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2011 Literature Review Archives - Detection and Attribution
Christidis, N., P.A. Stott, and S.J. Brown. 2011. The role of human activity in the recent warming of extremely warm daytime temperatures. Journal of Climate, Vol 24, p. 1922-1930, doi: 10.1175/2011jcli4150.1. Also, Zwiers, F., X. Zhang, and Y. Feng. 2011. Anthropogenic influence on long return period daily temperature extremes at regional scales. Journal of Climate, Vol 24, p. 881-892, doi: 10.1175/2010jcli3908.1.
Anthropogenic influence on maximum daytime temperatures has been detected in two recent studies at global and regional scales.
Formal detection and attribution studies of daily temperature extremes have previously detected anthropogenic influence on the recent increased severity of extremely warm nights and, less robustly, decreased severity of extremely cold days and nights. Two recent papers apply slightly different formal detection and attribution techniques to further explore the causes of changes in observed temperature extremes.
Christidis and colleagues use an optimal detection technique to examine the role of anthropogenic activity in the warming of extremely warm daytime temperatures over the second half of the twentieth century. Fingerprints (i.e. spatial patterns) of extreme temperatures in model runs (from HadCM3) forced with anthropogenic, natural and all (anthropogenic plus natural) forcings are compared with the observed patterns of temperature extremes over the period 1950-99. The spatial patterns of observed extremely warm daytime temperatures are not explained by natural forcings alone and are significantly related to anthropogenic forcing. Regional trends computed for North America, Europe, Asia and Australia reveal that anthropogenic forcings have caused extremely warm days to become hotter. Zwiers, and colleagues from Environment Canada, focus on detecting the causes of extreme temperatures (annual daily maximum and minimum nighttime and daytime temperatures) and their expected return periods) at regional scales. Simulations from seven global climate models run separately with anthropogenic and all forcings were used to evaluate the observed temperature response to these forcings over the period 1961-2000. Globally, anthropogenic and ‘all’ signals are detected in nighttime and daytime maximum (consistent with the findings of Christidis et al., above) and minimum temperatures. The anthropogenic signal is strongest in nighttime minimum and maximum temperatures and daytime minimum temperatures and has increased the wait time for cold extremes from 20 yrs in the 1960s to 30-35 yrs and decreased the wait time for warm extremes from 20 yrs in the 1960s to 10-15 yrs. The authors also detect anthropogenic influence on extreme temperatures at the regional scale with changes in the warmest nights detected in almost all regions.
Jones, G.S., N. Christidis, and P.A. Stott. 2011. Detecting the influence of fossil fuel and bio-fuel black carbon aerosols on near surface temperature changes. Atmospheric Chemistry and Physics 11:799-816.
The contribution of combined fossil fuel and biofuel black carbon (fBC) to global temperature change over the past century was shown to have been significant but small compared to the effect of GHGs. A formal detection and attribution analysis could not detect a signal from fBC over the last ~100 years, but could over the period 1950-99. This is the first time that the impact of fBC on climate change has been detected separately from that of other anthropogenic aerosols.
There is increasing interest in trying to quantify how much warming, at global and regional scales, might be avoided through reducing emissions of short-lived climate warming agents such as black carbon. Quantifying the contribution of black carbon to past climate change is helpful to understanding how reducing current emissions might ameliorate future climate change. This paper by Jones and colleagues at the U.K. Hadley Centre reports on the results of experiments with the Hadley climate model HadGEM1 to investigate how much influence fossil fuel and biofuel black carbon (fBC) has had on recent climate change. Ensembles simulations were run for different combinations of forcing factors: 1. well-mixed GHGs, 2. all anthropogenic factors (including GHGs, sulphate, fBC and biomass burning aerosols, ozone and land-use change), 3. anthropogenic and natural factors, and 4. all forcing factors minus fBC. The fBC is assumed to be purely black carbon in composition with only radiative absorbing properties. The simulated global response to fBC emissions was significant warming over the 1900-2007 period of 0.14 ±0.10K (compared to 1.06 ±0.07K from GHGs), with stronger warming over recent decades that varied with the time period chosen. The warming is mostly evident over the Northern Hemisphere reflecting the history of regional emissions. A formal detection and attribution analysis confirmed that the observed warming since 1900 has detectable anthropogenic and natural influences but was unable to detect a signal from fBC. However, fBC was found to have a detectable contribution to warming over the second half of the 20th century, although not over the slightly later period of 1957-2006. The authors suggest these differing detection results may be due to the larger influence of fBC in the earlier period. This is the first time that the influence of fBC on climate change has been detected separately from that of other aerosols.
