IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3precipitation. There are also known biases in precipitation measurements,mostly leading to rain undercatch. Little evidence of paleoclimatic andhistorical changes in heavy precipitation is available to place recentvariations into context.Observed ChangesThe AR4 (Trenberth et al., 2007) concluded that it was likely that therehad been increases in the number of heavy precipitation events (e.g.,95th percentile) over the second half of the 20th century within manyland regions, even in those where there had been a reduction in totalprecipitation amount, consistent with a warming climate and observedsignificant increasing amounts of water vapor in the atmosphere.Increases had also been reported for rarer precipitation events (1-in-50year return period), but only a few regions had sufficient data to assesssuch trends reliably. However, the AR4 (Trenberth et al., 2007) also statedthat “Many analyses indicate that the evolution of rainfall statisticsthrough the second half of the 20th century is dominated by variationson the interannual to inter-decadal time scale and that trend estimatesare spatially incoherent (Manton et al., 2001; Peterson et al., 2002; Griffithset al., 2003; Herath and Ratnayake, 2004)”. Overall, as highlighted inAlexander et al. (2006), the observed changes in precipitation extremeswere found at the time to be much less spatially coherent and statisticallysignificant compared to observed changes in temperature extremes:although statistically significant trends toward stronger precipitationextremes were generally found for a larger fraction of the land areathan trends toward weaker precipitation extremes, statistically significantchanges in precipitation indices for the overall land areas with datawere only found for the Simple Daily Intensity index, and not for otherconsidered indices such as Heavy Rainfall Days (Alexander et al., 2006).Recent studies have updated the assessment of the AR4, with moreregional results now available (Table 3-2). Overall, this additional evidenceconfirms that more locations and studies show an increase than adecrease in extreme precipitation, but that there are also wide regionaland seasonal variations, and trends in many regions are not statisticallysignificant (Table 3-2).Recent studies on past and current changes in precipitation extremes inNorth America, some of which are included in the recent assessment ofthe CCSP report (Kunkel et al., 2008), have reported an increasing trendover the last half century. Based on station data from Canada, theUnited States, and Mexico, Peterson et al. (2008a) reported that heavyprecipitation has been increasing over 1950-2004, as well as the averageamount of precipitation falling on days with precipitation. For thecontiguous United States, DeGaetano (2009) showed a 20% reductionin the return period for extreme precipitation of different return levelsover 1950-2007; Gleason et al. (2008) reported an increasing trend inthe area experiencing a much above-normal proportion of heavy dailyprecipitation from 1950 to 2006; and Pryor et al. (2009) provided evidenceof increases in the intensity of events above the 95th percentile duringthe 20th century, with a larger magnitude of the increase at the end of thecentury. The largest trends toward increased annual total precipitation,number of rainy days, and intense precipitation (e.g., fraction derivedfrom events in excess of the 90th percentile value) were focused on theGreat Plains/northwestern Midwest (Pryor et al., 2009). In the core ofthe North American monsoon region in northwest Mexico, statisticallysignificant positive trends were found in daily precipitation intensityand seasonal contribution of daily precipitation greater than its 95thpercentile in the mountain sites for the period 1961-1998. However, nostatistically significant changes were found in coastal stations (Cavazoset al., 2008). Overall, the evidence indicates a likely increase in observedheavy precipitation in many regions in North America, despite statisticallynon-significant trends and some decreases in some subregions (Table 3-2).This general increase in heavy precipitation accompanies a generalincrease in total precipitation in most areas of the country.There is low to medium confidence in trends for Central and SouthAmerica, where spatially varying trends in extreme rainfall events havebeen observed (Table 3-2). Positive trends in many areas but negativetrends in some regions are evident for Central America and northernSouth America (Dufek and Ambrizzi, 2008; Marengo et al., 2009b; Reand Ricardo Barros, 2009; Sugahara et al., 2009). For the western coastof South America, a decrease in extreme rainfall in many areas and anincrease in a few areas are observed (Haylock et al., 2006b).There is medium confidence in trends in heavy precipitation in Europe,due to partly inconsistent signals across studies and regions, especiallyin summer (Table 3-2). Winter extreme precipitation has increased in partof the continent, in particular in central-western Europe and EuropeanRussia (Zolina et al., 2009), but the trend in summer precipitation hasbeen weak or not spatially coherent (Moberg et al., 2006; Bartholy andPongracz, 2007; Maraun et al., 2008; Pavan et al., 2008; Zolina et al.,2008; Costa and Soares, 2009; Kyselý, 2009; Durão et al., 2010; Roddaet al., 2010). Increasing trends in 90th, 95th, and 98th percentiles of dailywinter precipitation over 1901-2000 were found (Moberg et al., 2006),which has been confirmed by more detailed country-based studies forthe United Kingdom (Maraun et al., 2008), Germany (Zolina et al.,2008), and central and eastern Europe (Bartholy and Pongracz, 2007;Kyselý, 2009), while decreasing trends have been found in some regionssuch as northern Italy (Pavan et al., 2008), Poland (Lupikasza, 2010),and some Mediterranean coastal sites (Toreti et al., 2010). Uncertaintiesare overall larger in southern Europe and the Mediterranean region,where there is low confidence in the trends (Table 3-2). A recent study(Zolina et al., 2010) has indicated that there has been an increase ofabout 15 to 20% in the persistence of wet spells over most of Europeover the last 60 years, which was not associated with an increase of thetotal number of wet days.There is low to medium confidence in trends in heavy precipitation inAsia, both at the continental and regional scale for most regions (Table3-2; see also Alexander et al., 2006). A weak increase in the frequencyof extreme precipitation events is observed in northern Mongolia(Nandintsetseg et al., 2007). No systematic spatially coherent trends inthe frequency and duration of extreme precipitation events have been142