could play a role in modulating local population responses.
Rahmstorf, S., Cazenave, A., Church, J.A. et al. 2007. Recent climate observations compared to projections. Science Vol 316:709.
Results of an international study that compares trends in global temperature and sea levels since 1990 with those projected by climate models suggest that IPCC projections for future climate change are not exaggerated, and may in fact be underestimates of what is likely to actually happen.
The authors of the study noted that the observed rise in carbon dioxide (CO2) concentrations was almost identical to that projected in CO2 scenarios developed by IPCC. However, the observed 0.33°C rise in temperature since 1990 was at the upper end of that projected by the models for the same period. Likewise, the observed sea level rise between 1993 and 2006 was 0.3 mm/year, about 50% faster than the best estimate projected by IPCC. The authors caution that 16 years is too short an interval to accurately attribute the cause of the recent rapid change in temperature and sea level. Hence internal climate variability or non-CO2 climate forcings may be factors in the rise. However, at a minimum, these results suggest that the range of IPCC projections are not likely to have overestimated the risk of future climate change.
Rayback, S.A. and Henry, G.H.R. 2006. Reconstruction of Summer Temperature for a Canadian High Arctic Site from Retrospective Analysis of the Dwarf Shrub, Cassiope tetragona. Arctic, Antarctic, and Alpine Research, 38 (2), 228-238.
A recently published study by two UBC scientists used the dwarf shrub Cassiope tetragona to reconstruct climate over the past century on Ellesmere Island. Such work seeks to supplement the scarce observational climate data that exist in the Canadian High Arctic from weather stations. In this study, chronologies based on plant morphology were constructed for periods ranging from 80-118 years. Multiple regression models were then used to develop a 100 year long (1895-1994) reconstruction of summer (July-September) air temperatures. Results showed increasing temperatures from about 1905 to the early 1960s followed by a subsequent cooling trend through the 1970s. The reconstruction ended with a shift towards warmer temperatures beginning in the early 1980s. This trend is consistent with other Arctic-wide studies and with observations over the past 50 years suggesting that Cassiope Tetragona is a suitable temperature proxy and one that contains a large-scale temperature signal.
Rayner, N.A., P. Brohan, D.E. Parker, C.K. Folland, J.J. Kennedy, M. Vanicek, T.J. Ansell, and S.F.B. Tett, 2006. Improved analysis of changes and uncertainties in sea surface temperature measured In Situ since mid-nineteenth century: the HadSST2 dataset. J. Climate, 19 (3), 446-469.
Using the new International Comprehensive Ocean-Atmosphere Data Set (ICOADS), this study examines changes in sea surface temperature (SST) between 1850 and 2005. This dataset corrects for bias introduced by different measurement methods and platforms, and better gridding methodologies are used. The results show that the global SST has increased by 0.67°C; Northern Hemisphere waters have warmed by 0.71°C; and Southern Hemisphere oceans warmed by 0.64°C. These trends are non-linear, with most of the change occurring after 1900.
Rignot, E. and Kanagaratnam, P. 2006. Changes in the Velocity Structure of the Greenland Ice Sheet. Science 311: 986-990.
Rignot and Kanagaratnam used satellite radar interferometry data to measure the change of glacial velocity of Greenland between 1996 and 2005 due to the artic warming in the last decade. Results indicate that the flow of several large glaciers accelerated, which extended from southeast and northwest in 1996 to 2000 to central east and west in 2005. Moreover, the affected area expanded rapidly from below 66°N to 70°N during the study period. This glacier acceleration drains the Greenland Ice Sheet to the sea. In addition to increasing surface melting and runoff under a warmer climate, all these factors enhance further melting of the Greenland Ice Sheet. The authors estimated that the ice loss of Greenland doubled in the last decade from 90 to 220 km3/yr, which is equivalent to 0.23 to 0.57 mm/year of sea-level rise. As current models do not include the factor of current glacial acceleration in Greenland, Rignot and Kanagaratnam believed that model projections can only provide lower limits to the potential contribution of Greenland to sea level rise.
Rignot, E., J.E. Box, E. Burgess, and E. Hanna. 2008. Mass balance of the Greenland ice sheet from 1958 to 2007, Geophys. Res. Lett., 35, L20502, doi:10.1029/2008GL035417.
Variations in the total mass balance of the Greenland ice sheet closely follow climate fluctuations with a strong relationship to ice discharge.
A new study has linked the increase in the rate of melting of the Greenland ice sheet to the warming of summer mean air temperatures over the past 15 years. This study updates the authors' earlier work on ice sheet mass loss by comparing annually and spatially resolved data for ice discharge and surface mass balance for the period 1958-2007. Ice discharge values were derived from satellite data and ice thickness from radio echo soundings. The surface mass balance was derived from a model used to estimate snow melt and accumulation rates. The total mass balance was estimated as the difference between surface mass balance and discharge for 37 drainage units over the ice sheet. Since 1958 the temporal variability in the mass balance of the ice sheet is significant, is well correlated with climatic fluctuations. Since 1996, the total annual mass deficit has tripled to 267 Gigatonnes/year. This research suggests that variations in ice discharge dominate the total mass budget (rather than surface melting). Furthermore, variations in ice discharge are strongly correlated to the surface mass balance.
Rignot, E., J.L. Bamber, M.R. Van Den Broeke, C. Davis, Y. Li, W.J. Van de Berg and E. Van Meijgaard, 2008. Recent Antarctic ice mass loss from radar interferometry and regional climate modeling, Nature Geoscience, on-line publication, 5 pages.
A new study, based on the most extensive measurements to date, concludes that the Antarctic ice sheet as a whole is shrinking. Mass loss increased by 75% between 1996 and 2006 while snowfall accumulation did not change significantly between 1980 and 2004.