Kaufman, R.K., H. Kauppi, M. Mann and J.H. Stock. 20110. Reconciling anthropogenic climate change with observed temperature 1998-2008. PNAS Vol 108 No 29 pp 11791-3. Solomon, S., J.S. Daniel, R.R. Neely, J.P. Vernier, E.G. Dutton and L.W. Thomason. 2011. The persistently variable ‘background’ stratospheric aerosol layer and global climate change. Science Express 21 July, 2011/10.1126/science.1206027.
Two studies show that observed global temperatures over the last decade or so, including the oft-noted ‘lack of global warming since 1998’, are consistent with expectations based on changes in human and natural factors. Rapid growth in sulphur emissions from Asia are shown to have largely offset warming from greenhouse gases, allowing the impact of natural factors to be revealed.
The apparent lack of a warming trend for the specific period of 1998 – 2008 (which begins with one of the hottest years on record) has been used by some to claim that concerns about anthropogenic global warming are unfounded. A paper published recently in the journal The Proceedings of the National Academy of Sciences by Kaufman and colleagues looks specifically at this decade and investigates the influence of natural and anthropogenic factors on global average surface temperature over this time period. To do this, the authors employ a previously published statistical (regression) model updated with more recent data. The data used to estimate the model include annual observations of atmospheric concentrations of five major greenhouse gases (GHGs), and activity based estimates of anthropogenic sulphur emissions (anthropogenic factors), as well as time series for solar insolation, southern oscillation index (SOI, representative of ENSO activity), and volcanic sulphates (natural factors). All variables except SOI were converted to radiative forcing and for sulphur emissions, both direct and indirect radiative forcing effects were accounted for. The model was used to simulate global average surface temperature from 1999-2008. To identify the effects of human activity on global temperature, the model was used with post-1998 values of natural factors held at their 1998 level while allowing GHG concentrations and sulphur emissions to evolve as observed. Conversely, holding anthropogenic factors at their 1998 values and allowing solar insolation, SOI and volcanic sulphates to evolve as observed generated a simulation of the response of global temperature to natural factors. The results of this work by Kaufman and colleagues show that the net impact of human activity over this time period has been slight (a small positive effect) because the cooling effect of rising sulphur emissions (primarily from China) has largely counteracted the warming effects of GHGs. In terms of the influence of natural factors, the authors note the cooling influence of the declining phase of the 11 year solar cycle, and a change from El Niño to La Niña conditions. Of particular interest is that this study, like many others, assumes that the radiative forcing from volcanic sulphate aerosols in the stratosphere is approaching zero, as the stratospheric aerosol layer recovers from the impact of the last major volcanic eruption (Mt. Pinatubo). Solomon and colleagues present evidence to the contrary in a just published paper in Science. With data from four independent data sets they show that in fact, the ‘background’ stratospheric aerosol layer has changed significantly over the past decade primarily as a result of ongoing minor volcanic eruptions. Furthermore, using an intermediate complexity climate model they show that these recent changes in stratospheric aerosol concentrations have caused recent global warming rates to be slower than they would otherwise have been. Together these two studies confirm that recent changes in global average temperature are consistent with our understanding of the effects of changes in various climate forcers and remind us of the need to include all radiative forcing terms in examinations of short-term, decadal changes in climate.
Min, S.K. X. Zhang, F.W. Zwiers, G.C. Hegerl. 2011. Human contribution to more-intense precipitation extremes. Nature, Vol 470 (February 17, 2011) pp 378–381.
A statistical comparison of precipitation extremes over the late 20th century in the observed record and multi-model simulations indicate that the observed increase in heavy precipitation over Northern Hemisphere land areas can be attributed to human-induced increases in greenhouse gases in the atmosphere.
