Climate Change Indicators in the United States: U.S. and Global Temperature
www.epa.gov/climatechange/indicators - Updated May 2014
U.S. and Global Temperature
This indicator describes trends in average surface temperature for the United States and the world.
Background
Temperature is a fundamental measurement for describing the climate, and the temperature in
particular places can have wide-ranging effects on human life and ecosystems. For example, increases in
air temperature can lead to more intense heat waves, which can cause illness and death, especially in
vulnerable populations. Annual and seasonal temperature patterns also determine the types of animals
and plants that can survive in particular locations. Changes in temperature can disrupt a wide range of
natural processes, particularly if these changes occur more quickly than plant and animal species can
adapt.
Concentrations of heat-trapping greenhouse gases are increasing in the Earth's atmosphere (see the
Atmospheric Concentrations of Greenhouse Gases indicator). In response, average temperatures at the
Earth's surface are rising and are expected to continue rising. However, because climate change can
shift the wind patterns and ocean currents that drive the world's climate system, some areas are
warming more than others, and some have experienced cooling.
About the Indicator
This indicator examines U.S. and global surface temperature patterns from 1901 to the present. U.S.
surface measurements come from weather stations on land, while global surface measurements also
incorporate observations from buoys and ships on the ocean, thereby providing data from sites
spanning much of the surface of the Earth. For comparison, this indicator also displays satellite
measurements that can be used to estimate the temperature of the Earth's lower atmosphere since
This indicator shows anomalies, which compare recorded annual temperature values against a long-
term average. For example, an anomaly of +2.0 degrees means the average temperature was 2 degrees
higher than the long-term average. This indicator uses the average temperature from 1901 to 2000 as a
baseline for comparison. Annual anomalies are calculated for each weather station, starting from daily
and monthly average temperatures. 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 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.
• Since 1901, the average surface temperature across the contiguous 48 states has risen at an
average rate of 0.14°F per decade (see Figure 1). Average temperatures have risen more quickly
1979.
Key Points
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Climate Change Indicators in the United States: U.S. and Global Temperature
www.epa.gov/climatechange/indicators - Updated May 2014
since the late 1970s (0.31 to 0.48°F per decade). Seven of the top 10 warmest years on record
for the contiguous 48 states have occurred since 1998, and 2012 was the warmest year on
record.
• Worldwide, 2001-2010 was the warmest decade on record since thermometer-based
observations began. Global average surface temperature has risen at an average rate of 0.15°F
per decade since 1901 (see Figure 2), similar to the rate of warming within the contiguous 48
states. Since the late 1970s, however, the United States has warmed faster than the global rate.
• Some parts of the United States have experienced more warming than others (see Figure 3). The
North, the West, and Alaska have seen temperatures increase the most, while some parts of the
Southeast have experienced little change. However, not all of these regional trends are
statistically significant.
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Figure 1. Temperatures in the Contiguous 48 States, 1901-2013
_ 3
Lower troposphere
Earth's surface (measured by satellite)
¦ ¦ UAH RSS
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1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
Year
This figure shows how annual average temperatures in the contiguous 48 states have changed since
1901. Surface data come from land-based weather stations. Satellite measurements cover the lower
troposphere, which is the lowest level of the Earth's atmosphere. "UAH" and "RSS" represent two
different methods of analyzing the original satellite measurements. 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, 20141
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Climate Change Indicators in the United States: U.S. and Global Temperature
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 2. Temperatures Worldwide, 1901-2013
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Earth's surface Lower troposphere
(land and ocean) {measured by satelite)
¦ ¦ UAH RSS
This figure shows how annual average temperatures worldwide have changed since 1901. Surface data
come from a combined set of land-based weather stations and sea surface temperature measurements.
Satellite measurements cover the lower troposphere, which is the lowest level of the Earth's atmosphere.
"UAH" and "RSS" represent two different methods of analyzing the original satellite measurements. 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, 20142
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Climate Change Indicators in the United States: U.S. and Global Temperature
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 3, Rate of Temperature Change in the United States, 1901-2012
Rate of temperature change (°F per century):
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i i i
-2-10 1 2
Gray Interval; -0.1 to 0.1 °F
This figure shows how annual average air temperatures have changed in different parts of the United
States since the early 20th century (since 1901 for the contiguous 48 states, 1905for Hawaiii, and 1918 for
Alaska).
Data source: NGAA, 20133
Indicator Notes
Data from the early 20ln century are somewhat less precise than more recent data because there were
fewer stations collecting measurements at the time, especially in the Southern Hemisphere. However,
the overall trends are stili reliable. Where possible, the data have been adjusted to account for any
biases that might be introduced by factors such as station moves, urbanization near the station, changes
in measuring instruments, and changes in the exact times at which measurements are taken.
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Climate Change Indicators in the United States: U.S. and Global Temperature
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 surface temperature anomalies shown here were calculated
based on monthly values from a network of long-term monitoring stations. Satellite data were analyzed
by two independent groups—the Global Hydrology and Climate Center at the University of Alabama in
Huntsville (UAH) and Remote Sensing Systems (RSS)—resulting in slightly different trend lines.
1 NOAA (National Oceanic and Atmospheric Administration). 2014. National Climatic Data Center. Accessed May
2014. www.ncdc.noaa.gov/oa/ncdc.html.
2 NOAA (National Oceanic and Atmospheric Administration). 2014. National Climatic Data Center. Accessed May
2014. 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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