Drought

Global warming exacerbates drought through changes in both precipitation and temperature, and these changes can vary greatly by region.

One precipitation-related driver of drought is the concentration of the year’s precipitation into fewer but heavier downpours. This is occurring across the United States (NCA Chpt 2 Fig 2.16, p.50). Heavier downpours means that moisture is more likely to escape as runoff than be absorbed in the soil, a factor that contributes to drought.

Other precipitation trends that contribute to drought are dependent on latitude and current local conditions. In the very big picture, climate change is projected to cause dry areas to become drier, especially in the mid-latitudes and sub-tropics, while wet areas are projected to get wetter (IPCC AR5 WGI SPM, p.20). Consistent with this trend, the Western U.S., a historically dry area, is currently the driest in 800 years (Schwalm et al. 2012).

Global warming also raises local temperatures and drives more frequent and intense heat waves. This increases evaporation from the soil (Sherwood et al. 2014), and causes the early melt of snow pack in the spring. Melting snows typically slowly release water over the course of the spring and summer, providing a valuable water storage service (NCA Chpt 1, p.9). Early melt and high temperatures mean that by the hottest part of the summer, the water may be gone and drought conditions set in.

The different ways climate change drives drought can be observed across the United States. The Western U.S. has experienced drought worsened by loss of snow pack (NCA Chpt 1, p.9). In 2011, Texas experienced drought conditions worsened by record heat waves and overall high temperatures (Hansen et al. 2012; Rupp et al. 2012) that dry out soils. The extreme drought in California during 2013 was affected by all of these impacts: low precipitation, low snowpack, and high temperatures.