Precipitation extremes are expected to increase with increasing global temperatures because a warmer atmosphere can hold more moisture in accordance with physical laws. Min et al. use a regression-based technique (optimal detection) to determine whether observed increases in precipitation extremes over the second half of the 20th century can be attributed to anthropogenic temperature increases. Observed changes in annual maximum daily and five-day consecutive precipitation amounts are compared with simulated changes from eight climate models over the period 1951-1999. The climate models were forced with either historical anthropogenic forcing only (ANTH; this includes greenhouse gases and aerosols) or historical anthropogenic and natural forcing (ALL; this includes ANTH and solar and volcanic variations). Data availability limits the analysis to Northern Hemisphere land areas. The observations show increasing trends in both precipitation extremes over roughly 60% of the land area with sufficient data. A similar trend is found in the multi-model mean from the ANTH simulations but they are not as strong; trends in the ALL multi-model mean extremes are even less pronounced with more spatial variability in their sign. The authors note that precipitation extremes in the ALL simulations are likely weaker and more variable because natural forcing alone would have caused long-term cooling and drying over this period. Optimal detection was used to compare observed and simulated patterns of extreme precipitation through time for four regions (Northern Hemisphere, northern mid-latitudes, northern tropics and northern mid-latitudes plus northern tropics). The results indicate that an anthropogenic signal can be detected in observed extreme precipitation (one-day more robustly than five-day) over this period (i.e. the simulated patterns or fingerprints are present in the observations). The ALL signal is detectable in the one-day precipitation extremes but less robustly than the ANTH signal alone. Spatially, the signals are only detectable over hemispheric or continental scales. The simulations underestimate the magnitude of changes in observed precipitation extremes which causes the authors to conclude that projected future changes in precipitation extremes may also be underestimated.
Pall, P., T. Aina, D. Stone, P.A. Stott, T. Nozawa, A.G.J. Hilberts, D. Lohmann and M.R. Allen. 2011. Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000. Nature, Vol 470 (February 17, 2011), pp 382-385.
Study examines whether a specific extreme event – in this case floods in the U.K. in autumn, 2000 – can be linked to human-induced climate change. Using several thousand high-resolution climate model simulations, they are able to attribute a higher risk (>20% and possibly as high as 90%) of floods in England and Wales in the autumn of 2000 to anthropogenic greenhouse gas emissions.
There is growing interest in attributing individual damaging weather-related events to anthropogenic climate change. Climate models used in attribution studies do not typically resolve short-lived, small-scale weather systems. Furthermore, weather-related damaging events such as floods are associated with a number of complex and often untraceable anthropogenic and natural factors (e.g. pre-existing conditions). Pall and colleagues present a new method to quantify the risk of extreme climate-related events related to anthropogenic climate change and use it to evaluate the contribution of 20th century anthropogenic greenhouse gas emissions to the risk of flood occurrence in England and Wales in the fall of 2000 (a period that saw widespread, damaging floods). A seasonal-forecast-resolution climate model (HadAM3-N144; atmosphere only) is used to generate simulations under two scenarios: (1) a realistic scenario representing actual climate conditions by prescribing the atmospheric composition of GHGs and other atmospheric pollutants; and (2) a hypothetical scenario representing climate conditions if anthropogenic 20th century greenhouse gas emissions had not occurred (i.e. are reduced to year 1900 levels). In order to simulate sufficient variability to capture rare, unpredictable events such as this flood, several thousand simulations of autumn 2000 weather were generated for both scenarios. Analysis revealed that the autumn 2000 floods were related to heavy multi-day precipitation events falling on catchments that became saturated. Total daily precipitation time series from all of the climate model simulations were used to drive a daily river run-off model in order to provide better estimates of flood risk. Return times for runoffs of all magnitudes were lower in the simulations that incorporated greenhouse gas forcing (i.e. extreme events were more probable) than in those where GHGs were removed. The authors calculate that twentieth century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales in autumn 2000 by at least 20 percent and in some cases as high as 90%, assuming that other factors that alter the risk of a severe daily river runoff event are constant.
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