Book 2.indb - US Climate Change Science Program

3CHAPTERHydrological Variability and ChangeLead Author: Edward R. Cook,* Lamont-Doherty EarthObservatory, Columbia UniversityAbrupt Climate ChangeContributing Authors: Patrick j. Bartlein,* University of OregonNoah Diffenbaugh, Purdue UniversityRichard Seager,* Lamont-Doherty Earth Observatory, ColumbiaUniversityBryan N. Shuman, University of WyomingRobert S. Webb,* NOAA Earth System Research Laboratoryjohn W. Williams, University of WisconsinConnie Woodhouse, University of Arizona* SAP 3.4 Federal Advisory Committee memberkEy FINDINGS• Protracted droughts, and their impacts on agricultural production and water supplies, are among thegreatest natural hazards facing the United States and the globe today and in the foreseeable future.• Floods predominantly reflect both antecedent conditions and meteorological events and are often morelocalized relative to drought in both time and space. On subcontinental-to-continental scales, droughtsoccur more frequently than floods and can persist for decades and even centuries.• On interannual to decadal time scales, droughts can develop faster than the time scale needed for humansocieties to adapt to the change. Thus, a severe drought lasting several years can be regarded as an abruptchange, although it may not reflect a permanent change of state of the climate system.• Droughts and episodes of regional-scale flooding can both be linked to the large-scale atmosphericcirculation patterns over North America, and often occur simultaneously in different parts of the country,compounding their impact on human activities.• Empirical studies and climate model experiments conclusively show that droughts over North Americahave been significantly influenced by the state of tropical sea-surface temperatures (SSTs). Of particularrelevance to North America, cool La Niña-like SSTs in the eastern equatorial Pacific frequently causedevelopment of droughts over the Southwestern United States and Northern Mexico. Warm subtropicalNorth Atlantic SSTs play a secondary role in forcing drought in southwestern North America.• Historic droughts over North America have been severe, the “Dust Bowl” drought of the 1930s beingthe canonical example, but those droughts were not nearly as prolonged as a series of “megadroughts”reconstructed from tree rings since Medieval times (ca. 1,000 years ago) up to about A.D. 1600. Modelingexperiments indicate that these megadroughts were likely partly forced by cool SSTs in the easternequatorial Pacific as well. However, their exceptional duration has not been adequately explained nor hasany involvement in forcing from SST changes in other oceans.• These megadroughts are significant because they occurred in a climate system that was not being perturbedin a major way by human activity (i.e., the ongoing anthropogenic changes in greenhouse gas concentrations,atmospheric dust loadings, and land-cover changes).• Even larger and more persistent changes in hydroclimatic variability worldwide are indicated throughout theHolocene (the past 11,500 years) by a diverse set of paleoclimatic indicators including some with annualto-decadalresolution (e.g., speleothems, varved-lake records, high-resolution lake-sediment records). Theglobal-scale controls associated with those changes were quite different from those of the past millenniumand today, but they show the additional range of natural variability and abrupt hydroclimatic change thatcan be expressed by the climate system, including widespread and protracted (multi-century) droughts.67