A number of studies have been conducted to evaluate if the Antarctic ice sheet is growing, shrinking or stable, with no clear consensus among researchers about its overall trend. In a recent study, based on the most extensive measurements to date, the authors conclude that the Antarctic ice sheet as a whole is shrinking and at a rate that has been increasing from 1996 to 2006. The study used satellite radar observations from 1992 to 2006, covering 85% of Antarctica's coastline, to estimate the total ice mass flux into the ocean, and a regional atmospheric climate model (the RACMO2/ANT) for the period 1980 to 2004 to calculate the interior snow accumulation. Ice flux and snowfall are compared for all major outlet glaciers, ice streams and tributaries of importance. The detailed results show that the biggest ice losses are in West Antarctica, around the Amundsen Sea and in the Antarctic Western Peninsula. These losses are mainly attributed to an upwelling of warm circumpolar deep waters. The largest ice sheet covering East Antarctica remained relatively stable, with a near-zero mass balance: some sectors are gaining ice mass, others are losing some. Over the 1992-2006 time period, the Antarctica ice sheet as a whole was losing ice mass, and the mass loss increased by 75% in the last 10 years. Snowfall results integrated over all Antarctica show no significant change. The authors conclude by saying that changes in glacier flow therefore have a significant, if not dominant, impact in ice sheet mass balance.
Robock, Alan and Li, Haibin. 2006. Solar dimming and CO2 effects on soil moisture trends. Geophysical Research Letters 33, L20708, doi: 10.1029/2006GL027585.
Summer soil moisture increased significantly from 1958 to the mid 1990s in Ukraine and Russia, a trend that cannot be explained by changes in precipitation and temperature alone. The authors investigate the potential contribution of solar dimming and increasing carbon dioxide to this trend using a state-of-the-art land surface model (a modified version of the Community Land Model 3.0). When a slow dimming was applied to the model experiment, using a 0.5% per year decrease in solar insolation between 1961 and 1980 (and 1960 level concentrations of atmospheric CO2), results projected a 5% reduction in evapotranspiration for the Ukraine, and 9% reduction for Russia. This increased to 16% and 20%, respectively, when the rate of dimming was increased to -1% per year. Repeating these experiments using rising CO2 concentrations as observed since 1960 produced results that only slightly reduced evapotranspiration relative to that for constant CO2 levels. Comparison with observed changes in soil moisture suggest that the best fit occurs for experiments with enhanced solar dimming. While the CO2 fertilization effects seems to have been very small for this region, authors note that it could be much more significant in regions where evapotranspiration is composed of primarily transpiration (e.g., Amazon rainforest).
Santer, B.D., Thorne, P.W., Haimberger, L. et al., 2008. Consistency of modeled and observed temperature trends in the tropical troposphere. Int. J. Climatolgy, DOI: 10.1002/joc.1756.
New study notes that, when corrected for analysis errors, observed trends in tropospheric temperatures over the tropics relative to those for surface temperatures are consistent with those predicted by climate models.
In recent years, there has been considerable debate about whether or not observed trends in tropospheric temperatures are consistent with model projections. While most models, when forced by past changes in greenhouse gas and aerosol concentrations, project that the tropospheric temperatures should warm more than those at the surface, analyses of the observed data appeared not to show this enhanced tropospheric warming. This apparent discrepancy was particular significant over the tropics, causing some critics to declare this to be proof that climate model projections were unreliable. A new study by a team of 17 climate data analysis experts, led by American scientist Ben Santer, now suggests that, if the proper corrections are made to the observed data to account for verifiable biases, these discrepancies largely disappear. One key factor in this reconciliation was the correction of surface data obtained from ocean buoys and satellites, which resulted in slightly weaker surface warming in tropical regions. Another was the use of an improved procedure for correcting tropospheric temperature data obtained from satellites to remove biases present between the sub-sets of data recorded by different satellites. These results are a reminder that discrepancies between model predictions and observations can often be due to poor quality of the observed data or the methods used to analyze them.
Saunders, M.A. and A.S. Lea. 2008. Large contribution of sea surface warming to recent increase in Atlantic hurricane activity. Nature, 451:7178, pp. 557-561; Wang, C. and S.-K. Lee. 2008. Global warming and United States landfalling hurricanes. GRL, 35: L02708; doi:10.1029/2007GL032396.
Two recent studies attempt to better explain the contribution of changes in vertical atmospheric wind shear and sea surface temperatures to past Atlantic hurricane activity. One study looks only at the high quality Atlantic hurricane data of the past 40 years, noting a significant rise in activity during the past decade. Authors conclude that changes in wind shear and SSTs in the main hurricane development region (MDR) can explain most of the observed variability in activity, and that rising SSTs alone can explain about 40% of the large increase in hurricane activity observed in the most recent decade. The second study examines the relationships between these two factors and US hurricane land falls over the past 150 years. Authors argue that the lack of evidence for any significant long term change in land-falling Atlantic hurricane activity may be due to the offsetting effects of rising wind shear and rising SSTs.
While researchers have suggested that both vertical wind shear and sea surface temperatures (SST) are important factors in hurricane formation, there is considerable debate about the relative importance of these factors in long term trends of Atlantic hurricane behaviour. Two recent studies have added to that debate. In one of these, British researchers Saunders and Lea use a statistical model to investigate the contribution of local ocean warming and wind shear within the main development region (MDR) for Atlantic hurricanes to the recent rise in hurricane activity. The authors show that their model can replicate about 75-80% of the variance in observed tropical Atlantic hurricane activity between 1965 and 2005 - the period with most reliable hurricane records. Active hurricane seasons are associated with below-norm Aug-Sept 925-hPa trade winds (their proxy for wind shear) and above-norm Aug-Sept SST in the Atlantic hurricane MDR. About 40% of the large increase in hurricane activity during the most recent decade can be explained by rising SSTs alone. The authors note that the ability of climate models to reproduce the observed relationship between hurricanes and sea surface temperature will serve as a useful means of assessing whether they are likely to provide reliable projections of future changes in Atlantic hurricane activity. In the second study, American scientists Wang and Lee take a much longer look at trends - from 1851 to 2006. Because of the paucity of reliable ocean hurricane data prior to 1965, they consider only data for US hurricane landfalls. While data for this subset of hurricane activity still has important quality concerns, it is more reliable than that for activity across the Atlantic Basin. The authors show that there appears to have been a weak (not statistically significant) decreasing trend in U.S. landfalling hurricanes over this time period, despite a global rise in SST. They argue that the lack of a significant long-term upward trend in hurricane landfall is due to an increase in vertical wind shear in the MDR that offsets the concurrent effects of rising SSTs. This strengthening wind shear arises because of the pattern of global changes in SSTs, with stronger warmer over the Pacific and Indian Oceans than over the Atlantic Ocean. During time periods when the local warming in the Atlantic hurricane MDR is stronger, wind shear decreases and increases the risk of active hurricane activity. Thus, whether or not future global warming increases or decreases Atlantic hurricane activity will also depend on relative warming over the tropical oceans.
