IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentTemperature extremes were the type of extremes projected to changewith most confidence in the AR4 (IPCC, 2007a). This is confirmedregarding the sign of change with more recent analyses (Figures 3-3and 3-4), although there is a large spread with respect to the magnitudeof changes both due to emission scenario and climate model uncertainty(Figures 3-5a,b). If changes in temperature extremes scale with changesin mean temperature (i.e., simple shifts of the probability distribution),we infer that it is virtually certain that hot extremes will increase andcold extremes will decrease over the 21st century with respect to the1960-1990 climate. Changes in the tails of the temperature distributionsmay not scale with changes in the mean in some regions (Section 3.1.6),though in most such reported cases hot extremes tend to increase andcold extremes decrease more than mean temperature, and thus theabove statement for extremes (virtually certain increase in hot extremesand decrease in cold extremes) still applies. Central and eastern Europeis a region where the evidence suggests that projected changes intemperature extremes result from both changes in the mean as well asfrom changes in the shape of the probability distributions (Schär et al.,2004). The main mechanism for the widening of the distribution islinked to the drying of the soil in this region (Sections 3.1.4 and 3.1.6).Furthermore, remote surface heating may induce circulation changesthat modify the temperature distribution (Haarsma et al., 2009). Otherlocal, mesoscale, and regional feedback mechanisms, in particular withland surface conditions (beside soil moisture, also with vegetation andsnow; Section 3.1.4) and aerosol concentrations (Ruckstuhl and Norris,2009) may enhance the uncertainties in temperature projections. Someof these processes occur at a small scale unresolved by the models(Section 3.2.3). In addition, lack of observational data (e.g., for soilmoisture and snow cover; see Section 3.2.1) reduces the possibilities toevaluate climate models (e.g., Roesch, 2006; Boe and Terray, 2008; Hallet al., 2008; Brown and Mote, 2009). Because of these various processesand associated uncertainties, mean global warming does not necessarilyimply warming in all regions and seasons (see also Section 3.1.6).Regarding mesoscale processes, lack of information also affectsconfidence in projections. One example is changes in heat waves in theMediterranean region that are suggested to have the largest impact incoastal areas, due to the role of enhanced relative humidity in healthimpacts (Diffenbaugh et al., 2007; Fischer and Schär, 2010). But it is notclear how this pattern may or may not be moderated by sea breezes(Diffenbaugh et al., 2007).In summary, since 1950 it is very likely that there has been anoverall decrease in the number of cold days and nights and anoverall increase in the number of warm days and nights at theglobal scale, that is, for land areas with sufficient data. It is likelythat such changes have also occurred at the continental scale inNorth America, Europe, and Australia. There is medium confidencein a warming trend in daily temperature extremes in much of Asia.Confidence in historical trends in daily temperature extremes inAfrica and South America generally varies from low to mediumdepending on the region. Globally, in many (but not all) regionswith sufficient data there is medium confidence that the length ornumber of warm spells or heat waves has increased since themiddle of the 20th century. It is likely that anthropogenicinfluences have led to warming of extreme daily minimum andmaximum temperatures at the global scale. Models projectsubstantial warming in temperature extremes by the end of the21st century. It is virtually certain that increases in the frequencyand magnitude of warm days and nights and decreases in the colddays and nights will occur through the 21st century at the globalscale. This is mostly linked with mean changes in temperatures,although changes in temperature variability can play an importantrole in some regions. It is very likely that the length, frequency,and/or intensity of warm spells or heat waves (defined withrespect to present regional climate) will increase over most landareas. For the SRES A2 and A1B emission scenarios a 1-in-20 yearannual hottest day is likely to become a 1-in-2 year annualextreme by the end of the 21st century in most regions, except inthe high latitudes of the Northern Hemisphere where it is likelyto become a 1-in-5 year annual extreme. In terms of absolutevalues, 20-year extreme annual daily maximum temperature (i.e.,return value) will likely increase by about 1 to 3°C by mid-21stcentury and by about 2 to 5°C by the late 21st century, dependingon the region and emissions scenario (Figure 3-5). Moderatetemperature extremes on land are projected to warm faster thanglobal annual mean temperature in many regions and seasons.Projected changes at subcontinental scales are less certain thanis the case for the global scale. Regional changes in temperatureextremes will differ from the mean global temperature change.Mean global warming does not necessarily imply warming in allregions and seasons.3.3.2. PrecipitationThis section addresses changes in daily extreme or heavy precipitationevents. Reductions in mean (or total) precipitation that can lead todrought (i.e., associated with lack of precipitation) are considered inSection 3.5.1. Because climates are so diverse across different parts ofthe world, it is difficult to provide a single definition of extreme or heavyprecipitation. In general, two different approaches have been used:(1) relative thresholds such as percentiles (typically the 95th percentile)and return values; and (2) absolute thresholds [e.g., 50.8 mm (2 inches)day -1 of rain in the United States, and 100 mm day -1 of rain in China].For more details on the respective drawbacks and advantages of thesetwo approaches, see Section 3.1 and Box 3-1. Note that we do notdistinguish between rain and snowfall (both considered as contributorsto overall extreme precipitation events) as they are not treated separatelyin the literature, but do distinguish changes in hail from other precipitationtypes. Increases in public awareness and changes in reporting practiceshave led to inconsistencies in the record of severe thunderstorms andhail that make it difficult to detect trends in the intensity or frequencyof these events (Kunkel et al., 2008). Furthermore, weather events suchas hail are not well captured by current monitoring systems and, insome parts of the world, the monitoring network is very sparse (Section3.2.1), resulting in considerable uncertainty in the estimates of extreme141