|Drought severity across the U.S. on July 22, 2014. Source: New York Times.|
Droughts are often through to be periods of abnormally low rain, but they are actually quite multifaceted with considerable geographic variability. There's more to the story than precipitation as the conditions that lead to low soil moisture are also affected by temperature and other weather, climate, and soil factors. Although there are many different ways to both define and determine the severity of drought, the most widely used index is the Palmer Drought Severity Index (PDSI), which attempts to account for a variety of factors that affect soil moisture.
Plots like the one above derive from the U.S. Drought Monitor, which blends a number of drought measures and expert judgement.
The Southwest is currently in the grips of widespread drought, with drought conditions rated as exceptional over portions of California and Nevada (the "more" region above). To the first order, this drought reflects the influence of climate variability. As concluded by Hoerling et al. (2013) in the Assessment of Climate Change in the Southwest United States:
"It is likely that most of the recent dryness over the Southwest is associated with natural, decadal coolness in tropical Pacific sea-surface temperatures, and is mostly unrelated to influences of increased greenhouse gases and aerosols."In other words, more persistent La Nina conditions have played an important role in the long-term drought conditions. This is not to say that global warming has had no influence on the drought. It is an exacerbating factor with higher temperatures, by contributing to soil drying, increasing in drought coverage and intensity.
Thus, we should be cautious in attributing the current drought to global warming. On the other hand, we also shouldn't assume that all is well and good in the coming century. The evidence suggests a decline in water resources over the Southwest over the long term. As discussed by Gershunov et al. (2013) in that same Assessment of Climate Change in the Southwest United States:
"Drought, as expressed in Colorado River flow, is projected to become more frequent, more intense, and longer lasting, resulting in water deficits not seen during the instrumented record (high confidence)"
"In terms of soil moisture, drought is expected to generally intensify in the dry season due to warming (high confidence)"