Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science ProgramChapter 3A tree can provideinformation aboutpast climate in itsannual ring widthsbecause its growthrate is almost alwaysclimate-dependentto some degree.heralds a period of anthropogenic drying asopposed to the continuation of natural decadaland multidecadal variations.3. North American DroughtOver the Past MillenniaHistorical climate records provide considerableevidence for the past occurrence of exceptionalmultiyear droughts on the North Americancontinent and their impacts on American history.In addition, modeling experiments haveconclusively demonstrated the importance oflarge-scale tropical SSTs on forcing much ofthe observed hydroclimatic variability overNorth America and other global land areas.What is still missing from this narrative is abetter understanding of just how bad droughtscan become over North America. Is the 1930sDust Bowl drought the worst that can conceivablyoccur over North America? Or, is therethe potential for far more severe droughts todevelop in the future? Determining the potentialfor future droughts of unprecedented severitycan be investigated with climate models (Seageret al., 2007c), but the models still containtoo much uncertainty in them to serve as adefinitive guide. Rather, what we need is animproved understanding of the past occurrenceof drought and its natural range of variability.The instrumental and historical data only goback about 130 years with an acceptable degreeof spatial completeness over the United States(see the 19th century instrumental data maps inHerweijer et al., 2006), which does not provideus with enough time to characterize the fullrange of hydroclimatic variability that has occurredin the past and could conceivably occurin the future independent of any added effectsdue to greenhouse warming. To do so, we mustlook beyond the historical data to longer naturalarchives of past climate information.3.1 Tree Ring Reconstructions of PastDrought over North AmericaIn the context of how North American droughthas varied over the past 2,000 years, anespecially useful source of “proxy” climate informationis contained in the annual ring-widthpatterns of long-lived trees (Fritts, 1976). A treecan provide information about past climate in itsannual ring widths because its growth rate is almostalways climate-dependent to some degree.Consequently, the annual ring-width patterns oftrees provide proxy expressions of the actualclimate affecting tree growth in the past, andthese expressions can therefore be used toreconstruct past climate. The past 2,000 yearsis also particularly relevant here because theEarth’s climate boundary conditions are notmarkedly different from those of today, save forthe 20th century changes in atmospheric tracegas composition and aerosols that are thoughtto be responsible for recent observed warming.Consequently, a record of drought variabilityfrom tree rings in North America over thepast two millennia would provide a far morecomplete record of extremes for determininghow bad conditions could become in the future.Again, this assessment would be independent ofany added effects due to greenhouse warming.An excellent review of drought in the Centraland Western United States, based on tree ringsand other paleoproxy sources of hydroclimaticvariability, can be found in Woodhouse andOverpeck (1998). In that paper, the authorsintroduced the concept of the “megadrought,”a drought that has exceeded the intensity andduration of any droughts observed in the morerecent historical records. They noted that therewas evidence in the paleoclimate records forseveral multidecadal megadroughts prior to1600 that “eclipsed” the worst of the 20thcentury droughts including the Dust Bowl. Thereview by Woodhouse and Overpeck (1998) waslimited geographically and also restricted bythe lengths of tree-ring records of past droughtavailable for study. At that time, a gridded set ofsummer drought reconstructions, based on thePalmer Drought Severity Index (PDSI; Palmer,1965), was available for the conterminousUnited States, but only back to 1700 (Cook etal., 1999). Those data indicated that the DustBowl was the worst drought to have hit theUnited States over the past three centuries.However, a subset of the PDSI reconstructionsin the western, southeastern, and Great Lakesportions of the United States also extended backto 1500 or earlier. This enabled Stahle et al.(2000) to describe in more detail the temporaland spatial properties of the late 16th centurymegadrought noted earlier by Woodhouse andOverpeck (1998) and compare it to droughtsin the 20th century. In concurrence with thoseearlier findings, Stahle et al. (2000) showed86