Drought in The United States - Causes

Causes

See also: Drought, Precipitation (meteorology), and Rain See also: United States rainfall climatology

Generally, rainfall is related to the amount of water vapour in the Earth's atmosphere, combined with the upward forcing of the air mass containing that water vapor. If either moisture, lift, or atmospheric instability are reduced, rain will be more scant which in time will lead to drought. This can be triggered by an above average prevalence of high pressure systems, winds carrying continental, rather than oceanic air masses (i.e. reduced water content), and ridges of high pressure areas form with behaviors which prevent or restrict the developing of thunderstorm activity or rainfall over one certain region. Oceanic and atmospheric weather cycles such as ENSO make drought a regular recurring feature of the Americas along the Pacific coast. Droughts can develop anywhere in the United States. El Nino and La Nina alters rainfall patterns in the West and along the Gulf coast. The North Atlantic Oscillation, Atlantic Multidecadal Oscillation (AMO), and Pacific Decadal Oscillation (PDO) are also significantly tied to droughts in the United States. For example, 52 percent of drought timing and location across the United States is explained by the PDO and AMO. When both in their positive phase, the most extensive droughts occur across the United States.

Human activity can directly trigger exacerbating factors such as over farming, excessive irrigation, deforestation, and erosion adversely impact the ability of the land to capture and hold water. While these tend to be relatively isolated in their scope, activities resulting in global climate change are expected to trigger droughts with a substantial impact on agriculture throughout the world, and especially in developing nations. Although global warming will result in increased world rainfall, warmer temperatures also bring increased evaporation and increased drought frequency. Along with drought in some areas, flooding and erosion will increase in others. Some proposed solutions to global warming that focus on more active techniques, solar radiation management through the use of a space sunshade for one, may also carry with them increased chances of drought.