Book 2.indb - US Climate Change Science Program

Abrupt Climate Changechanges than those found in the instrumentalrecord are required to test the models andapproaches than could be used.• Better understanding of physical processes.The complexity of the response of extremehydrologic events to climatic variations,including as it does the impacts on both thefrequency and magnitude of meteorologicalextremes, and mediation by land cover andwatershed characteristics that themselves arechanging, suggests that further diagnosticstudies of the nature of the response shouldbe encouraged.7. Other Aspects ofHydroclimate ChangeThe atmosphere can hold more water vapor as itwarms (as described by the Clausius-Clapeyronequation), to the tune of about 7% per degreeCelsius of warming. With only small changesprojected for relative humidity (Soden et al.,2002), the specific humidity content of theatmosphere will also increase with warming atthis rate. This is in contrast to the global meanprecipitation increase of about 1–2% per degreeCelsius of warming. The latter is caused whenevaporation increases to balance increaseddownward longwave radiation associated withthe stronger greenhouse trapping. For both ofthese constraints to be met, more precipitationhas to fall in the heaviest of precipitation eventsas well explained by Trenberth et al. (2003).The change in precipitation intensity seemsto be a hydrological change that is alreadyevident (Kunkel et al., 2008; CCSP SAP 3.3,Secs. 2.2.2.2 and 2.2.2.3). Groisman et al.(2004) demonstrate that daily precipitationrecords over the last century in the UnitedStates show a striking increase, beginningaround 1990, in the proportion of precipitationwithin very heavy (upper 1% of events) andextreme (upper 0.1%) of events. In the annualmean there is a significant trend to increasedintensity in the southern and central plainsand in the Midwest, and there is a significantpositive trend in the Northeast in winter. Incontrast, the Rocky Mountain States show anunexplained significant trend to decreasingintensity in winter.Groisman et al. (2005) show that the observedtrend to increasing precipitation intensity isseen across much of the world, and both theyand Wilby and Wigley (2002) show that climatemodel projections of the current century showthat this trend will continue. Groisman et al.(2005) make the point that the trends in intensityare greater than the trends in mean precipitation,that there is good physical reason to believethat they are related to global warming, and thatthey are likely to be more easily detected thanchanges in the mean precipitation.Increases in precipitation intensity can havesignificant social impacts as they increase thepotential for flooding and overloading of sewersand wastewater treatment plants. See Rosenzweiget al. (2007) for a case study of New YorkCity’s planning efforts to deal with water-relatedaspects of climate change. Increasing precipitationintensity can also lead to an increase ofsediment flux, including potentially harmfulpathogens, into water-supply reservoirs, thusnecessitating more careful water-qualitymanagement, a situation already being facedby New York City. (See http://www.amwa.net/cs/climatechange/newyorkcity for a usefuldiscussion of how a major metropolitan area isalready beginning to address this issue.)Another aspect of hydroclimatic change thatcan be observed in many regions is the generaldecrease in snowpack and snow cover (Moteet al., 2005; Déry and Brown, 2007; Dyer andMote, 2006; see also Lettenmaier et al., 2008;CCSP SAP 4.3, Sec. 4.2.4). Winter snowfalland the resulting accumulated snowpackdepend on temperature in complicated ways.Increasing temperatures favor greater moistureavailability and total precipitation (in muchthe same way that precipitation intensitydepends on temperature) and hence greatersnow accumulation (if winter temperaturesare cold enough), but greater snowmelt andhence a reduced snowpack if temperaturesincrease enough. Regions with abundant winterprecipitation and winter temperatures close tofreezing could therefore experience an overallincrease in winter precipitation as temperaturesincrease but also an overall decrease in snowcover as the balance of precipitation shifts fromIncreases inprecipitationintensity canhave significantsocial impactsas they increasethe potentialfor floodingand overloadingof sewers andwastewatertreatment plants.113