IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environmentfound in Eastern and Southeast Asia (Choi et al., 2009), central andsouth Asia (Klein Tank et al., 2006), and Western Asia (X. Zhang et al.,2005; Rahimzadeh et al., 2009). However, statistically significant positiveand negative trends were observed at subregional scales within theseregions. Heavy precipitation increased in Japan during 1901-2004 (Fujibeet al., 2006), and in India (Rajeevan et al., 2008; Krishnamurthy et al.,2009) especially during the monsoon seasons (Sen Roy, 2009; Pattanaikand Rajeevan, 2010). Both statistically significant increases anddecreases in extreme precipitation have been found in China over theperiod 1951-2000 (Zhai et al., 2005) and 1978-2002 (Yao et al., 2008).In Peninsular Malaysia during 1971-2005 the intensity of extremeprecipitation increased and the frequency decreased, while the trend inthe proportion of extreme rainfall over total precipitation was notstatistically significant (Zin et al., 2009). Heavy precipitation increasedover the southern and northern Tibetan Plateau but decreased in thecentral Tibetan Plateau during 1961-2005 (You et al., 2008).In southern Australia, there has been a likely decrease in heavyprecipitation in many areas, especially where mean precipitation hasdecreased (Table 3-2). There were statistically significant increases inthe proportion of annual/seasonal rainfall stemming from heavy raindays from 1911-2008 and 1957-2008 in northwest Australia (Gallantand Karoly, 2010). Extreme summer rainfall over the northwest of theSwan-Avon River basin in western Australia increased over 1950-2003while extreme winter rainfall over the southwest of the basin decreased(Aryal et al., 2009). In New Zealand, the trends are positive in the westernNorth and South Islands and negative in the east of the country (Mullanet al., 2008).There is low to medium confidence in regional trends in heavyprecipitation in Africa due to partial lack of literature and data, and dueto lack of consistency in reported patterns in some regions (Table 3-2).The AR4 (Trenberth et al., 2007) reported an increase in heavyprecipitation over southern Africa, but this appears to depend on theregion and precipitation index examined (Kruger, 2006; New et al.,2006; Seleshi and Camberlin, 2006; Aguilar et al., 2009). Central Africaexhibited a decrease in heavy precipitation over the last half century(Aguilar et al., 2009); however, data coverage for large parts of theregion was poor. Precipitation from heavy events has decreased inwestern central Africa, but with low spatial coherence (Aguilar et al.,2009). Rainfall intensity averaged over southern and west Africa hasincreased (New et al., 2006). There is a lack of literature on changes inheavy precipitation in East Africa (Table 3-2). Camberlin et al. (2009)analyzed changes in components of rainy seasons’ variability over thetime period 1958-1987 in this region, but did not specifically addresstrends in heavy precipitation. There were decreasing trends in heavyprecipitation over parts of Ethiopia during the period 1965-2002(Seleshi and Camberlin, 2006).Changes in hail occurrence are generally difficult to quantify because hailoccurrence is not well captured by monitoring systems and because ofhistorical data inhomogeneities. Sometimes, changes in environmentalconditions conducive to hail occurrence are used to infer changes in hailoccurrence. However, the atmospheric conditions are typically estimatedfrom reanalyses or from radiosonde data and the estimates are associatedwith high uncertainty. As a result, assessment of changes in hail frequencyis difficult. For severe thunderstorms in the region east of the RockyMountains in the United States, Brooks and Dotzek (2008) found strongvariability but no clear trend in the past 50 years. Cao (2008) identifieda robust upward trend in hail frequency over Ontario, Canada. Kunz etal. (2009) found that both hail damage days and convective instabilityincreased during 1974-2003 in a state in southwest Germany. Xie et al.(2008) identified no trend in the mean annual hail days in China from1960 to the early 1980s but a statistically significant decreasing trendafterwards.Causes of Observed ChangesThe observed changes in heavy precipitation appear to be consistentwith the expected response to anthropogenic forcing (increase due toenhanced moisture content in the atmosphere; see, e.g., Section 3.2.2.1)but a direct cause-and-effect relationship between changes in externalforcing and extreme precipitation had not been established at the timeof the AR4. As a result, the AR4 only concluded that it was more likelythan not that anthropogenic influence had contributed to a global trendtowards increases in the frequency of heavy precipitation events overthe second half of the 20th century (Hegerl et al., 2007).New research since the AR4 provides more evidence of anthropogenicinfluence on various aspects of the global hydrological cycle (Stott et al.,2010; see also Section 3.2.2), which is directly relevant to extremeprecipitation changes. In particular, an anthropogenic influence onatmospheric moisture content is detectable (Santer et al., 2007; Willettet al., 2007; see also Section 3.2.2). Wang and Zhang (2008) show thatwinter season maximum daily precipitation in North America appears tobe statistically significantly influenced by atmospheric moisture content,with an increase in moisture corresponding to an increase in maximumdaily precipitation. This behavior has also been seen in model projectionsof extreme winter precipitation under global warming (Gutowski et al.,2008b). Climate model projections suggest that the thermodynamicconstraint based on the Clausius-Clapeyron relation is a good predictorfor extreme precipitation changes in a warmer world in regions wherethe nature of the ambient flows change little (Pall et al., 2007). Thisindicates that the observed increase in extreme precipitation in manyregions is consistent with the expected extreme precipitation responseto anthropogenic influences. However, the thermodynamic constraintmay not be a good predictor in regions with circulation changes, such asmid- to higher latitudes (Meehl et al., 2005) and the tropics (Emori andBrown, 2005), and in arid regions. Additionally, changes in precipitationextremes with temperature also depend on changes in the moistadiabatictemperature lapse rate, in the upward velocity, and in thetemperature when precipitation extremes occur (O’Gorman andSchneider, 2009a,b; Sugiyama et al., 2010). This may explain why therehave not been increases in precipitation extremes everywhere, althougha low signal-to-noise ratio may also play a role. However, even in143