EPA's Report on the Environment: External Review Draft
U.S. and Global Mean Temperature and Precipitation
Note to reviewers of this draft revised ROE: This indicator reflects data through 2012. EPA anticipates updating this indicator in 2014.
Air temperature and precipitation are two important properties of climate and can have wide-ranging effects on human well-being 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. Rainfall, snowfall, and the timing of snowmelt can all affect the amount of water available for drinking, irrigation, and industry. Temperature and precipitation patterns also determine the types of animals and plants (including crops) that can survive in particular locations. Changes in temperature and precipitation 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. As average temperatures at the Earth’s surface rise, more evaporation occurs, which, in turn, increases overall precipitation. Therefore, a warming climate is expected to increase precipitation in many areas. However, because climate change causes shifts in wind patterns and ocean currents that drive the world’s climate system, some areas are experiencing more warming than others, some might experience cooling, and precipitation patterns will vary across the world. In addition, because higher temperatures lead to more evaporation, increased precipitation will not necessarily increase the amount of water available for drinking, irrigation, and industry. Increased evaporation can also produce more intense precipitation events—for example, heavier rain and snow storms that can damage crops and increase flood risk—even if the total amount of precipitation in an area does not increase.
This indicator shows trends in temperature and precipitation based on instrumental records from 1901 to 2012 (except for Alaska and Hawaii, where records begin in 1918 and 1905, respectively). Air temperature and precipitation trends are summarized for the contiguous U.S., as well as for 11 climate regions of the U.S., including Alaska and Hawaii (these climate regions are different from the 10 EPA Regions). For context, this indicator shows trends in global temperature (over land and sea) and global precipitation (over land) from 1901 to 2012. For comparison purposes, this indicator also displays U.S. and global data from satellites that have measured the temperature of the Earth’s lower atmosphere since 1979.
This indicator shows annual anomalies, or differences, compared with the average temperature and precipitation from 1901 to 2000. For example, an anomaly of +2.0 degrees means the average temperature was 2 degrees higher than the 1901-2000 long-term average. Anomalies were initially calculated for each weather station. Daily temperature measurements at each site were used to calculate monthly anomalies, which then were averaged to find an annual temperature anomaly for each year. Precipitation calculations started with anomalies for total monthly precipitation, in millimeters; these monthly anomalies were added to get an annual anomaly for each year, which was then converted to a percent anomaly—i.e., the percent departure from the average annual precipitation during the baseline period. 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 Exhibits 1, 2, 3, 5, 6, and 7) or displaying them on a map (Exhibits 4 and 8). This method ensures that the results are not biased toward areas that may have many stations close together.
Long-term trends in temperature and precipitation were calculated from the annual data by linear regression (the straight line that fits the data best). For each of the 11 climate regions, this indicator also shows a smoothed time series, which was created using a nine-year weighted average.
What the Data Show
Since 1901, the average surface temperature across the contiguous 48 states has risen at an average rate of 0.14°F per decade (1.4°F per century) (Exhibit 1). Average temperatures have risen more quickly since the late 1970s (0.36 to 0.55°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.
Warming has occurred throughout the U.S., with seven of the 11 climate regions showing a statistically significant increase of more than 1°F since the start of the 20th century (Exhibit 3). The North, the West, and Alaska have seen temperatures increase the most, while some parts of the Southeast have experienced little change (Exhibits 3 and 4).
Trends in global temperature provide context for interpreting U.S. trends. Instrumental records from land stations and ships indicate that global mean surface temperature has risen at an average rate of 0.15°F per decade since 1901 (Exhibit 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. Worldwide, 2001–2010 was the warmest decade on record since thermometer-based observations began. Satellite measurements of the Earth’s lower atmosphere reveal temperature trends similar to those observed through ground-based monitoring (Exhibits 1 and 2).
As mean temperatures have risen, mean precipitation also has increased. This is expected because evaporation increases with increasing temperature, and there must be an increase in precipitation to balance the enhanced evaporation (IPCC, 2007). Since 1901, total annual precipitation has increased at an average rate of 5 percent per century over the contiguous U.S. (Exhibit 5), although there has been considerable regional variability (Exhibits 7 and 8). Of the 11 climate regions, four have experienced statistically significant increases in precipitation, while one (Alaska) experienced a significant decrease. Globally, precipitation over land has increased at a rate of 2.2 percent per century since 1901 (Exhibit 6).
- Biases may have occurred as a result of station relocations, development (e.g., urbanization) near the station, changes in instruments and times of measurement, and other changes. Where possible, data have been adjusted to account for changes in these variables.
- Data from the early 20th 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 still reliable.
Anomaly data were provided by the National Oceanic and Atmospheric Administration’s (NOAA’s) National Climatic Data Center (NCDC), which calculated global, U.S., and regional temperature and precipitation time series based on monthly values from a network of long-term monitoring stations. Data from individual stations were obtained from the U.S. Historical Climate Network and the Global Historical Climate Network, which are NCDC’s online databases (NOAA, 2013).
For More Information
- NOAA'S National Climatic Data Center
- This indicator also appears in EPA's Climate Change Indicators in the United States
- Learn how this indicator fits into a conceptual diagram for: Wetland Loss
- This indicator relates to: Greenhouse Gases, Physical and Chemical Attributes
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