Schiefer, E., B. Menounos and R. Wheate. 2007. Recent volume loss of British Columbia glaciers, Canada. GRL Vol 34, L16503, doi:10.1029/2007GL030780.
Accelerated glacier thinning confirmed in British Columbia.
Most mountain glaciers have lost mass over the past few decades and this has contributed to sea level rise and also raised concerns about reductions in freshwater availability. Accurate glacier inventories are essential for tracking changes in glaciers over time and assessing the implications of continued glacier loss. This study by Canadian scientists with the University of Northern B.C. reports on recent glacier volume loss for all of B.C. using digital elevation models derived from both spaceborne radar and aerial photography. In their analysis, the authors draw attention to elevation biases in data from the Shuttle Radar Topography Mission (SRTM). The authors discuss the magnitude and sources of elevation bias in the SRTM elevations and report that biases are typically in the order of -12m/km (i.e. elevations derived from the STRM are too low). For the period 1985-1999, the bias-corrected thinning rate is -0.78 ± 0.19 m/year which yields an annual volume loss rate of 22.48 ±5.53 km3/year based on an estimated glacier cover of 28,826 km2. Without correction for elevation bias, the estimated volume change is 34.7 km3/year (approx. 1.5 times higher than the bais-corrected value). The authors therefore caution other investigators about using uncorrected SRTM data for glacier change studies. This rate of glacier thinning over the 1985-1999 period was compared to that for the two previous decades for the Coastal Mountains, the only mountains for which earlier data were available. The authors report that the rate of glacier loss for these mountains approximately doubled between these two time periods.
Sheffield, J. and E.F. Wood. 2007. Global trends and Variability in Soil Moisture and Drought Characteristics, 1950-2000, from Observation-Driven Simulations of the Terrestrial Hydrologic Cycle. Journal of Climate, Vol. 21. DOI:10.1175/2007JCLI1822.1.
Despite an overall wetting trend over the period 1950-2000, there has been a switch since the 1970s to a drying trend, globally and in many regions, especially in high northern latitudes, concurrent with increasing temperatures. Although drought duration, intensity and severity are primarily driven by variability in precipitation, temperature is being recognized as increasingly important.
Drought can be regarded as one of the most damaging of natural disasters in human, environmental and economic terms. It occurs as a result of extremes in climate that are driven by natural variability and more recently influenced by anthropogenic activities. This study examined the global and regional trends in drought from 1950-2000 using a soil moisture-based drought index over global terrestrial areas, excluding Greenland and the Antarctic. The soil moisture fields were derived from a model simulation of the terrestrial hydrologic cycle driven by a hybrid reanalysis-observation meteorological forcing dataset. The study found an overall increasing trend in global soil moisture driven by increasing precipitation which is most evident over North America. Continental variations, however, were apparent and significant. Drought extent increased over West Africa and southern Asia, whereas it decreased over Australia and the Americas. Globally, variations were mainly driven by ENSO variability. The AMO appears to play an important role as well, however, the multi-decadal scale of the AMO meant the 50 year length of the record was too short to make any definite conclusions about its role in drought trends. Despite the overall decreasing trend in global drought extent over the period 1950-2000, there was a switch since the 1970s to a drying trend, globally and in many regions, especially in high northern latitudes, concurrent with increasing temperatures. Although drought is driven primarily by variability in precipitation, the role of temperature was found in this study to have an effect that appears to have an increased significance in the last decade or so. This finding suggests the potential for enhanced drought occurrence during the 21st century with continuation of temperature increases.
Stroeve, J., M. Holland, W. Meier, T. Scambos and M. Sereze. Arctic sea ice decline: faster than forecast. 2007. GRL Vol 34, L09501, doi: 10.1029/2007GL029703.
Two studies provide evidence that future climate change, as projected by climate models, may be conservative.
A new analysis shows that Arctic sea ice is melting at a significantly faster rate than projected by most advanced computer models. The shrinking of summertime ice extent is about thirty years ahead of the climate model projections. Current projections of sea ice retreat may therefore be underestimated.
A study published this week in the journal Geophysical Research Letters by a team of U.S. scientists compared observed changes in sea ice extent over the 1953 to 2006 period with simulations produced by models used in the IPCC Fourth Assessment. The authors made use of a newly available data record for the Arctic compiling data from different sources. The observed trend in September sea ice extent (minimum sea ice conditions) from 1953 to 2006 is -7.8 ± 0.6%/decade, three times larger than the multi-model mean trend of -2.5 ± 0.2%.decade. March trends have not been as large (-1.8 ± 0.1%/decade) but again were three times the multi-model mean trend. More striking was that the largest negative trend from any individual model run was -5.4 ± 0.4%/decade, still significantly less than observed. The qualitative agreement between the models and the observations in terms of the overall decline in ice extent provide strong evidence of a response to GHG forcing. However, given that as a group the models underestimate the sea ice response to climate warming, the authors suggest that the Arctic could become seasonally ice free earlier than the IPCC projected range of 2050 to beyond 2100. Current summer sea ice minima are about 30 years ahead of the model mean forecast.
