Climate Change Indicators in the United States Drought

Climate Change Indicators in the United States: Drought
www.epa.gov/climate-indicators - Updated August 2016

Drought

This indicator measures drought conditions of U.S. lands.

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 that results in a serious water
shortage for some activity, population, or ecological system. Drought can also be thought of as an
extended imbalance between precipitation and evaporation.

As average temperatures have risen because of climate change, the Earth's water cycle has sped up
through an increase in the rate of evaporation. An increase in evaporation makes more water available
in the air for precipitation, but contributes to drying over some land areas, leaving less moisture in the
soil. Thus, as the climate continues to change, many 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. As a result, since the 1950s, some regions of the world have
experienced longer and more intense droughts, particularly in southern Europe and West Africa, while
other regions have seen droughts become less frequent, less intense, or shorter (for example, in central
North America).1

Drought conditions can negatively 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 and dried-up streams to major
water shortages. Prolonged droughts pose a particular threat to indigenous populations because of their
economic and cultural dependence on land and water supplies. Warming and drought can threaten
medicinal and culturally important plants and animals and can reduce water quality and availability,
making tribal populations particularly vulnerable to waterborne illnesses.2 Lower streamflow and
groundwater 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
precipitation, soil moisture, stream flow, vegetation health, and other variables.3 Figure 1 shows annual
values of the most widely used index, the Palmer Drought Severity Index, which is calculated from
precipitation and temperature measurements at weather stations. An index value of zero represents the
average moisture conditions observed between 1931 and 1990 at a given location. A positive value
means conditions are wetter than average, while a negative value is drier than average. Index values
from locations across the contiguous 48 states have been averaged together to produce the national
values shown in Figure 1.

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Climate Change Indicators in the United States: Drought
www.epa.gov/climate-indicators - Updated August 2016

For a more detailed perspective on recent trends, Figure 2 shows a newer index called the Drought
Monitor, which is based on several indices (including Palmer), along with additional factors such as snow
water content, groundwater levels, reservoir storage, pasture/range conditions, and other impacts. The
Drought Monitor uses codes from DO to D4 (see table below Figure 2) to classify drought severity. This
part of the indicator covers all 50 states and Puerto Rico.

• Average drought conditions across the nation have varied since records began in 1895. The
1930s and 1950s saw the most widespread droughts, while the last 50 years have generally
been wetter than average (see Figure 1).

• Over the period from 2000 through 2015, roughly 20 to 70 percent of the U.S. land area
experienced conditions that were at least abnormally dry at any given time (see Figure 2). The
years 2002-2003 and 2012-2013 had a relatively large area with at least abnormally dry
conditions, while 2001, 2005, and 2009-2011 had substantially less area experiencing drought.

• During the latter half of 2012, more than half of the U.S. land area was covered by moderate or
greater drought (see Figure 2). In several states, 2012 was among the driest years on record.4
See Temperature and Drought in the Southwest for a closer look at recent drought conditions in
one of the hardest-hit regions.

Key Points

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Climate Change Indicators in the United States: Drought

www.epa.gov/climate-indicators - Updated August 2016

Figure 1. Average Drought Conditions in the Contiguous 48 States, 1895-2015

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Climate Change Indicators in the United States: Drought

www.epa.gov/climate-indicators - Updated August 2016

Figure 2. U.S. Lands Under Drought Conditions, 2000-2015

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Climate Change Indicators in the United States: Drought
www.epa.gov/climate-indicators - Updated August 2016

Categories of Drought Severity

Category

Description

Possible Impacts

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.

Moderate drought

Some damage to crops or pastures; streams,
reservoirs, or wells low; some water shortages
developing or imminent; voluntary water use
restrictions requested.

Severe drought

Crop or pasture losses likely; water shortages
common; water restrictions imposed.

Extreme drought

Major crop/pasture losses; widespread water
shortages or restrictions.

Exceptional drought

Exceptional and widespread crop/pasture
losses; shortages of water in reservoirs,
streams, and wells, creating water emergencies.

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
website at: http://droughtmonitor.unl.edu.

Indicator Notes

Because this indicator focuses on national trends, it does not show how drought conditions vary by
region. For example, even if half of the country suffered from severe drought, Figure 1 could show an
average index value close to zero if the rest of the country was wetter than average. Thus, Figure 1
might understate the degree to which droughts are becoming more severe in some areas while other
places receive more rain as a result of climate change.

The U.S. Drought Monitor (Figure 2) offers a closer look at the percentage of the country that is affected
by drought. This index is relatively new, however, and thus too short-lived to be used for assessing long-
term climate trends or exploring how recent observations compare with historical patterns. With several
decades of data collection, future versions of this indicator should be able to paint a more complete
picture of trends over time.

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Climate Change Indicators in the United States: Drought
www.epa.gov/climate-indicators - Updated August 2016

Overall, this 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 Figure 1 were obtained from the National Oceanic and Atmospheric Administration's National
Centers for Environmental Information, which maintains a large collection of climate data online at:
www7.ncdc.noaa.gov/CDO/CDODivisionalSelect.isp. Data for Figure 2 were provided by the National
Drought Mitigation Center. Historical data in table form are available at:
http://droughtmonitor.unl.edu/MapsAndData.aspx.

1 IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis.
Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge
University Press, www.ipcc.ch/report/ar5/wgl.

2 Gamble, J.L, J. Balbus, M. Berger, K. Bouye, V. Campbell, K. Chief, K. Conlon, A. Crimmins, B. Flanagan, C.
Gonzalez-Maddux, E. Hallisey, S. Hutchins, L. Jantarasami, S. Khoury, M. Kiefer, J. Kolling, K. Lynn, A. Manangan,
M. McDonald, R. Morello-Frosch, M.H. Redsteer, P. Sheffield, K. Thigpen Tart, J. Watson, K.P. Whyte, and A.F.
Wolkin. 2016. Chapter 9: Populations of concern. The impacts of climate change on human health in the United
States: A scientific assessment. U.S. Global Change Research Program, https://health2016.globalchange.gov.

3 Heim, R.R. 2002. A review of twentieth-century drought indices used in the United States. B. Am. Meteorol. Soc.
83(8):1149-1165.

4 NOAA (National Oceanic and Atmospheric Administration). 2013. State of the climate: Drought: December 2012.
Accessed July 2013. www.ncdc.noaa.gov/sotc/drought/2012/12.

5 NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental
Information. Accessed January 2016. www7.ncdc.noaa.gov/CDO/CDODivisionalSelect.isp.

6 National Drought Mitigation Center. 2016. Maps and data. Accessed January 2016.
http://droughtmonitor.unl.edu/MapsAndData.aspx.

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