Climate Change Indicators in the United States A Closer Look Temperature and Drought in the Southwest

Climate Change Indicators in the United States: A Closer Look: Temperature and
Drought in the Southwest - www.epa.gov/climate-indicators - Updated August 2016

A Closer Look: Temperature and Drought in the Southwest

The American Southwest might evoke images of a hot, dry landscape—a land of rock, canyons, and
deserts baked by the sun. Indeed, much of this region has low annual rainfall and seasonally high
temperatures that contribute to its characteristic desert climate. Yet this landscape actually supports a
vast array of plants and animals, along with millions of people who call the Southwest home. All of these
plants, animals, and people need water to survive.

Water is already scarce in the Southwest, so every drop is a precious resource. People in the Southwest
are particularly dependent on surface water supplies like Lake Mead, which are vulnerable to
evaporation. Thus, even a small increase in temperature (which drives evaporation) or a decrease in
precipitation in this already arid region can seriously threaten natural systems and society. Droughts
also contribute to increased pest outbreaks and wildfires, both of which damage local economies, and
they reduce the amount of water available for generating electricity—for example, at the Hoover Dam.1

While two indicators in this report present information about unusually high or low temperatures and
drought on a national scale (see the High and Low Temperatures indicator and the Drought indicator),
this feature highlights the Southwest because of its particular sensitivity to temperature and drought.
Parts of the Southwest are also experiencing long-term reductions in mountain snowpack (see the
Snowpack indicator), which accounts for a large portion of the region's water supply. This feature
focuses on six states that are commonly thought of as "southwestern" and characterized at least in part
by arid landscapes and scarce water supplies: Arizona, California, Colorado, Nevada, New Mexico, and
Utah. Temperature and drought data come from a network of thousands of weather stations overseen
by the National Weather Service.

The map in Figure 1 shows how average annual temperatures in the Southwest from 2000 to 2015
differed from the average over the entire period since widespread temperature records became
available (1895-2015).

Figures 2 and 3 show two ways of measuring drought in the Southwest: the Drought Monitor and the
Palmer Drought Severity Index. The Palmer Index is calculated from precipitation and temperature
measurements at weather stations, and has been used widely for many years. The Drought Monitor is a
more recent and more detailed index based on several other indices (including Palmer), along with
additional factors such as snow water content, groundwater levels, reservoir storage, pasture/range
conditions, and other impacts. See the Drought indicator for more information about these indices.

• Every part of the Southwest experienced higher average temperatures between 2000 and 2015
than the long-term average (1895-2015). Some areas were nearly 2°F warmer than average (see

Key Points

Figure 1).

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Climate Change Indicators in the United States: A Closer Look: Temperature and
Drought in the Southwest - www.epa.gov/climate-indicators - Updated August 2016

•	Large portions of the Southwest have experienced drought conditions since weekly Drought
Monitor records began in 2000. For extended periods from 2002 to 2005 and from 2012 through
2015, nearly the entire region was abnormally dry or even drier (see Figure 2).

•	Based on the long-term Palmer Index, drought conditions in the Southwest have varied since
1895. The early 1900s and the 1950s experienced considerable drought, the 1980s were
relatively wet, and the last decade has seen the most persistent droughts on record (see Figure
3).

Figure 1. Average Temperatures in the Southwestern United States, 2000-2015 Versus Long-
Term Average

0.8	1	1.2	1.4	1.6	1.8	2

This map shows how the average air temperature from 2000 to 2015 has differed from the long-term
average (1895-2015), To provide more detailed information, each state has been divided into climate
divisions, which are zones that share similar climate features.

Data source: NOAA, 2Q162

&EPA

2

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Climate Change Indicators in the United States: A Closer Look: Temperature and
Drought in the Southwest - www.epa.gov/climate-indicators - Updated August 2016

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

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jfHk Climate Change Indicators in the United States: A Closer Look: Temperature and

Drought in the Southwest - www.epa.gov/climate-indicators - Updated August 2016

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020

Year

This chart shows annual values of the Palmer Drought Severity Index, averaged over six states in the
Southwest (Arizona, California, Colorado, Nevada, New Mexico, and Utah). Positive values represent
wetter-than-average conditions, while negative values represent drier-than-average conditions. A value
between -2 and -3 indicates moderate drought', -3 to -4 is severe drought, and -4 or below indicates
extreme drought. The thicker line is a nine-year weighted average.

Data source: NOAA, 2Q164

Notes

Natural variability, changes in irrigation practices, and other diversions of water for human use can
influence certain drought-related measurements. Soil moisture, ground water, and streamflow are part
of Drought Monitor calculations (Figure 2), and they are all sensitive to human activities.

Data Sources

Data for Figures 1 and 3 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: www.ncei.noaa.gov. Data for Figure 2 were provided by the National Drought Mitigation
Center, which gives historical data in table form at: http://droughtmonitor.unl.edu/MapsAndData.aspx.

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Climate Change Indicators in the United States: A Closer Look: Temperature and
Drought in the Southwest - www.epa.gov/climate-indicators - Updated August 2016

1 MacDonald, G.M. 2010. Water, climate change, and sustainability in the Southwest. P. Natl. Acad. Sci. USA
107(50):21256-21262.

2 NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental
Information. Accessed January 2016. www.ncdc.noaa.gov.

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

4 NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers for Environmental
Information. Accessed January 2016. www.ncdc.noaa.gov.

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