Tian, J., D.M. Nelson and F.S. Hu. 2006. Possible linkages of late-Holocene drought in the North American midcontinent to Pacific Decadal Oscillation and solar activity. GRL vol.33, L23702.
This paper used paleorecords (reconstructed from varves) from a small Minnesota Lake to look at drought episodes in the North American midcontinent over the past ~ 3100 years. Their results agree with those of other studies in showing that droughts events were not uncommon during the late Holocene and that some of these droughts might have been widespread across the North American midcontinent. In comparison with drought episodes experienced in the 20th century, the paleorecords show that droughts of greater severity and duration occurred repeatedly, especially prior to 300 AD. In addition, the authors found that before 1900 AD there was much greater variability in drought occurrence, the 20th century showing anomalously low values atypical of the last 3100 years. The authors then attempt to evaluate the controlling mechanisms of such droughts. SST variability of the North Pacific, through the Pacific Decadal Oscillation (PDO), is thought to have had a major influence on moisture transport to that region during the late Holocene. The authors also found that sunspot minima coincided with wetter and cooler periods.
Kaab, A., Chiarle, M., Raup, B. and Schneider, C., 2007. Climate change impacts on mountain glaciers and permafrost. Global and Planetary Change 56 (1-2): vii-ix.
A special issue of Global and Planetary Change considers the observed effects of climate change on mountain glaciers and permafrost in various regions of the world. These studies add to the large amounts of evidence showing clear signs of glacier retreat in most mountain regions of the world ranging from Alaska to New Zealand, as derived from new monitoring and modelling methodologies.
For instance, in Alaska, a 2.0°C average temperature increase since the mid-20th century has led to the retreat and/or thinning of 98% of the glaciers examined while the average glacier extent in the Tien Shan region of Kazakhstan decreased by 32% between 1955 and 1999. In South America, a decrease of 10% in glacier area loss was observed in the Peruvian Cordillera while a study of the Swiss glacier inventory showed that glacier area loss per decade accelerated by a factor of 7 from 1985 to 1998/99 compared to the 1850-1973 period. Another study extending back to the 1850's showed a larger decrease in glacier volume over the last 150 years in the New Zealand Alps (49% decrease) compared to in the well-documented European Alps (35% decrease). The studies cited here are good examples of how the monitoring of glaciers and permafrost can be useful indicators of climate change and advances in new earth observation and geoinformatic technologies will substantially advance our knowledge of these processes.
Lawrence, D.M., Slater, A.G., Tomas, R.A., Holland, M.M. and C. Deser. 2008. Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss. GRL VOL. 35, L11506, doi:10.1029/2008GL033985, 2008.
A new modeling study suggests that the rapid retreat of arctic sea ice may have consequences for the arctic terrestrial climate and the state of the permafrost far inland.
The record sea ice minimum of 2007, together with the observation that sea ice area is a robust inverse predictor of arctic land temperature, motivated a group of scientists to assess the adjacent land climate. The authors used the Community Climate Systems Model (CCSM3) (which both replicates the recent rate of sea ice loss and simulates present day sea ice conditions well) to assess the land temperature response to rapid ice loss events. Model simulations found an acceleration of land warming which can trigger rapid degradation of warm permafrost and increase the vulnerability of colder permafrost to degradation if the warming trend continues. Simulating nine sea ice loss events showed warming through most of the terrestrial western Arctic extending 1500 km inland. One important phenomenon predicted is the formation of permanently unfrozen ground (talik) at ~0.2m depth as a result of permanent heat accumulation (downwelling summer heating is greater than winter cooling). Once established, talik formation leads to rapid permafrost degradation and their occurrence may serve as a harbinger of rapid terrestrial changes.
Lee, T.C.K., Zwiers, F.Z. and M. Tsao. 2008. Evaluation of proxy-based millennial reconstruction methods. Climate Dynamics 31:263-281. DOI 10.1007/s00382-007-0351-9.
Methods of temperature reconstruction for the past millennium from proxy data have improved since the iconic 'hockey stick' graph was published in the IPCC's Third Assessment Report in 2001. While the new methods provide reconstructions that suggest that the climate was more variable in the past than shown in the iconic TAR graph, all available reconstructions confirm the unusual nature of recent warming in the context of the past millennium.
There is widespread scientific agreement that the 20th century was substantially warmer than the preceding millennium, a consensus that was recently reaffirmed in the IPCC's Fourth Assessment Report released in 2007. In that report, this conclusion was reached after assessment of a number of new temperature reconstructions based on improved and expanded paleoclimate data sets compared to those available at the time of the Third Assessment Report (TAR). After publication of the TAR, some criticism was targeted at the iconic 'hockey stick' graph that was published in the Summary for Policymakers, a reconstruction of the climate of the past 1000 years produced by Dr. Michael Mann. Although the greater number of studies now available to reconstruct millennial temperature variations is a positive advance in science, the fact that results from these reconstructions vary in some important respects left questions about the source of these discrepancies. A paper recently published in the journal Climate Dynamics and co-authored by Environment Canada scientist Francis Zwiers and two scientists with the University of Victoria investigated the source of these discrepancies, asking in particular whether they result from differences in the methods used for reconstructing historical temperatures from proxy data sources. The paper examines several existing methods and also presents a new method, developed by Environment Canada and the University of Victoria. The authors show that the method used to produce the iconic hockey stick graph that appeared in the IPCC's Third Assessment report does have some deficiencies relative to most other methods used to reconstruct historical climate from proxy data. However, the authors also show that the conclusion - that there has been a sharp rise in global temperatures during the 20th century - is not in question. Methods that appear superior for reconstructing interannual variations in Northern Hemisphere mean surface air temperature are identified, and include the new method proposed in this paper. A variety of methods are, however, shown to perform well when reconstructing temperature variability at decadal time scales. The authors conclude that differences between reconstructions are more likely to arise from the specific choices of proxies used in different reconstructions than from the choice of method, although there are some methods that are best avoided.
