IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3activity (Donnelly and Woodruff, 2007; Frappier et al., 2007b; Nott et al.,2007; Nyberg et al., 2007; Scileppi and Donnelly, 2007; Neu, 2008;Woodruff et al., 2008a,b; Mann et al., 2009; Yu et al., 2009), but generallydo not provide robust evidence that the observed post-industrial tropicalcyclone activity is unprecedented.The AR4 Summary for Policymakers concluded that it is likely that anincrease had occurred in intense tropical cyclone activity since 1970 insome regions (IPCC, 2007b). The subsequent CCSP assessment report(Kunkel et al., 2008) concluded that it is likely that the frequency oftropical storms, hurricanes, and major hurricanes in the North Atlantichas increased over the past 100 years, a time in which Atlantic SSTs alsoincreased. Kunkel et al. (2008) also concluded that the increase inAtlantic power dissipation is likely substantial since the 1950s. Based onresearch subsequent to the AR4 and Kunkel et al. (2008), which furtherelucidated the scope of uncertainties in the historical tropical cyclone data,the most recent assessment by the World Meteorological Organization(WMO) Expert Team on Climate Change Impacts on Tropical Cyclones(Knutson et al., 2010) concluded that it remains uncertain whether pastchanges in any tropical cyclone activity (frequency, intensity, rainfall)exceed the variability expected through natural causes, after accountingfor changes over time in observing capabilities. The present assessmentregarding observed trends in tropical cyclone activity is essentiallyidentical to the WMO assessment (Knutson et al., 2010): there is lowconfidence that any observed long-term (i.e., 40 years or more) increasesin tropical cyclone activity are robust, after accounting for past changesin observing capabilities.Causes of the Observed ChangesIn addition to the natural variability of tropical SSTs, several studieshave concluded that there is a detectable tropical SST warming trenddue to increasing greenhouse gases (Karoly and Wu, 2005; Knutson etal., 2006; Santer et al., 2006; Gillett et al., 2008a). The region where thisanthropogenic warming has occurred encompasses tropical cyclogenesisregions, and Kunkel et al. (2008) stated that it is very likely that humancausedincreases in greenhouse gases have contributed to the increasein SSTs in the North Atlantic and the Northwest Pacific hurricane formationregions over the 20th century.Changes in the mean thermodynamic state of the tropics can be directlylinked to tropical cyclone variability within the theoretical framework ofpotential intensity theory (Bister and Emanuel, 1998). In this framework,the expected response of tropical cyclone intensity to observed climatechange is relatively straightforward: if climate change causes anincrease in the ambient potential intensity that tropical cyclones movethrough, the distribution of intensities in a representative sample ofstorms is expected to shift toward greater intensities (Emanuel, 2000;Wing et al., 2007). The fractional changes associated with such a shiftin the distribution would be largest in the upper quantiles of thedistribution as the strongest tropical cyclones become stronger (Elsneret al., 2008).Given the evidence that SST in the tropics has increased due toincreasing greenhouse gases, and the theoretical expectation thatincreases in potential intensity will lead to stronger storms, it is essentialto fully understand the relationship between SST and potential intensity.Observations demonstrate a strong positive correlation between SSTand the potential intensity. This relationship suggests that SST increaseswill lead to increased potential intensity, which will then ultimately leadto stronger storms (Emanuel, 2000; Wing et al., 2007). However, there isa growing body of research suggesting that local potential intensity iscontrolled by the difference between local SST and spatially averagedSST in the tropics (Vecchi and Soden, 2007a; Xie et al., 2010; Ramsayand Sobel, 2011). Since increases in SST due to global warming are notexpected to lead to continuously increasing SST gradients, this recentresearch suggests that increasing SST due to global warming, by itself,does not yet have a fully understood physical link to increasingly strongtropical cyclones.The present period of heightened tropical cyclone activity in the NorthAtlantic, concurrent with comparative quiescence in other ocean basins(e.g., Maue, 2009), is apparently related to differences in the rate of SSTincreases, as global SST has been rising steadily but at a slower ratethan has the Atlantic (Holland and Webster, 2007). The present period ofrelatively enhanced warming in the Atlantic has been proposed to bedue to internal variability (Zhang and Delworth, 2009), anthropogenictropospheric aerosols (Mann and Emanuel, 2006), and mineral (dust)aerosols (Evan et al., 2009). None of these proposed mechanisms providea clear expectation that North Atlantic SST will continue to increase ata greater rate than the tropical mean SST.Changes in tropical cyclone intensity, frequency, genesis location,duration, and track contribute to what is sometimes broadly defined as‘tropical cyclone activity.’ Of these metrics, intensity has the most directphysically reconcilable link to climate variability within the frameworkof potential intensity theory, as described above (Kossin and Vimont,2007). Statistical correlations between necessary ambient environmentalconditions (e.g., low vertical wind shear and adequate atmosphericinstability and moisture) and tropical cyclogenesis frequency have beenwell documented (DeMaria et al., 2001) but changes in these conditionsdue specifically to increasing greenhouse gas concentrations do notnecessarily preserve the same statistical relationships. For example, theobserved minimum SST threshold for tropical cyclogenesis is roughly26°C. This relationship might lead to an expectation that anthropogenicwarming of tropical SST and the resulting increase in the areal extent ofthe region of 26°C SST should lead to increases in tropical cyclonefrequency. However, there is a growing body of evidence that theminimum SST threshold for tropical cyclogenesis increases at about thesame rate as the SST increase due solely to greenhouse gas forcing(e.g., Ryan et al., 1992; Dutton et al., 2000; Yoshimura et al., 2006;Bengtsson et al., 2007; Knutson et al., 2008; Johnson and Xie, 2010).This is because the threshold conditions for tropical cyclogenesis arecontrolled not just by surface temperature but also by atmosphericstability (measured from the lower boundary to the tropopause), whichresponds to greenhouse gas forcing in a more complex way than SST160