Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science ProgramChapter 3• There is no clear evidence to date of human-induced global climate change on North Americanprecipitation amounts. However, since the IPCC AR4 report, further analysis of climate modelscenarios of future hydroclimatic change over North America and the global subtropics indicates thatsubtropical aridity is likely to intensify and persist due to future greenhouse warming. This projecteddrying extends poleward into the United States Southwest, potentially increasing the likelihood ofsevere and persistent drought there in the future. If the model results are correct, then this dryingmay have already begun, but currently cannot be definitively identified amidst the considerable naturalvariability of hydroclimate in Southwestern North America.CHAPTER 3. RECOMMENDATIONS• Research is needed to improve existing capabilities to forecast short- and long-term droughtconditions and to make this information more useful and timely for decision making. In the future,drought forecasts should be based on an objective multimodel ensemble prediction system toenhance their reliability, and the types of information should be expanded to include soil moisture,runoff, and hydrological variables. (See also the Western Governors’ Association (2004) NationalIntegrated Drought Information System Report.)• The trend toward increasing subtropical aridity indicated by climate model projections needsto be investigated further to determine the degree to which it is likely to happen. If the modelprojections are correct, strategies for response to this pending aridity, on both regional andglobal scales, are urgently needed.• Improved understanding of the dynamical causes of long-term changes in oceanic conditions, theatmospheric responses to these ocean conditions, and the role of soil moisture feedbacks areneeded to advance drought prediction capabilities. Ensemble drought prediction is needed tomaximize forecast skill, and downscaling is needed to bring coarse-resolution drought forecastsfrom General Circulation Models down to the resolution of a watershed. (See also the NationalIntegrated Drought Information System Implementation Team, 2007.)• High-resolution paleoclimatic reconstructions of past drought have been fundamental to theevaluation of causes over North America in historic times and over the past millennium. Thisresearch should be expanded geographically to encompass as much of the global land masses aspossible for the development and testing of predictive models.• The record of past drought from tree rings and other proxies has revealed a succession ofmegadroughts prior to A.D. 1600 that easily eclipsed the duration of any droughts known to haveoccurred over North America since that time. Understanding the causes of these extraordinarymegadroughts is vitally important.• An understanding of the seasonality of drought and the relationships between winter and growingseason droughts during periods of megadroughts documented in paleoclimatic records is needed.In particular, knowledge about the North American monsoon and how its variability is linked toSSTs and winter precipitation variability over decadal and longer time scales in the SouthwesternUnited States and Northern Mexico is critical.• On longer time scales, significant land-cover changes have occurred in response to persistentdroughts, and the role of land-cover changes in amplifying or damping drought conditions shouldbe evaluated.• Improved understanding of the links among gradual changes in climate (e.g., Meridional OverturningCirculation, or MOC), the role of critical environmental thresholds, and abrupt hydrologic changesis needed to enhance society’s ability to plan and manage risks.• The relationship between climate changes and abrupt changes in water quality and biogeochemicalresponses is not well understood and needs to be a priority area of study for modern processand paleoclimate research.68