9.4.3.3 Attributable Changes in the Risk of Extremes

Many important impacts of climate change may manifest themselves through a change in the frequency or likelihood of occurrence of extreme events. While individual extreme events cannot be attributed to external influences, a change in the probability of such events might be attributable to external influences (Palmer, 1999; Palmer and Räisänen, 2002). One study estimates that anthropogenic forcings have significantly increased the risk of extremely warm summer conditions over southern Europe, as was observed during the 2003 European heat wave. Stott et al. (2004) apply a methodology for making quantitative statements about change in the likelihood of such specific types of climatic events (Allen, 2003; Stone and Allen, 2005a), by expressing the contribution of external forcing to the risk of an event exceeding a specific magnitude. If P1 is the probability of a climatic event (such as a heat wave) occurring in the presence of anthropogenic forcing of the climate system, and P0 is the probability of it occurring if anthropogenic forcing had not been present, then the fraction of the current risk that is attributable to past greenhouse gas emissions (fraction of attributable risk; FAR) is given by FAR = 1 – P0 / P1 (Allen, 2003). Stott et al. (2004) apply the FAR concept to mean summer temperatures of a large part of continental Europe and the Mediterranean. Using a detection and attribution analysis, they determine that regional summer mean temperature has likely increased due to anthropogenic forcing, and that the observed change is inconsistent with natural forcing. They then use the HadCM3 model to estimate the FAR associated with a particular extreme threshold of regional summer mean temperature that was exceeded in 2003, but in no other year since the beginning of the record in 1851. Stott et al. (2004) estimate that it is very likely that human influence has more than doubled the risk of the regional summer mean temperature exceeding this threshold (Figure 9.13).

Figure 9.13. Change in risk of mean European summer temperatures exceeding a threshold of 1.6°C above 1961 to 1990 mean temperatures, a threshold that was exceeded in 2003 but in no other year since the start of the instrumental record in 1851. (Top) Frequency histograms of the estimated likelihood of the risk (probability) of exceeding a 1.6°C threshold (relative to the 1961–1990 mean) in the 1990s in the presence (red curve) and absence (green curve) of anthropogenic change, expressed as an occurrence rate. (Bottom) Fraction of attributable risk (FAR). The vertical line indicates the ‘best estimate’ FAR, the mean risk attributable to anthropogenic factors averaged over the distribution. The alternation between grey and white bands indicates the deciles of the estimated FAR distribution. The shift from the green to the red distribution in (a) implies a FAR distribution with mean 0.75, corresponding to a four-fold increase in the risk of such an event (b). From Stott et al. (2004).

This study considered only continental mean seasonally averaged temperatures. Consideration of shorter-term and smaller-scale heat waves will require higher resolution modelling and will need to take complexities such as land surface processes into account (Schär and Jendritzky, 2004). Also, Stott et al. (2004) assume no change in internal variability in the region they consider (which was the case in HadCM3 21st-century climate projections for summer mean temperatures in the region they consider), thereby ascribing the increase in risk only to an increase in mean temperatures (i.e., as shown in Box TS.5, Figure 1, which illustrates how a shift in the mean of a distribution can cause a large increase in the frequency of extremes). However, there is some evidence for a weak increase in European temperature variability in summer (and a decrease in winter) for the period 1961 to 2004 (Scherrer et al., 2005), which could contribute to an increase in the likelihood of extremes. Schär et al. (2004) show that the central European heat wave of 2003 could also be consistent with model-predicted increases in temperature variability due to soil moisture and vegetation feedbacks. In addition, multi-decadal scale variability, associated with basin-scale changes in the Atlantic Ocean related to the Meridional Overturning Circulation (MOC) could have contributed to changes in European summer temperatures (Sutton and Hodson, 2005), although Klein Tank et al. (2005) show evidence that patterns of change in European temperature variance in spring and summer are not consistent with patterns of change in temperature variance expected from natural variability. Meteorological aspects of the summer 2003 European heat wave are discussed in Box 3.6.