IPCC Report.pdf - Adam Curry

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environmentobserved and projected tropical cyclone variability when comparingweaker and stronger tropical cyclones (e.g., Webster et al., 2005; Elsneret al., 2008; Bender et al., 2010), while records of the strongest stormsare potentially less reliable than those of their weaker counterparts(Landsea et al., 2006).In addition to intensity, the structure and areal extent of the wind fieldin tropical cyclones, which can be largely independent of intensity, alsoplay an important role on potential impacts, particularly from stormsurge (e.g., Irish and Resio, 2010), but measures of storm size are largelyabsent in historical data. Other relevant tropical cyclone measuresinclude frequency, duration, and track. Forming robust physical linksbetween all of the metrics briefly mentioned here and natural orhuman-induced changes in climate variability is a major challenge.Significant progress is being made, but substantial uncertainties stillremain due largely to data quality issues (see Section 3.2.1 and below)and imperfect theoretical and modeling frameworks (see below).Observed ChangesDetection of trends in tropical cyclone metrics such as frequency,intensity, and duration remains a significant challenge. Historical tropicalcyclone records are known to be heterogeneous due to changingobserving technology and reporting protocols (e.g., Landsea et al., 2004).Further heterogeneity is introduced when records from multiple oceanbasins are combined to explore global trends because data quality andreporting protocols vary substantially between regions (Knapp andKruk, 2010). Progress has been made toward a more homogeneousglobal record of tropical cyclone intensity using satellite data (Knappand Kossin, 2007; Kossin et al., 2007), but these records are necessarilyconstrained to the satellite era and so only represent the past 30 to 40years.Natural variability combined with uncertainties in the historical datamakes it difficult to detect trends in tropical cyclone activity. There havebeen no significant trends observed in global tropical cyclone frequencyrecords, including over the present 40-year period of satellite observations(e.g., Webster et al., 2005). Regional trends in tropical cyclone frequencyhave been identified in the North Atlantic, but the fidelity of these trendsis debated (Holland and Webster, 2007; Landsea, 2007; Mann et al.,2007a). Different methods for estimating undercounts in the earlier partof the North Atlantic tropical cyclone record provide mixed conclusions(Chang and Guo, 2007; Mann et al., 2007b; Kunkel et al., 2008; Vecchiand Knutson, 2008). Regional trends have not been detected in otheroceans (Chan and Xu, 2009; Kubota and Chan, 2009; Callaghan andPower, 2011). It thus remains uncertain whether any observed increasesin tropical cyclone frequency on time scales longer than about 40 yearsare robust, after accounting for past changes in observing capabilities(Knutson et al., 2010).Frequency estimation requires only that a tropical cyclone be identifiedand reported at some point in its lifetime, whereas intensity estimationrequires a series of specifically targeted measurements over the entireduration of the tropical cyclone (e.g., Landsea et al., 2006). Consequently,intensity values in the historical records are especially sensitive tochanging technology and improving methodology, which heightens thechallenge of detecting trends within the backdrop of natural variability.Global reanalyses of tropical cyclone intensity using a homogenoussatellite record have suggested that changing technology has introduceda non-stationary bias that inflates trends in measures of intensity(Kossin et al., 2007), but a significant upward trend in the intensity ofthe strongest tropical cyclones remains after this bias is accounted for(Elsner et al., 2008). While these analyses are suggestive of a linkbetween observed global tropical cyclone intensity and climate change,they are necessarily confined to a roughly 30-year period of satelliteobservations, and cannot provide clear evidence for a longer-term trend.Time series of power dissipation, an aggregate compound of tropicalcyclone frequency, duration, and intensity that measures total energyconsumption by tropical cyclones, show upward trends in the NorthAtlantic and weaker upward trends in the western North Pacific over thepast 25 years (Emanuel, 2007), but interpretation of longer-term trendsin this quantity is again constrained by data quality concerns. Thevariability and trend of power dissipation can be related to SST andother local factors such as tropopause temperature and vertical windshear (Emanuel, 2007), but it is a current topic of debate whether localSST or the difference between local SST and mean tropical SST is themore physically relevant metric (Swanson, 2008). The distinction is animportant one when making projections of changes in power dissipationbased on projections of SST changes, particularly in the tropical Atlanticwhere SST has been increasing more rapidly than in the tropics as awhole (Vecchi et al., 2008). Accumulated cyclone energy, which is anintegrated metric analogous to power dissipation, has been decliningglobally since reaching a high point in 2005, and is presently at a 40-year low point (Maue, 2009). The present period of quiescence, as wellas the period of heightened activity leading up to the high point in 2005,does not clearly represent substantial departures from past variability(Maue, 2009).Increases in tropical water vapor and rainfall (Trenberth et al., 2005; Lauand Wu, 2007) have been identified and there is some evidence forrelated changes in tropical cyclone-related rainfall (Lau et al., 2008a),but a robust and consistent trend in tropical cyclone rainfall has not yetbeen established due to a general lack of studies. Similarly, an increasein the length of the North Atlantic hurricane season has been noted(Kossin, 2008), but the uncertainty in the amplitude of the trends andthe lack of additional studies limits the utility of these results for ameaningful assessment.Estimates of tropical cyclone variability prior to the modern instrumentalhistorical record have been constructed using archival documents(Chenoweth and Devine, 2008), coastal marsh sediment records, andisotope markers in coral, speleothems, and tree rings, among othermethods (Frappier et al., 2007a). These estimates demonstrate centennialtomillennial-scale relationships between climate and tropical cyclone159