IPCC Report.pdf - Adam Curry

137in annual extremes (20-year extreme values) based on 12 GCMs fortemperature extremes and 14 GCMs for precipitation extremes employingthe SRES A2, A1B, and B1 emissions scenarios. This analysis projectsincreases in the temperature of the 1-in-20 year annual extreme hottestday of about 2 to 6°C (depending on region and scenario; Figure 3-5adapted from Kharin et al., 2007) and strong reductions in the returnperiods of this extreme event by the end of the 21st century. However,as noted above, the limited number of relevant detection and attributionstudies suggests that models may overestimate some changes intemperature extremes, and our assessments take this into account byreducing the level of certainty in the assessments from what would bederived by uncritical acceptance of the projections in Figure 3-5. Theassessments are also weakened to reflect the possibility that someimportant processes relevant to extremes may be missing or be poorlyrepresented in models, as well as the fact that the model projectionsconsidered in this study did not correspond to the full CMIP3 ensemble.Hence, we assess that in terms of absolute values, the 20-year extremeannual daily maximum temperature (i.e., return value) will likelyincrease by about 2 to 5°C by the late 21st century, and by about 1 to3°C by mid-21st century, depending on the region and emissions scenario(considering the B1, A1B, and A2 scenarios; Figure 3-5a). Furthermore,we assess that globally under the A2 and A1B scenarios a 1-in-20 yearannual extreme hot day is likely to become a 1-in-2 year annual extremeby the end of the 21st century in most regions, except in the high latitudesof the Northern Hemisphere where it is likely to become a 1-in-5 yearannual extreme (Figure 3-5b, based on material from Kharin et al.,2007). Further, we assess that under the more moderate B1 scenario acurrent 1-in-20 year extreme would likely become a 1-in-5 year event(and a 1-in-10 year event in Northern Hemisphere high latitudes).Next, regional assessments of projected changes in temperature extremesare provided. More details are found in Table 3-3. For North America, theCCSP reached the following conclusions (using IPCC AR4 likelihoodterminology) regarding projected changes in temperature extremes bythe end of the 21st century (Gutowski et al., 2008a):1) Abnormally hot days and warm nights and heat waves are very likelyto become more frequent.2) Cold days and cold nights are very likely to become much lessfrequent.3) For a mid-range scenario (A1B) of future greenhouse gas emissions,a day so hot that it is currently experienced only once every 20years would occur every 3 years by the middle of the century overChapter 3Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentFraction of Warm DaysANN DJF JJA Standard Deviation6 3 0 3 6Fraction of Cold Days Standard Deviation3.25 1.75 0 1 3.25Percentage Days with Tmax>30 Percentage of Days40 20 0 20 40Figure 3-3 | Projected annual and seasonal changes in three indices for daily Tmax for 2081-2100 with respect to 1980-1999, based on 14 GCMs contributing to the CMIP3.Left column: fraction of warm days (days in which Tmax exceeds the 90th percentile of that day of the year, calculated from the 1961-1990 reference period); middle column:fraction of cold days (days in which Tmax is lower than the 10th percentile of that day of the year, calculated from the 1961-1990 reference period); right column: percentage ofdays with Tmax >30°C. The changes are computed for the annual time scale (top row) and two seasons (December-January-February, DJF, middle row, and June-July-August, JJA,bottom row) as the fractions/percentages in the 2081-2100 period (based on simulations for emission scenario SRES A2) minus the fractions/percentages of the 1980-1999 period(from corresponding simulations for the 20th century). Warm day and cold day changes are expressed in units of standard deviations, derived from detrended per year annual orseasonal estimates, respectively, from the three 20-year periods 1980-1999, 2046-2065, and 2081-2100 pooled together. Tmax >30°C changes are given directly as differences inpercentage points. Color shading is only applied for areas where at least 66% (i.e., 10 out of 14) of the GCMs agree on the sign of the change; stippling is applied for regionswhere at least 90% (i.e.,13 out of 14) of the GCMs agree on the sign of the change. Adapted from Orlowsky and Seneviratne (2011); updating Tebaldi et al. (2006) for additionalnumber of indices and CMIP3 models, and including seasonal time frames. For more details, see Appendix 3.A.