Climate Change Indicators in the United States: U.S. and Global Precipitation
www.epa.gov/climatechange/indicators - Updated May 2014
U.S. and Global Precipitation
This indicator describes trends in average precipitation for the United States and the world.
Background
Precipitation can have wide-ranging effects on human well-being and ecosystems. Rainfall, snowfall, and
the timing of snowmelt can all affect the amount of water available for drinking, irrigation, and industry,
and can also determine what types of animals and plants (including crops) can survive in a particular
place. Changes in precipitation can disrupt a wide range of natural processes, particularly if these
changes occur more quickly than plant and animal species can adapt.
As average temperatures at the Earth's surface rise (see the U.S. and Global Temperature indicator),
more evaporation occurs, which, in turn, increases overall precipitation. Therefore, a warming climate is
expected to increase precipitation in many areas. However, just as precipitation patterns vary across the
world, so will the precipitation effects of climate change. By shifting the wind patterns and ocean
currents that drive the world's climate system, climate change will also cause some areas to experience
decreased precipitation. In addition, higher temperatures lead to more evaporation, so increased
precipitation will not necessarily increase the amount of water available for drinking, irrigation, and
industry (see the Drought indicator).
About the Indicator
This indicator examines U.S. and global precipitation patterns from 1901 to the present, based on
rainfall and snowfall measurements from land-based weather stations worldwide.
This indicator shows annual anomalies, or differences, compared with the average precipitation from
1901 to 2000. These anomalies are presented in terms of percent change compared with the baseline.
Annual anomalies are calculated for each weather station. Anomalies for broader regions have been
determined by dividing the country (or the world) into a grid, averaging the data for all weather stations
within each cell of the grid, and then averaging the grid cells together (for Figures 1 and 2) or displaying
them on a map (Figure 3). This method ensures that the results are not biased toward regions that
happen to have many stations close together.
• On average, total annual precipitation has increased over land areas in the United States and
worldwide (see Figures 1 and 2). Since 1901, global precipitation has increased at an average
rate of 0.2 percent per decade, while precipitation in the contiguous 48 states has increased at a
rate of 0.5 percent per decade.
• Some parts of the United States have experienced greater increases in precipitation than others.
A few areas such as Hawaii and parts of the Southwest have seen a decrease in precipitation
(see Figure 3).
Key Points
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d% Climate Change Indicators in the United States: U.S. and Global Precipitation
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 1. Precipitation in the Contiguous 48 States, 1901-2012
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d% Climate Change Indicators in the United States: U.S. and Global Precipitation
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 2. Precipitation Worldwide, 1901-2012
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This figure shows how the total annual amount of precipitation over land worldwide has changed since
1901. This graph uses the 1901-2000 average as a baseline for depicting change. Choosing a different
baseline period would not change the shape of the data over time.
Data source: NOAA, 20132
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Climate Change Indicators in the United States: U.S. and Global Precipitation
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 3. Rate of Precipitation Change in the United States, 1901-2012
Rate of change In precipitation (% per century):
t—1—I—T
-60-50-40-10-20 -10 0 10 20 30 40 50 60
Gray Interval: -2 to 2%
This figure shows the rate of change in total annual precipitation in different parts of the United States
since the early 20th century (since 1901 for the contiguous 48 states, 1905 for Hawaii, and 1918 for
Alaska).
Data source: NQAA, 20133
Indicator Notes
Data from the early 20tn century are somewhat less precise because there were fewer stations collecting
measurements at the time. To ensure that overall trends are reliable, the data have been adjusted
where possible to account for any biases that might be introduced by factors such as station moves or
changes in measurement instruments.
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Climate Change Indicators in the United States: U.S. and Global Precipitation
www.epa.gov/climatechange/indicators - Updated May 2014
Data Sources
The data for this indicator were provided by the National Oceanic and Atmospheric Administration's
National Climatic Data Center, which maintains a large collection of climate data online at:
www.ncdc.noaa.gov/oa/ncdc.html. The precipitation anomalies shown here were calculated based on
monthly values from a network of long-term monitoring stations.
1 NOAA (National Oceanic and Atmospheric Administration). 2013. National Climatic Data Center. Accessed April
2013. www.ncdc.noaa.gov/oa/ncdc.html.
2 NOAA (National Oceanic and Atmospheric Administration). 2013. National Climatic Data Center. Accessed April
2013. www.ncdc.noaa.gov/oa/ncdc.html.
3 NOAA (National Oceanic and Atmospheric Administration). 2013. National Climatic Data Center. Accessed April
2013. www.ncdc.noaa.gov/oa/ncdc.html.
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