Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Programsnow to rain, along with an earlier occurrenceof spring snowmelt. Such trends seem to beunderway in many regions (Moore et al., 2007),but particularly in the Western United States(Mote et al., 2005, 2008).As a consequence of reduced snowpack andearlier spring snowmelt, a range of otherhydrologic variables can be affected, includingthe amount and timing of runoff, evapotranspiration,and soil moisture (Hamlet et al., 2007;Moore et al., 2007). Although gradual changesin snowcover and snowmelt timing could bethe rule, the transition from general winterlongsnowcover, to transient snowcover, tooccasional snow cover, could appear to be quiteabrupt, from the perspective of the hydrologyof individual watersheds.Most studies of past and modern impacts onwater resources focus on abrupt changes inthe physical system such as the duration of icecover and timing of snow melt, lake thermalstructure, evaporation, or water level, withconsiderably less attention on abrupt changesin water quality (e.g., Lettenmaier et al., 2008;CCSP SAP 4.3, Sec. 4.2.5). Assessing recentclimate impacts on water quality has beencomplicated by human land use. For example,analysis of contemporary data in the northernGreat Plains suggests that climate impacts aresmall relative to land use (Hall et al., 1999). Asimilar conclusion has been reached in Europebased on the paleoclimate literature, wherehumans have been impacting the environmentfor thousands of years (Hausmann et al.,2002). Some of the best evidence for climatechanges resulting in changes in water qualityand on aquatic biological communities comesfrom work in the Experimental Lakes Area inCanada where land use changes have been morelimited (Schindler, 1996a,b). This work showedhow climate changes affect ion concentration,nutrients, and dissolved organic carbon concentrations,often amplifying acidification andother external perturbations. Other evidencesuggests that climate warming might affectwater quality (phytoplankton biomass andnutrient concentrations) indirectly by affectinglake thermal structure (Lebo et al., 1994;Gerten and Adrian, 2000). The climate changesmay lead to abrupt changes in salinity and waterquality for drinking, irrigation, and livestock.The recent paleolimnological records of abruptchanges in salinity have been inferred fromchanges in diatoms in the sediments of MoonLake, ND (Laird et al., 1996), and the AralSea (Austin et al., 2007); however, determiningif the magnitude of these abrupt changesrepresents a significant degradation of waterquality is difficult to discern.8. ConclusionsDrought is among the greatest of recurringnatural hazards facing both the people of theUnited States and humanity worldwide todayand in the foreseeable future. Its causes arecomplex and not completely understood,but its impact on agriculture, water supply,natural ecosystems, and other human needsfor survival can be severe and long lasting inhuman terms, making it one of the most pressingscientific problems to study in the field ofclimatic change. Floods, though generally morelocalized in time and space than droughts, arealso a major natural hazard, and share withdroughts many of same large-scale controls andthe potential for experiencing major changes inthese controls in the future.Droughts can develop faster than the timescale needed for human societies and naturalsystems to adapt to the increase in aridity.Thus, a severe drought lasting several yearsmay be experienced as an abrupt change todrier conditions even though wetter conditionswill eventually return. The 1930s Dust Bowldrought, which resulted in a mass exodus fromthe parched Great Plains to more favorable areasin the West, is one such example. The droughteventually ended when the rains returned, butthe people did not. For them it was a truly abruptand permanent change in their lives. Thus,it is a major challenge of climate research tofind ways to help reduce the impact of futuredroughts through improved prediction and themore efficient use of the limited available waterresources.For examples of truly abrupt and long-lastingchanges in hydroclimatic variability overmidcontinental North America and elsewherein the world, we must go back in time to themiddle Holocene, when much larger changesin the climate system occurred. The climateChapter 3114