Nicholls, N., 2008. Recent trends in seasonal and temporal behaviour of the El Nino - Southern Oscillation. Geophys. Res. Letters 35, L19703, doi:10.1029/2008GL034499.
As far as El Nino-Southern Oscillation goes, there is no evidence of changes in either variability or persistence trends over the 1958-2007 period. This finding is in keeping with the finding of the IPCC AR4, which states models do not project changes in El Nino like conditions in the 21st century.
A recent study by an Australian scientist examined trends in variability and persistence in El Nino - Southern Oscillations (ENSO) over the period 1958-2007. Two different ENSO indices were used to look for trends in seasonal and temporal behaviour, the NINO3.4 which looks at the sea surface temperatures in the equatorial Pacific and the Southern Oscillation Index (SOI) which compares station pressures between Tahiti and Darwin, Australia. The results show that there is no trend in the seasonal pattern, or in the variability and strength across seasons. The only trend detected was towards a more El Nino-like pattern in the SOI, but only for the March - September period, reflecting a change in the Darwin station pressure. ENSO tends to peak during the November - February period, so this change is a curiosity, and thus requires further investigation as to its cause. The finding of no change in trends is in keeping with the IPCC AR4 report which states that, for models that simulate present El Nino conditions well, there is no consistent indication "of discernible changes in projected ENSO amplitude or frequency in the 21st century".
Perovich, D.K., J.A. Richter-Menge, K.F. Jones, and B. Light. 2008. Sunlight, water, and ice: Extreme Arctic sea ice melt during the summer of 2007. Geophys. Res. Lett., 35, L11501, doi:10.1029/2008GL034007. Schweiger, A. J., J. Zhang, R. W. Lindsay, and M. Steele. 2008. Did unusually sunny skies help drive the record sea ice minimum of 2007?Geophys. Res. Lett., 35, L10503, doi:10.1029/2008GL033463.
Steps are taken towards a comprehensive explanation of the extreme melting of Arctic sea-ice in the summer of 2007. Ice-albedo feedbacks are shown to be important while the sunny skies of 2007 are not contributors.
Two recent articles contribute pieces to the puzzle regarding the record-setting retreat of the extent of Arctic sea ice cover in the summer of 2007. Possible causes that have been suggested are warming trends, changes in atmospheric circulation, export of older ice through Fram Straight, low clouds, clear skies, advection of ocean heat from the Pacific and heating of the ocean. All of these have implications for the future trajectory of sea ice cover. The role of ocean surface temperatures is explored in the paper by Perovich et al. Observations of ice growth and melt show a dramatic increase in bottom melt of ice in the Beaufort Sea that is six times the annual average of the 1990s in the summer of 2007. The authors believe that the radiation input to the open water of the Beaufort Sea was the primary source of heat (400-500 percent higher than average) to the ocean resulting in more ice melting. Furthermore, the thinning of the sea ice from excessive bottom melt allowed more solar radiation to be transmitted to the ocean. These processes are consistent with the classic ice-albedo feedback. In Schweiger et al., the authors confirm that an anomalous high pressure cell did contribute to relatively clear skies during the summer of 2007 (a 57% reduction from average cloud amounts). A dynamic sea ice model, Pan-arctic Ice -Ocean Modeling and Assimilation System (PIOMAS), was then used to test whether the unusually sunny skies contributed to the sea ice anomaly. Model runs investigating the role of clouds (using a simplified comparison of 2007 downwelling of short and long wave radiative fluxes against the average) showed that the "sunny skies " did not contribute to the record sea ice minimum in 2007. The full assessment of the total impact of clouds is yet to be done. Overall, the thinning of the surviving ice suggests a greater vulnerability of summer sea ice in the future to forces that would contribute to melting.
Thompson, D.W., J.J. Kennedy, J.M. Wallace, and P.D. Jones. 2008. A large discontinuity in the mid-twentieth century in observed global-mean surface temperature. Nature 453:646-650; Forest, C.E. and R.W. Reynolds. 2008. Hot questions of temperature bias. Nature 453:601-602; Schiermeier, Q. 2008. Climate anomaly is an artifact. Nature 453:569.
Researchers indicate that changes in shipped-based ocean temperature observations during the Second World War introduced a bias in average global temperature records that ended with an artificial abrupt cooling anomaly between 1945 and 1946. This may explain some of the apparent mid-century cooling anomaly noted in global temperature records.
For many years, experts have tried to explain why global temperatures appear to have cooled for a period of time during the mid 20th century, even though greenhouse gas concentrations were rising at the time. A new study now suggests that not all of that apparent cooling may have been real. The team of American and British researchers involved in the study first filtered the global temperature records to remove effects of known natural fluctuations caused by ocean oscillations and volcanoes. There remained one abrupt anomaly in the ocean data record between 1945 and 1946 that remained unexplained by such natural variability. More careful examination of the data indicated that, prior to WW II, most of the sea surface temperatures were collected by British and American ships. Past studies have confirmed that there is a difference of a few tenths of a degree between the different methods used (with the American data being slightly warmer). However, during the war, the British stopped taking temperature data. Hence the reconstruction of the global data base has relied primarily on the slightly warmer American data during this time. When the British recommenced their observations in 1945, it removed the warm bias of the preceding years caused by using primarily American ship data - and thus introduced an abrupt cooling shift of as much as 0.3°C into the global averages at that point in time. The authors point out that, while many other related errors in data collection have been corrected, that error had not. They are quick to point out that, since this error has no effect on data collected prior to 1940 or during the past few decades, this newly identified data artifact has no effect on global temperature trends for the century. However, it is important in studies that try to explain the mid century cooling, and in the use of observed data to test climate models.
