IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3over South America and Australia, between the 1960s-1970s and1980s-1990s. The time scales of variability in modes such as the AMOand PDO are so long that it is difficult to diagnose any change in theirbehavior in modern data, although some evidence suggests that thePDO may be affected by anthropogenic forcing (Meehl et al., 2009b).The AR4 (Hegerl et al., 2007) concluded that trends over recent decadesin the NAO and SAM are likely related in part to human activity. Thenegative NAO phase of the last few years, however, with the lack offormal attribution studies, means that attribution of changes in the NAOto human activity in recent decades now can only be considered aboutas likely as not (expert opinion). Attribution of the SAM trend to humanactivity is still assessed to be likely (expert opinion) although mainlyattributable to trends in stratospheric ozone concentration (Hegerl etal., 2007).The AR4 noted that there was considerable spread among the modelprojections of the NAO, leading to low confidence in NAO projectedchanges, but the magnitude of the increase for the SAM is generallymore consistent across models (Meehl et al., 2007b). However, the abilityof coupled models to simulate the observed SAM impact on climatevariability in the Southern Hemisphere is limited (e.g., Miller et al., 2006;Vera and Silvestri, 2009). Variations in the longer time-scale modes ofvariability (AMO, PDO) might affect projections of changes in extremesassociated with the various natural modes of variability and globaltemperatures (Keenlyside et al., 2008).Sea level pressure is projected to increase over the subtropics and midlatitudes,and decrease over high latitudes (Meehl et al., 2007b). Thiswould equate to trends in the NAO and SAM, with a poleward shift ofthe storm tracks of several degrees latitude and a consequent increasein cyclonic circulation patterns over the Arctic and Antarctica. In theSouthern Hemisphere, two opposing effects, stratospheric ozone recoveryand increasing greenhouse gases, can be expected to affect the modessuch as the SAM (Arblaster et al., 2011). During the 21st century, althoughstratospheric ozone concentrations are expected to recover, tending tolead to a weakening of the SAM, models consistently project polarvortex intensification to continue due to the increases in greenhousegases, except in summer where the competing effects of stratosphericozone recovery complicate this picture (Arblaster et al., 2011).A recent study (Woollings et al., 2010) found a tendency toward a morepositive NAO under anthropogenic forcing through the 21st centurywith one model, although they concluded that confidence in the modelprojections was low because of deficiencies in its simulation of current-dayNAO regimes. Goodkin et al. (2008) predict continuing high variability,on multi-decadal scales, in the NAO with continued global warming.Keenlyside et al. (2008) proposed that variations associated with themulti-decadal modes of variability may offset warming due to increasedgreenhouse gas concentrations over the next decade or so. Conway etal. (2007) reported that model projections of future IOD behavior showedno consistency. Kay and Washington (2008) reported that under someemissions scenarios, changes in a dipole mode in the Indian Oceancould change rainfall extremes in southern Africa.In summary, it is likely that there has been an anthropogenicinfluence on recent trends in the SAM (linked with trends instratospheric ozone rather than changes in greenhouse gases),but it is only about as likely as not that there have beenanthropogenic influences on observed trends in the NAO. Issueswith the ability of models to simulate current behavior of thesenatural modes, the influence of competing factors (e.g.,stratospheric ozone, greenhouse gases) on current and futuremode behavior, and inconsistency between the model projections(and the seasonal dependence of these projections), means thatthere is low confidence in the ability to project changes in themodes including the NAO, SAM, and IOD. Models do, however,consistently project a strengthening of the polar vortex in theSouthern Hemisphere from increasing greenhouse gases,although in summer stratospheric ozone recovery is expected tooffset this intensification.3.4.4. Tropical CyclonesTropical cyclones occur in most tropical oceans and pose a significantthreat to coastal populations and infrastructure, and marine interestssuch as shipping and offshore activities. Each year, about 90 tropicalcyclones occur globally, and this number has remained roughly steadyover the modern period of geostationary satellites (since around themid-1970s). While the global frequency has remained steady, there canbe substantial inter-annual to multi-decadal frequency variability withinindividual ocean basins (e.g., Webster et al., 2005). This regional variability,particularly when combined with substantial inter-annual to multi-decadalvariability in tropical cyclone tracks (e.g., Kossin et al., 2010), presents asignificant challenge for disaster planning and mitigation aimed atspecific regions.Tropical cyclones are perhaps most commonly associated with extremewind, but storm-surge and freshwater flooding from extreme rainfallgenerally cause the great majority of damage and loss of life (e.g.,Rappaport, 2000; Webster, 2008). Related indirect factors, such as thefailure of the levee system in New Orleans during the passage ofHurricane Katrina (2005), or mudslides during the landfall of HurricaneMitch (1998) in Central America, represent important related impacts(Case Study 9.2.5). Projected sea level rise will further compound tropicalcyclone surge impacts. Tropical cyclones that track poleward can undergoa transition to become extratropical cyclones. While these storms havedifferent characteristics than their tropical progenitors, they can still beaccompanied by a storm surge that can impact regions well away fromthe tropics (e.g., Danard et al., 2004).Tropical cyclones are typically classified in terms of their intensity, which isa measure of near-surface wind speed (sometimes categorized accordingto the Saffir-Simpson scale). The strongest storms (Saffir-Simpson category3, 4, and 5) are comparatively rare but are generally responsible for themajority of damage (e.g., Landsea, 1993; Pielke Jr. et al., 2008).Additionally, there are marked differences in the characteristics of both158