IPCC Report.pdf - Adam Curry

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentThree types of metrics have been considered to avoid these problems, and thereby allow an answer to this question. One approach is tocount the number of record-breaking events in a variable and to examine such a count for any trend. However, one would still face theproblem of what to do if, for instance, hot extremes are setting new records, while cold extremes are not occurring as frequently as inthe past. In such a case, counting the number of records might not indicate whether the climate was becoming more or less extreme,rather just whether there was a shift in the mean climate. Also, the question of how to combine the numbers of record-breaking eventsin various extremes (e.g., daily precipitation and hot temperatures) would need to be considered. Another approach is to combineindicators of a selection of important extremes into a single index, such as the Climate Extremes Index (CEI), which measures the fractionof the area of a region or country experiencing extremes in monthly mean surface temperature, daily precipitation, and drought. The CEI,however, omits many important extremes such as tropical cyclones and tornadoes, and could, therefore, not be considered a completeindex of ‘extremeness.’ Nor does it take into account complex or multiple extremes, nor the varying thresholds that relate extremes toimpacts in various sectors.A third approach to solving this dilemma arises from the fact that extremes often have deleterious economic consequences. It maytherefore be possible to measure the integrated economic effects of the occurrence of different types of extremes into a commoninstrument such as insurance payout to determine if there has been an increase or decrease in that instrument. This approach wouldhave the value that it clearly takes into account those extremes with economic consequences. But trends in such an instrument will bedominated by changes in vulnerability and exposure and it will be difficult, if not impossible, to disentangle changes in the instrumentcaused by non-climatic changes in vulnerability or exposure in order to leave a residual that reflects only changes in climate extremes.For example, coastal development can increase the exposure of populations to hurricanes; therefore, an increase in damage in coastalregions caused by hurricane landfalls will largely reflect changes in exposure and may not be indicative of increased hurricane activity.Moreover, it may not always be possible to associate impacts such as the loss of human life or damage to an ecosystem due to climateextremes to a measurable instrument.None of the above instruments has yet been developed sufficiently as to allow us to confidently answer the question posed here. Thuswe are restricted to questions about whether specific extremes are becoming more or less common, and our confidence in the answersto such questions, including the direction and magnitude of changes in specific extremes, depends on the type of extreme, as well ason the region and season, linked with the level of understanding of the underlying processes and the reliability of their simulation inmodels.Brown et al., 2008). As a consequence, more complete and homogenousinformation about changes is now available for at least some variablesand regions (Nicholls and Alexander, 2007; Peterson and Manton,2008). For instance, the development of global databases of dailytemperature and precipitation covering up to 70% of the global landarea has allowed robust analyses of extremes (see Alexander et al.,2006). In addition, analyses of temperature and precipitation extremesusing higher temporal resolution data, such as that available in theGlobal Historical Climatology Network – Daily data set (Durre et al., 2008)have also proven robust at both a global (Alexander et al., 2006) andregional scale (Sections 3.3.1 and 3.3.2). Nonetheless, as highlightedabove, for many extremes, data remain sparse and problematic, resultingin lower ability to establish changes, particularly on a global basis andfor specific regions.3.2.2. The Causes behind the ChangesThis section discusses the main requirements, approaches, andconsiderations for the attribution of causes for observed changes inextremes. In Sections 3.3 to 3.5, the causes of observed changes inspecific extremes are assessed. A global summary of these assessmentsis provided in Table 3-1. Climate variations and change are induced byvariability internal to the climate system, and changes in externalforcings, which include natural external forcings such as changes in solarirradiance and volcanism, and anthropogenic forcings such as aerosoland greenhouse gas emissions principally due to the burning of fossilfuels, and land use and land cover changes. The mean state, extremes,and variability are all related aspects of the climate, so external forcingsthat affect the mean climate would in general result in changes inextremes. For this reason, we provide in Section 3.2.2.1 a brief overviewof human-induced changes in the mean climate to aid the understandingof changes in extremes as the literature directly addressing the causesof changes in extremes is quite limited.3.2.2.1. Human-Induced Changes in the Mean Climatethat Affect ExtremesThe occurrence of extremes is usually the result of multiple factors,which can act either on the large scale or on the regional (and local)scale (see also Section 3.1.6). Some relevant large-scale impacts of125