IPCC Report.pdf - Adam Curry

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3FAQ 3.1 | Is the Climate Becoming More Extreme?While there is evidence that increases in greenhouse gases have likely caused changes in some types of extremes, there is no simpleanswer to the question of whether the climate, in general, has become more or less extreme. Both the terms ‘more extreme’ and ‘lessextreme’ can be defined in different ways, resulting in different characterizations of observed changes in extremes. Additionally, from aphysical climate science perspective it is difficult to devise a comprehensive metric that encompasses all aspects of extreme behavior inthe climate.One approach for evaluating whether the climate is becoming more extreme would be to determine whether there have been changesin the typical range of variation of specific climate variables. For example, if there was evidence that temperature variations in a givenregion had become significantly larger than in the past, then it would be reasonable to conclude that temperatures in that region hadbecome more extreme. More simply, temperature variations might be considered to be becoming more extreme if the differencebetween the highest and the lowest temperature observed in a year is increasing. According to this approach, daily temperature overthe globe may have become less extreme because there have generally been greater increases in mean daily minimum temperaturesglobally than in mean daily maximum temperatures, over the second half of the 20th century. On the other hand, one might concludethat daily precipitation has become more extreme because observations suggest that the magnitude of the heaviest precipitation eventshas increased in many parts of the world. Another approach would be to ask whether there have been significant changes in thefrequency with which climate variables cross fixed thresholds that have been associated with human or other impacts. For example, anincrease in the mean temperature usually results in an increase in hot extremes and a decrease in cold extremes. Such a shift in thetemperature distribution would not increase the ‘extremeness’ of day-to-day variations in temperature, but would be perceived asresulting in a more extreme warm temperature climate, and a less extreme cold temperature climate. So the answer to the questionposed here would depend on the variable of interest, and on which specific measure of the extremeness of that variable is examined. Aswell, to provide a complete answer to the above question, one would also have to collate not just trends in single variables, but alsoindicators of change in complex extreme events resulting from a sequence of individual events, or the simultaneous occurrence ofdifferent types of extremes. So it would be difficult to comprehensively describe the full suite of phenomena of concern, or to find a wayto synthesize all such indicators into a single extremeness metric that could be used to comprehensively assess whether the climate as awhole has become more extreme from a physical perspective. And to make such a metric useful to more than a specific location, onewould have to combine the results at many locations, each with a different perspective on what is ‘extreme.’Continued next pagetropical cyclone observations are taken to support short-term forecastingneeds. Improvements in observing techniques are often implementedwithout any overlap or calibration against existing methods to documentthe impact of the changes on the climate record. Additionally, advancesin technology have enabled better and more complete observations. Forexample, the introduction of aircraft reconnaissance in some basins inthe 1940s and satellite data in the 1960s had a profound effect on ourability to accurately identify and measure tropical cyclones, particularlythose that never encountered land or a ship. While aircraft reconnaissanceprograms have continued in the North Atlantic, they were terminated inthe Western Pacific in 1987. The introduction of geostationary satelliteimagery in the 1970s, and the introduction (and subsequent improvement)of new tropical cyclone analysis methods (such as the Dvorak techniquefor estimating storm intensity), further compromises the homogeneityof historical records of tropical cyclone activity.Regarding impacts to the physical environment, soil moisture is a keyvariable for which data sets are extremely scarce (e.g., Robock et al.,2000; Seneviratne et al., 2010). This represents a critical issue for thevalidation and correct representation of soil moisture (agricultural) aswell as hydrological drought (Box 3-3) in climate, land surface, andhydrological models, and the monitoring of ongoing changes in regionalterrestrial water storage. As a consequence, these need to be inferredfrom simple climate indices or model-based approaches (Box 3-3). Suchestimates rely in large part on precipitation observations, which have,however, inadequate spatial coverage for these applications in manyregions of the world (e.g., Oki et al., 1999; Fekete et al., 2004; Koster etal., 2004a). Similarly, runoff observations are not globally available,which results in significant uncertainties in the closing of the global andsome regional water budgets (Legates et al., 2005; Peel and McMahon,2006; Dai et al., 2009; Teuling et al., 2009), as well as for the globalanalysis of changes in the occurrence of floods (Section 3.5.2).Additionally, ground observations of snow, which are lacking in severalregions, are important for the investigation of physical impacts,particularly those related to the cryosphere and runoff generation (e.g.,Essery et al., 2009; Rott et al., 2010).All of the above-mentioned issues lead to uncertainties in observedtrends in extremes. In many instances, great care has been taken todevelop procedures to reduce the confounding influences of theseissues on the data, which in turn helps to reduce uncertainty, andprogress has been made in the last 15 years (e.g., Caesar et al., 2006;124