Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science ProgramChapter 3After the 1997–98El Niño, theWestern UnitedStates entered adrought that haspersisted until thetime of writing(July 2007).change the vegetation from forest to grasslandfor several millennia during the mid-Holocene(roughly 8,000 to 4,000 years ago). Thesechanges were driven primarily by variationsin the Earth’s orbit that altered the seasonaland latitudinal distribution of incoming solarradiation. Superimposed on these Holocenevariations were variations on centennial andshorter time scales that also were recordedby aeolian activity, and by geochemical andpaleolimnological indicators.The serious hydrological changes and impactsknown to have occurred in both historic andprehistoric times over North America reflectlarge-scale changes in the climate system thatcan develop in a matter of years and, in thecase of the more severe past megadroughts,persist for decades. Such hydrological changesfit the definition of abrupt change because theyoccur faster than the time scales needed forhuman and natural systems to adapt, leadingto substantial disruptions in those systems. Inthe Southwest, for example, the models projecta permanent drying by the mid-21st centurythat reaches the level ofaridity seen in historicaldroughts, and a quarter ofthe projections may reachthis level of aridity muchearlier. It is not unreasonableto think that, given thecomplexities involved, thestrategies to deal with decliningwater resources inthe region will take manyyears to develop and implement.If hardships are tobe minimized, it is time tobegin planning to deal withthe potential hydroclimaticchanges described here.2. Causes and Impacts ofHydrological VariabilityOver North America in theHistorical RecordAfter the 1997–98 El Niño, the Western UnitedStates entered a drought that has persisted untilthe time of writing (July 2007). The driestyears occurred during the extended La Niñaof 1998–2002. Although winter 2004–05 waswet, dry conditions returned afterwards andeven continued through the modest 2006–07 ElNiño. In spring 2007 the two massive reservoirson the Colorado River, Lakes Powell and Mead,were only half full. Droughts of this severityand longevity have occurred in the Westbefore, and Lake Mead (held back by HooverDam, which was completed in 1935) was justas low for a few years during the severe 1950sdrought in the Southwest. Studies of the instrumentalrecord make clear that western NorthAmerica is a region of strong meteorologicaland hydrological variability in which, amidstdramatic year-to-year variability, there areextended droughts and pluvials (wet periods)running from a few years to a decade. Thesedramatic swings of hydroclimatic variabilityhave tremendous impacts on water resources,agriculture, urban water supply, and terrestrialand aquatic ecosystems. Drought and its severitycan be numerically defined using indicesthat integrate temperature, precipitation, andother variables that affect evapotranspirationand soil moisture. See Heim (2002) for details.2.1 What Is Our Current Understandingof the Historical Record?Instrumental precipitation and temperature dataover North America only become extensivetoward the end of the 19th century. Records ofsea-surface temperatures (SSTs) are sufficientto reconstruct tropical and subtropical oceanconditions starting around A.D. 1856. The largespatial scales of SST variations (in contrast tothose of precipitation) allow statistical methodsto be used to “fill in” spatial and temporal gapsand provide near-global coverage from this time72