Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeboundary conditions responsible for thosechanges were quite different from those today,so the magnitude of change that we mightconceivably expect in the future under “natural”forcing of the climate system might not to be asgreat. However, the rising level of greenhousegas forcing that is occurring now and in theforeseeable future is truly unprecedented,even over the Holocene. Therefore, the abrupthydrologic changes in the Holocene ought tobe viewed as useful examples of the magnitudeof change that could conceivably occur in thefuture, and the mechanisms through which thatchange occurs.The need for improved drought prediction ontime scales of years to decades is clear now. Toaccomplish this will require that we developa much better understanding of the causesof hydroclimatic variability worldwide. It islikely that extended periods of anomaloustropical ocean SSTs, especially in the easternequatorial Pacific ENSO region, strongly influencethe development and duration of droughtover substantial land areas of the globe. Asthe IPCC AR4 concluded, “the paleoclimaticrecord suggests that multiyear, decadal andeven centennial-scale drier periods are likelyto remain a feature of future North Americanclimate, particularly in the area west of theMississippi River.” Multiple proxies indicatethe past 2,000 years included periods withmore frequent, longer and/or geographicallymore extensive droughts in North America thanduring the 20th century. However, the record ofpast drought from tree rings offers a soberingpicture of just how severe droughts can be undernatural climate conditions. Prior to A.D. 1600,a succession of megadroughts occurred thateasily eclipsed the duration of any droughtsknown to have occurred over North Americasince that time. Thus, understanding the causesof these extraordinary megadroughts is ofparamount importance. Increased solar forcingover the tropical Pacific has been implicated, ashas explosive volcanism, but the uncertaintiesremain large.However significant enhanced solar forcing hasbeen in producing past megadroughts, the levelof current and future radiative forcing due togreenhouse gases is very likely to be of muchgreater significance. It is thus disquieting toconsider the possibility that drought-inducingLa Niña-like conditions may become morefrequent and persistent in the future as greenhousewarming increases. We have no firmevidence that this is happening now, even withthe serious drought that has gripped the Westsince about 1998. Yet, a large number of climatemodels suggest that future subtropical dryingis a virtual certainty as the world warms and,if they are correct, indicate that it may havealready begun. The degree to which this is trueis another pressing scientific question that mustbe answered if we are to know how to respondand adapt to future changes in hydroclimaticvariability.115