Climate Change
Climate Change Indicators in the United States
This chart shows the percentage of U.S. lands classified under drought conditions from 2000 through 2009. The data cover all 50 states plus Puerto Rico.
Data source: National Drought Mitigation Center, 2010 4
| Categories of Drought Severity | ||
|---|---|---|
| Category | Description | Possible Impacts |
| D0 | Abnormally dry | Going into drought: short-term dryness slowing planting or growth of crops or pastures. Coming out of drought: some lingering water deficits; pastures or crops not fully recovered. |
| D1 | Moderate drought | Some damage to crops or pastures; streams, reservoirs, or wells low; some water shortages developing or imminent; voluntary water use restrictions requested. |
| D2 | Severe drought | Crop or pasture losses likely; water shortages common; water restrictions imposed. |
| D3 | Extreme drought | Major crop/pasture losses; widespread water shortages or restrictions. |
| D4 | Exceptional drought | Exceptional and widespread crop/pasture losses; shortages of water in reservoirs, streams, and wells, creating water emergencies. |
Key Points
- Because data from the U.S. Drought Monitor are only available for the most recent decade, there is no clear long-term trend in this indicator. With continued data collection, future versions of this indicator should be able to paint a more complete picture of long-term trends in drought.
- Over the period from 2000 through 2009, roughly 30 to 60 percent of the U.S. land area experienced drought conditions at any given time (see Figure 1). The years 2002, 2003, and 2007 were relatively high drought years, while 2001, 2005, and 2009 were relatively low drought years.
- "Abnormally dry area" (D0)—the mildest drought event—was the most commonly occurring level of drought in the United States between 2000 and 2009.
- As of early 2010, moderate to severe drought is affecting parts of several western states, along with a small portion of the Upper Midwest. 3
Background
There are many definitions and types of drought. Meteorologists generally define drought as a prolonged period of dry weather caused by a lack of precipitation, which results in a serious water shortage for some activity, group, or ecological system. Drought can also be thought of as an imbalance between precipitation and evaporation.
As average temperatures rise because of climate change, the Earth's water cycle is expected to speed up, increasing evaporation. Increased evaporation will make more water available in the air for precipitation, but contribute to drying over some land areas. As a result, storm-affected areas are likely to experience increased precipitation (see the U.S. and Global Precipitation indicator) and increased risk of flooding (see the Heavy Precipitation indicator), while areas located far from storm tracks are likely to experience less precipitation and increased risk of drought. Since the 1970s, drought-affected areas have increased on a global scale—more likely than not as a result of climate change caused by human activities. 1
Drought conditions can affect agriculture, water supplies, energy production, and many other aspects of society. The impacts vary depending on the type, location, intensity, and duration of the drought. For example, effects on agriculture can range from slowed plant growth to severe crop losses, while water supply impacts can range from lowered reservoir levels to major water shortages. Lower stream flow and ground water levels can also harm plants and animals, and dried-out vegetation increases the risk of wildfires.
About the Indicator
During the 20th century, many indices were created to measure drought severity by looking at trends in precipitation, soil moisture, stream flow, vegetation health, and other variables. 2 This indicator is based on the U.S. Drought Monitor, which integrates several of these indices. The Drought Monitor also considers additional factors such as snow water content, ground water levels, reservoir storage, pasture/range conditions, and other impacts.
The Drought Monitor uses codes from D0 to D4 (see table below Figure 1) to classify drought severity. This indicator measures the percent of U.S. land under each of these drought categories from 2000 through 2009. The indicator covers all 50 states and Puerto Rico.
Experts update the U.S. Drought Monitor weekly and produce maps that illustrate current conditions as well as short- and long-term trends. Major participants include the National Oceanic and Atmospheric Administration, the U.S. Department of Agriculture, and the National Drought Mitigation Center.
For a map of current drought conditions, visit the Drought Monitor Web site at: www.drought.unl.edu/dm/ monitor.html.
Indicator Confidence
Because of the relative newness of the U.S. Drought Monitor, it cannot be used to assess long-term trends. Other indicators are available that do show historical trends, but they have other weaknesses and cannot be compared across geographic regions or across time. 5
The drought classification scheme used for this indicator is produced by combining data from several different sources. These data are combined to reflect the collective judgment of experts and in some cases are adjusted to reconcile conflicting trends shown by different data sources over different time periods.
The indicator gives a broad overview of drought conditions in the United States. It is not intended to replace local or state information that might describe conditions more precisely for a particular region.
Data Sources
Data for this indicator were provided by the U.S. Drought Monitor. Historical data in table form are available at: http://droughtmonitor.unl.edu/
archive.html. Maps and current drought information can be found on the main Drought Monitor site at: http://droughtmonitor.unl.edu.
Indicator Documentation
- Download related technical information PDF (6 pp, 45K)