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Report on New Climate Change Science - August 11, 2011

Detection and Attribution

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.
(Reference: 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.)

Climate Change Projections

A new study shows that, should greenhouse gas concentrations continue to increase similar to those under a mid-range business-as-usual emission scenario, many areas of the globe are likely to move into a new heat regime over the next four decades where the coolest warm-season of the 21st century is hotter than the hottest warm-season of the late 20th century. Tropical areas are shown to experience the most immediate and robust emergence of unprecedented heat.

A robust prediction associated with continued increases in human emissions of greenhouse gases is that the frequency and intensity of extreme hot events will increase. A study by two scientists with Stanford University explored one aspect of this projected change: the timing of emergence of a novel heat regime in which the new minimum is hotter than the baseline maximum. To do this, they analyzed surface air temperature from the CMIP3 global climate model archive, using a total of 52 realizations from 24 models that contributed both 20th century simulations and projections based on the IPCC SRES A1B emission scenario. They estimate 3 metrics of severe heat emergence, calculated separately for June-July-August (JJA) and Dec-Jan-Feb ( DJF) for a number of time periods: 1) the percentage of seasons warmer than the late 20th century (1980-1999 period) maximum, 2) the ‘time of emergence’ when the ensemble mean warming above the late 20th century maximum emerges above the ensemble spread, and 3) the timing of the last occurrence in each model realization of a season cooler than the late 20th century maximum. Overall, they find that tropical areas exhibit the most immediate and robust emergence of unprecedented heat. Up to 70% of seasons in the early 21st century period (2010-2039) exceed the late 20th century maximum for both JJA and DJF, a percentage that increases to more than 90% for much of the tropics by late in the 21st century. In other regions, exceedance is generally greater in JJA than DJF, and extends to greater than 90% of seasons over much of extra-tropical Africa, southern Eurasia and western North America in the late 21st century. The median date (from the 52 realizations) marking the last occurrence of a season that is cooler than the late 20th century maximum occurs by the end of the 2050s over most of the tropics and large areas of northern Africa and southern Eurasia. The authors note that since actual GHG emissions over the early 21st century exceed those in the A1B emission scenario, these results may be conservative estimates of the emergence of unprecedented heat.
(Reference: Diffenbaugh, N.S. and M. Scherer. 2011. Observational and model evidence of global emergence of permanent, unprecedented heat in the 20th and 21st centuries. Climatic Change 107:615-624.)

Experiments with the Canadian regional climate model show that, over most regions of Canada, there will be increases in single and multi-day extreme precipitation amounts for a given return period; that is, a 1 in 20 yr, 50 yr or 100 yr event in the future will be associated with more rainfall than it is now. This increase in short and long duration extreme precipitation has implications for many water management activities.

Extreme precipitation events and related impacts such as floods are of particular concern for societies in planning for future climate change given the potential threats to infrastructure, the environment and human life. A new study has recently been published reporting on experiments with the fourth generation of the Canadian Regional Climate Model (CRCM) to simulate characteristics of precipitation extremes and their projected changes over Canada. Single and multi-day (i.e. 1-,2-,3-,5-,7- and 10-day) maximum precipitation amounts over the period April-September (to minimize the chance of mixing rain and snow events) are studied using an ensemble of five 30-yr integrations each for current climate (1961-1990) and future climate (2040-2071) using two methods of extreme event analysis (regional frequency analysis, RFA, and gridbox analysis, GBA). Projected changes to return levels (i.e. changes in amounts of rainfall) associated with 20-, 50- and 100-yr return periods for the period 2040-71 relative to 1961-90 are derived both in terms of percentage change and absolute changes. The results of the regional scale analysis, based on 12 climatic regions, suggest an increase in return level in a future climate for all regions, with the largest percentage changes but lowest absolute changes projected for northern regions. In general, similar patterns but larger changes were projected for the longer return periods although these results were also shown to be less statistically significant. Analyses at the gridcell level with both RFA and GBA confirm the projected increases in extreme single and multi-day precipitation amounts for most areas of Canada. As expected, analysis at this finer spatial scale revealed a more complex pattern of response, with small areas of large projected percent changes in extreme precipitation appearing in southern as well as northern regions. The increase in return levels of short- and long-duration extreme precipitation events has implications for management of water resources over the coming century.
(Reference: Mladjic, B., L. Sushama, M.N. Khaliq, R. Laprise, D. Caya and R. Roy. 2011. Canadian RCM projected changes to extreme precipitation characteristics over Canada. Journal of Climate 15 May, 2011, pp2565-2584.)

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Impacts and Adaptation

Estimates of extinction risk from studies documenting observed responses to recent climate change and from predictive studies of the impacts of future climate change converge at mean extinction probabilities, by 2100, of about 10-14%. This result provides support to assertions that anthropogenic climate change ranks as a significant threat to global biodiversity.

The growing number of studies documenting ecological changes in response to recent climate change presents an opportunity to test whether the magnitude and nature of recent responses match predictions. A recently published study does this, focusing on observations and predictions of extinction risk. Maclean and Wilson use a dataset of 130 observed and 188 predicted ecological responses to climate change obtained from studies appearing in major science journals from 2005 to 2009. Estimates of extinction risk for all 318 responses were derived using International Union for Conservation of Nature (IUCN) Red List Criteria (based on relationships between changes in population size and extinction probability). Various types of potential biases in the data set were accounted for, including biases related to type of climate impact studied, region of investigation and non-independence of extinction risk among taxa. After taking account of these possible biases, they found that predictive studies suggest a mean extinction probability over 90 years (out to the year 2100) of ~10% across taxa and regions while observation-based studies gave a mean extinction probability of ~14% for the same time period. Although the authors state these estimates should be treated with caution in light of the many difficulties in assessing impacts on biodiversity, the similarity in these estimates and their robustness to common sources of bias, provide confidence that realized ecological responses to climate change support predictions of future change. The authors also note that these estimates are lower than some published estimates based on the proportion of species ‘committed to extinction’ explaining that estimated extinction risk over a given time period would be expected to be lower than estimates of commitment to extinction.
(Reference: Maclean, M.D. and R.J. Wilson. 2011. Recent ecological responses to climate change support predictions of high extinction risk. PNAS Vol. 108 No. 30, pp 12337–12342.)

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