Thorne, P.W. 2008. The answer is blowing in the wind. Nature geoscience on-line. Allen, R.J., and Sherwood, S.C., 2008. Warming maximum in the tropical upper troposphere deduced from thermal winds. Nature 25 May 2008;doi:10.1038/ngeo208.
New light is shed on predicting tropical tropospheric temperature trends through the use of radiosonde wind data.
Over the past 30 years, there has been little consistency in the radiosonde and satellite data sets in their record of tropospheric warming. This uncertainty between the data sets has made it difficult to evaluate the performance of climate models, which do project a greater warming trend in the upper troposphere compared to the Earth's surface temperature trend. A new avenue of investigation has been reported that involved using wind data to infer tropospheric temperature change. Using radiosondes to take temperature and wind measurements throughout the troposphere has been in practice since the early 1900s. This has enabled meteorologists to understand processes in the troposphere and helped improve weather forecasts. However, since radiosondes were designed for weather forecasting they have not been designed to take measurements to the accuracy required to confirm climate model behavior. In this paper, the researchers use the trends in winds to infer temperature through the application of the thermal-wind equation. Winds are observed through the tracking of the radiosonde and are independent from temperature observations. Through this approach, the investigators found a maximum warming trend of 0.65±0.47 K per decade in the upper troposphere since 1970. This warming trend is consistent with model predictions and therefore increases overall confidence in current global climate models when used in applications such as projections of future climate change.
Turner, J., Lachlan-Cope, A., Colwell, S., Marshall, G.J. and Connolley W.M. 2006. Significant Warming of the Antarctic Winter Troposphere. Science 311: 1914-1917.
Turner et al. (with the British Antarctic Survey) make use of some recently improved radiosonde data sets for the Antarctic that were digitized and quality controlled through a project of the Scientific Committee on Antarctic research. The data are from 8 stations from coastal Antarctica as well as the inland Amundsen-Scott station at the South Pole. The authors report that a strong warming trend in the troposphere above the Antarctic is observable in the radiosonde data, with maximum warming in the winter season. The mean winter temperature trend for the nine stations over the period 1971-2003 was 0.15°C increase per decade at the surface and a 0.70°C increase per decade in the midtroposphere (at 600 hPa). The authors found no evidence for any changes in atmospheric circulation that could help explain the temperature trend, and therefore posit that in situ processes are responsible. Limitations in the ability of current GCMs to simulate Antarctic climate trends precludes assigning a cause to the tropospheric warming, however, the authors suggest that changes in cloud amount and/or properties, and increases in greenhouse gases, may well be playing a part. This reported warming of the Antarctic troposphere during winter is larger than any previously identified regional tropospheric warming on Earth.
Velicogna, I. and J. Wahr. 2006. Measurements of Time-Variable Gravity Show Mass Loss in Antarctica. ScienceExpress 2 March 2006, 10.1126/science.1123785.
Using the Gravity Recovery and Climate Experiment (GRACE) satellite, this study gives, for the first time, a comprehensive survey of the entire Antarctica ice sheet. Based on 34 monthly estimates of gravity fields, between April 2002 and August 2005, the authors estimate the mass change of the Antarctic ice sheet, as a whole, and of East Antarctic ice sheet (EAIS) and West Antarctic ice sheet (WAIS) separately. The authors correct the GRACE fields for internal errors, then for external leakage like continental hydrology outside Antarctica and ocean mass variability, and finally for post-glacial rebound (PGR) effects, which are as large as the ice variability (in the order of 192 ± 79 km3/year). They find that the Antarctica ice sheet mass decreased significantly between the summers of 2002 and 2005, at a rate of 152 ± 80 km3/year. This rate of loss corresponds to 0.4 ± 0.2 mm/year of global sea level rise. The results for EAIS and WAIS are respectively, for the same period, of 0 ± 56 km3/year and 148 ± 21 km3/year. This confirms the results of other studies that have showed important mass losses of WAIS.
Viau, A.E., Gajewski, K., Sawada, M.C. and Fines, P. 2006. Millennial scale temperature variations in North America during the Holocene. JGR 111, D09102, doi:10.1029/2005JD006031, 2006.
A team of U of Ottawa researchers have used pollen data from some 750 sites across North America to reconstruct July temperature trends and patterns for the continent over the past 14 000 years. They report that July temperatures during the current interglacial peaked between 6000 and 3000 years ago, and have subsequently decreased slightly until the 20th century. Superimposed on this trend is a long-term pattern of variability with a frequency of about 1150 years. Of particular note is that the observed temperature increase of the 20th century appears to be without precedence throughout the 14 000 year record.
Vincent, L.A and Mekis, E. Changes in Daily and Extreme Temperature and Precipitation Indices for Canada over the Twentieth Century. Atmosphere-Ocean, 44 (2) 2006, 177-193.
This paper reports on the work of two Environment Canada (Climate Research Branch) scientists to examine trends in Canada over the past century for various temperature and precipitation indices, in particular, indices of extreme events. This work updates previously published analyses by CRB scientists with data to 2003. Trends for 1950-2003 (all of Canada) and 1900-2003 (southern Canada) were generated and analyzed using homogenized data from 210 temperature and 495 precipitation stations across Canada. Between 1950 and 2003, results showed an increasing trend in warm events (number of warm days and nights) and a decreasing trend in cool events (number of cold days and nights). Similar results were obtained for the 1900 - 2003 period, but in addition, a significant decrease in the diurnal temperature range was also detected during this period. The number of frost days also decreased significantly over both time periods. The analysis of precipitation indices for 1950-2003 revealed more days with precipitation, a decrease in daily precipitation intensity and a decrease is the maximum number of consecutive dry days. Annual total snowfall significantly decreased in the south and increased in the north and north-east. The century-long changes generally parallel those observed during the last 54 years. No consistent change was found in most of the extreme precipitation indices in either time period, except for one; a significant increase in the number of days with heavy precipitation ( 10mm) was detected during 1900-2003. The results extend and support the previously published temperature and precipitation analyses by Bonsal et al. (2001) and Zhang et al. (2001) which supported the notions that in Canada, it is not so much getting warmer but getting less cold, and that the observed increases in precipitation totals are due mostly to an increase in the number of small to moderate events.
