Drought is a complex environmental impact and is affected strongly by the balance between precipitation and evapotranspiration and the concomitant effect on soil moisture. Global climate models predict increasing summer temperatures and decreasing summer precipitation in many continental areas, implying reductions in soil moisture. Long-term records of soil moisture are sparse, and the records that do exist do not show depletion of soil moisture, possibly due to reductions in solar radiation reaching the Earth's surface due to increased cloudiness (Robock et al., 2005). A surrogate indicator, derived from land-surface models, is the Palmer Drought Severity Index, which measures the duration and intensity of long-term drought-inducing patterns through thousands of data points such as rainfall, snowpack, stream flow, and other water supply indicators.1 The historical record of the Palmer Index from 1870 to 2002 shows that very dry areas have more than doubled globally since the 1970s, and the expansion after the 1980s is associated with surface warming (Dai et al., 2004). However, there are considerable year-to-year variations in soil moisture associated with the El Nino-Southern Oscillation and other modes of climate variability, and model projections of soil moisture for the 21st century do not provide a consistent indication of future changes (Trenberth et al., 2007). This uncertainty in future soil moisture projections leads to uncertainties about ecosystem dynamics and projections of agricultural productivity and, thus, presents a challenge for farmers, natural resource managers, and others trying to plan adaptation measures.

Attributing increases in severe droughts to human causes using observed data is difficult (e.g., Seager et al., 2009) and cannot currently be done unambiguously (Seager et al., 2007; Sun et al., 2007). For the United States, trend analyses indicate that droughts decreased in intensity, duration, and frequency over the period from 1915 to 2003, except in the Southwest (Andreadis and Lettenmaier, 2006; Sheffield and Wood, 2008). However, other analyses (Groisman and Knight, 2008) suggest increases in extended dry periods over the past 40 years. Model projections indicate that the area affected by drought will probably increase in the decades ahead (Bates and Kundzewicz, 2008) and that the number of dry days annually will also increase (Kundzewicz et al., 2007). In snowmelt-dominated systems, the risk of drought is expected to increase (Barnett et al., 2005b).

1 For an overview of the Palmer Drought Severity Index and limitations on its use, see Alley (1984).

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