Vincent, L.A., W.A. Van Wijngaarden and R. Hopkinson, 2007. Surface Temperature and Humidity Trends in Canada for 1953-2005. J. of Climate, vol. 20: 5100-5113.
A recent study confirms that Canada is definitely getting warmer, especially in the western and southern regions in winter and spring. The strongest warming is observed in the western boreal forest region, in winter. Significant warming trends are accompanied by a significant increase in air moisture content.
An Environment Canada scientist, Lucie Vincent, along with two other Canadian scientists, has looked at annual and seasonal surface air temperature, dewpoint, relative humidity and specific humidity trends over Canada for the period 1953-2005, using, for the first time, homogenized hourly values. In total, data from 75 climatological stations were used. The trends were calculated for eight different climatic regions and for the country as a whole. Also, seasonal trends in nighttime and daytime values were compared over the eight climatic regions. The results show that, overall, there has been significant warming in Canada (1.2°C over 1955-2005), with the largest warming occurring in winter (1.6°C) and spring (1.4°C). The winter warming is mainly observed in the west and across southern Canada, with many stations showing an increase of 2.5-4.0°C between 1953 and 2005. A strong warming of 4.0°C was observed in the western boreal forest region between 1955 and 2005. Over the same period, the springtime data show a lesser but significant warming of 1.5-2.5°C in some stations, with a pattern very similar to winter, In most cases, significant warming trends are accompanied by an increase in air moisture content, showing that warmer temperatures have increased the capacity of the air to hold more moisture. This is particularly evident in the west and in the Great Lakes and St. Lawrence region. Finally, for the period 1955-2005, there is no strong evidence that the warming observed in Canada is more pronounced in the nighttime than in the daytime. This finding is in agreement with earlier published results on Canadian temperature trends over a similar time period. However, the nighttime humidity has increased slightly more than the daytime humidity, particularly in spring and summer.
Vinnikov, K.Y., N.C. Groody, A. Robock, R.J. Stouffer, P.D. Jones, and M.D. Goldberg, 2006. Temperature trends at the surface and in the troposphere. J. Geophys. Res., 111, D03106, doi:10.1029/2005JD006392.
Temperature trends derived from satellite measurements are often cited by skeptics as proof that there is little positive trend in tropospheric temperatures. However, over the last few years, scientists have continued to improve the analysis of the satellite data which now show confirmation of tropospheric warming. As another verification of this warming, this paper presents data from the Microwave Sounding Unit (MSU) and determines climatic trends in the troposphere. Using new statistical techniques to calibrate the satellite measurements, the authors find that the tropospheric temperature trend is +0.2°C/ decade with a standard error of +/- 0.05°C/decade. This is slightly higher than the surface temperature record which is stated to be about 0.18°C/decade.
Wagner, T., Beirle, S., Grzegorski, M. and Platt, U. Global trends (1996-2003) of total column precipitable water observed by Global Ozone Monitoring Experiment (GOME) on ERS-2 and their relation to near-surface temperature. Journal of Geophysical Research 11, D12102, doi: 10.1029/2005JD006523, 2006.
A recent study attempted to answer the question as to whether 'relative humidity stays constant with climate change'. This study used data from the Global Ozone Monitoring Experiment (GOME) aboard the European Research Satellite. The methodology used to analyze atmospheric water vapour content uses the red spectral region, thus increasing sensitivity to water vapour changes over both land and water and generating a more accurate global picture. Global and yearly averaged total column precipitable water at various elevations along with globally averaged near-surface temperatures were obtained for the January 1996 to June 2003 period. Results showed that total column precipitable water values increased by roughly 0.19g/cm2 per K increase in near-surface temperatures, a value that rises to 0.29g/cm2 in tropical regions. While a high correlation between global maps of trend patterns for total water and near-surface temperatures was observed over the oceans, the trends were non-existent and in many cases reversed over continents. Over the nearly 7 year study, the total column precipitable water increased by 2.8% globally, while a 0.175 K increase in the global yearly averaged temperature was observed during the same period. Since water vapour content shows a correlation with temperature change (mostly positive but negative in certain cases) as a result of strong water vapour feedback, it verifies the assumption that the relative humidity does stay constant for a changed near-surface temperature.
Wild, M., J. Grieser, and C. Schär. 2008, Combined surface solar brightening and increasing greenhouse effect support recent intensification of the global land-based hydrological cycle, Geophysical Research Letters, volume 35, L17706, doi:10.1029/2008GL034842, 5 pages.
A study links solar brightening and the increasing greenhouse effect to the intensification of the hydrological cycle since the mid-1980s.
It is well known that the global hydrological cycle is mainly driven by the radiative energy available at the Earth's surface. In a recent study, European scientists investigated what role solar versus thermal radiation played in the recent intensification of the hydrological cycle. Using surface observations, satellite measurements and reanalysis data from the period 1986-2000, they calculate recent changes in the surface radiation balance and in annual land precipitation. Their results provide evidence that both solar and thermal radiative heating of land surfaces made similar contributions to the intensification of the hydrological cycle since the mid-1980s. The increase in solar radiation was attributed to "solar brightening" - the increasing transparency of the atmosphere that has resulted, largely, from reductions in atmospheric aerosol concentrations. The increase in thermal radiation at the surface is the result of the enhanced greenhouse effect. These results are interesting as they are in contrast to those from a previous study (Wild and al. 2005). The earlier work looked at the period from the 1960s to the 1980s, and found no evidence of an increase in surface net radiation. At that time, the greenhouse-induced increase in downward thermal radiation appears to have been outweighed by decreasing surface solar radiation ("global dimming"), thereby suppressing an intensification of the hydrological cycle. In their conclusion, the authors note that recent studies show evidence that climate models driven by estimated historical forcings simulate smaller increases in precipitation than observed. The results of this present study suggest that the recent brightening effect, which may not be fully reproduced in these simulations, could account for some of these discrepancies.
Willerslev, E., Cappellini, E., Boomsma, W. and many authors. 2007. Ancient biomoelcules from deep ice cores reveal a forested Southern Greenland. Science Vol 317 6 July 2007 pp 111 - 114.
A new study raises some questions about how much the Greenland Ice Sheet might have contributed to sea level rise during the last interglacial (warm) period. The evidence of this study indicates the Greenland Ice Sheet receded less and therefore made a smaller contribution to sea level rise than has generally been thought to have been the case. This would indicate less sensitivity to climate warming, but some caution is warranted given that the methods used in this study for dating the ice are new.
Publication of the results of a study that investigated a new method for recreating climate and environmental conditions on Greenland hundreds of thousands of years ago received considerable press coverage because of implications for the stability of the Greenland ice sheet under climate warming. The paper was primarily aimed at testing a new method for reconstructing past flora and fauna by recovering DNA and amino acids from the very bottom-most sections of deep ice cores. The recovered fragments of DNA were then compared to known DNA sequences and assigned a probable taxonomic classification. The authors applied this method to ice cores from southern Greenland (Dye-3 core), the central Greenland summit (GRIP core) and from a glacier on Ellesmere Island. The latter core was included as a control to test for potential contamination of the Greenland cores by long distance dispersal of DNA. They found that the analysis of basal ice does indeed provide a source of biotic DNA that can be used to recreate environmental conditions of the past. The recovery of biotic DNA from under the Dye-3 core provided evidence of an earlier northern boreal forest ecosystem at that site. However, the key question for some is when this forest existed. Since there are no established methods for dating basal ice, the authors obtained a tentative age estimate by applying a number of different dating techniques. All four of their methods suggested that the ice from southern Greenland (Dye-3) - and the fragments of biota contained within - were at least 450 thousand years old, pre-dating the last interglacial by at least 300,000 years. This would mean that the Greenland Ice Sheet (GIS) could not have receded as far as the location of Dye-3 during the last interglacial (130 to 116 thousand years ago) otherwise younger DNA would have been found there. If the dating is correct, then it would imply a smaller contribution from the GIS to the 4-6m sea level rise during the last interglacial than previously thought. The most recent IPCC assessment estimated a 2-4m contribution from large scale retreat of the GIS and other Arctic ice fields. The results of this study would indicate a smaller contribution from the GIS, which would then imply a much larger contribution from Antarctica in order to raise sea level sufficiently, something which would need to be supported by other evidence before becoming widely accepted.
Zhang, X., A. Sorteberg, J. Zhang, and R. Gerdes. 2008. Recent radical shifts of atmospheric circulations and rapid changes in Arctic climate system. GRL, Vol. 35, L22701, doi:10,1029/2008GL035607.
Understanding changes in the Arctic atmospheric circulation may help explain the rapidity of recent Arctic warming, and could be useful in future predictions of Arctic climate change.
Changes in the Arctic climate system have accelerated tremendously over the past few decades with the prime example being the extreme sea-ice loss observed in the late summers of 2007 and 2008. Anthropogenic climate change has been shown to have been a contributing factor in the recent reduction of Arctic sea ice. However, the overall acceleration in Arctic climate changes are not well correlated with the slow rise in greenhouse gases nor with the Arctic / North Atlantic Oscillation, which has showed a general positive trend since 2000 but has weakened in recent years. The fundamental physical process for the accelerated change in the Arctic remains unknown but could be due to a change in circulation, which is the hypothesis investigated in this study. Zhang et al. attempt to understand this process by comparing the recent climate behaviour with alterations in a broad spectrum of atmospheric circulation patterns over the Arctic. To do so, they use an analysis applied to mean sea level pressure data that provides simple representations of spatial states of atmospheric circulations that evolve over a 30-month running wintertime window. Using data from 1958 to 2006, the researchers found a shift in the centres of maximum climate variability from the North Atlantic northeast into the Barents Sea. They also found systematic spatial changes in the atmospheric circulation reflecting a change in the AO/NAO pattern. These changes in circulation pattern could be a result of the polar shift of storm tracks and the intensification of Arctic storm activity. The shifts in circulation could represent an impetus for the recent acceleration in the arctic climate system's response to climate change, and appear to support arguments for a tipping point of the Arctic climate system. The radical spatial shifts may also be used as a precursor to extreme change and therefore useful in future atmosphere prediction.
Zhang, Y., Chen, W. and Riseborough, D.W. 2006. Temporal and spatial changes of permafrost in Canada since the end of the Little Ice Age. Journal of Geophysical Research 111, D22103, doi:10.1029/2006JD007284.
Three Natural Resources Canada researchers have simulated the response of permafrost in Canada to changes in climate during the 19th and 20th century using the Northern Ecosystem Soil Temperature (NEST) model. The simulated southern boundary of the permafrost for recent decades is similar to that of the current published map, and the simulated active layer thickness and the depth to permafrost base are comparable to site measurements. The simulated results from the model experiment showed a 5.4% decrease in the total area underlain by permafrost from the 1850s to the 1990s, with two periods of rapid decrease from the 1920s to early 1940s and after the 1950s. The average thickness of the active layer, in the persistent permafrost region over the whole of Canada, showed a simulated increase of 0.65m to 0.87m, a 34% change, while the depth of the permafrost table also deepened by 0.39m. Permafrost degradation also occurred from the bottom-up as the mean depth of the permafrost base decreased by 3m, with most of the change occurring after the 1940s. The authors acknowledge that the next step would be to couple NEST with vegetation dynamics and biogeochemical models.
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