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Climate Change Indicators

Understanding the Connections Between Climate Change and Human Health

Access USGCRP's Climate and Health Assessment Online

The U.S. Global Change Research Program (USGCRP) report, The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment, was published in April 2016.

This comprehensive report captures the state of scientific knowledge about observed and projected impacts of climate change on human health in the United States.

The report is available

			online at: https://health2016.globalchange.gov.

Climate change poses many threats to the health and well-being of Americans, from increasing the risk of extreme heat events and heavy storms to increasing the risk of asthma attacks and changing the spread of certain diseases carried by ticks and mosquitoes. Some of these health impacts are already happening in the United States.

Scientists' understanding of the multiple ways that climate change increases risks to human health has advanced significantly in recent years. This section highlights key concepts from one of the latest climate and health assessments (see the box) to help illustrate how climate change can affect human health in the United States. In addition, this section demonstrates how EPAâ€™s climate change indicators advance the dialogue in connecting climate change and human health.

How Does Climate Change Affect Human Health?

Climate change can exacerbate existing health threats or create new public health challenges through a variety of pathways. Figure 1 summarizes these connections by linking climate impacts to changes in exposure, which can then lead to negative effects on health (health outcomes). This figure also shows how other factorsâ€”such as where people live and their age, health, income, or ability to access health care resourcesâ€”can positively or negatively influence peopleâ€™s vulnerability to human health effects. For example, a familyâ€™s income, the quality of their housing, or their communityâ€™s emergency management plan can all affect that familyâ€™s exposure to extreme heat, the degree to which their health is affected by this threat, and their ability to adapt to impacts of extreme heat (for more examples, see Figure 4).

Figure 1. Climate Change and Health Pathway

Figure 1 shows how climate change can affect people by changing their exposure to health threats (moving from top to bottom) and by influencing the environmental, institutional, social, and behavioral factors that affect a personâ€™s or communityâ€™s health (moving through the boxes on the sides).

What can Indicators Tell Us About Climate Change and Human Health?

As shown in Figure 1, the impacts of climate change on health are complex, often indirect, and dependent on multiple societal and environmental factors. Tracking changes in climate impacts and exposures improves understanding of changes in health risk, however, even if the actual health outcome is difficult to quantify. For example, the flooding pathway in Figure 2 shows how indicators of certain climate impacts like Sea Level, Heavy Precipitation, and Coastal Flooding could be used by state and local health officials to better understand changes in human exposure to contaminated waters (a health risk). By recognizing changing risks, these officials can better understand how climate change affects the number of people who get sick with gastrointestinal illnesses (a health outcome). Thus, even where health data or long-term records are unavailable or where the links between climate and health outcomes are complex, indicators play an important role in understanding climate-related health impacts.

Figure 2. Connecting Climate Change Indicators to Health Pathways

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The three examples above show how climate impacts can affect health. The numbered circles identify where climate change indicators provide key information on changes occurring at different points along the pathways. Other factors can play a role in determining a personâ€™s vulnerability to climate-related health outcomes;Â see Figure 1 and Figure 4.

Whoâ€™s at Risk?

The effects of climate change also affect peopleâ€™s mental health.Â In particular, climate- or weather-related disasters can increase the risk of adverse mental health consequences, especially if they result in damage to homes and livelihoods or loss of loved ones. The mental health impacts of these events can range from minimal stress and distress symptoms to clinical disorders, such as anxiety, depression, and post-traumatic stress.

Every American faces a risk of health impacts associated with climate change. Some people, however, face higher risks than others because of differences in the hazards to which they are exposed, their sensitivity to these hazards, and their ability to adapt (see Figure 3). Thus, it is important to be able to identify â€œpopulations of concern,â€? which include groups representing people of all ages, living in different places, who interact with their environment in different ways. Figure 4 shows some examples of how certain populations are more vulnerable to health impacts because of differences in their exposure, sensitivity, or ability to adapt to climate-related stresses.

It is important to remember that the different health impacts identified here do not occur in isolation; people can face multiple threats at the same time, at different stages in their lives, or accumulating over the course of their lives. Risks may increase as people are exposed to multiple health threats. For example, extremely hot days can lead to heat-related illness as well as poor air quality, by increasing the chemical reactions that produce smog. In addition, many of the factors that influence whether a person is exposed to health threats or whether they become ill, such as an individualâ€™s personal habits, living conditions, and access to medical care (see Figure 1), can also change over time.

Figure 3. Determinants of Vulnerability

Figure 4. Examples of Climate Change Vulnerability

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Additional Resources

Understanding the threats that climate change poses to human health can help people and communities work together to lower risks and be prepared. Climate change threatens human health, including mental health, as well as access to clean air, safe drinking water, nutritious food, and shelter.

The following EPA resources on how climate change affects your health can be found on theÂ Climate Change Impacts site:

Eight fact sheets on populations shown in Figure 4 that are particularly vulnerable to the health impacts of climate change.

A clickable map with examples of state impacts and resources to help individuals and communities prepare and respond to climate threats.

An online 10-question quiz to challenge you and your friends on climate and health knowledge.

A fact sheet highlighting health impacts of climate change at different stages of life (PDF, 1 pp, 1 MB), from infancy through adolescence to adulthood.

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Climate Change Indicators: Wildfires

This indicator tracks the frequency, extent, and severity of wildfires in the United States.

Figure 1. Wildfire Frequency in the United States, 1983â€“2015

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This figure shows the total number of wildfires per year from 1983 to 2015. These totals include all reported wildfires, which can be as small as just a few acres. The two lines represent two different reporting systems; though the Forest Service stopped collecting statistics (orange line) in 1997 and will not update them, those statistics are shown here for comparison.

Data source: NIFC, 2016;13 USDA Forest Service, 201414
Web update: AprilÂ 2016

Figure 2. Wildfire Extent in the United States, 1983â€“2015

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This figure shows annual wildfire-burned area (in millions of acres) from 1983 to 2015. The two lines represent two different reporting systems; though the Forest Service stopped collecting statistics (orange line) in 1997 and is not planning to update them, those statistics are shown here for comparison.

Data source: NIFC, 2016;15 Short, 201516
Web update: AugustÂ 2016

Figure 3. Damage Caused by Wildfires in the United States, 1984â€“2014

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This figure shows the distribution of acreage burned by large wildfires, based on the level of damage caused to the landscapeâ€“a measure of wildfire severity. Large wildfires are defined as fires with an area larger than 1,000 acres in the western United States and 500 acres in the eastern United States. The total acreage shown in Figure 3 is slightly less than the total in Figure 2 because Figure 3 is limited to large fires and because a few areas did not have sufficient satellite imagery to allow damage to be assessed.

Data source: MTBS, 201617
Web update: AugustÂ 2016

Interactive Map

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This map shows the average number of acres burned in each state per year as a proportion of that stateâ€™s total land area. Darker-shaded states have the largest proportion of acreage burned. Click each state to reveal the data. For reference, there are 640 acres in a square mile; therefore, an average burned area of 6.4 acres per square mile would mean that fires burned 1 percent of a stateâ€™s total land area. A few states did not have any fires that were large enough to be included in this analysis.

Data source: MTBS, 201618
Web update: AugustÂ 2016

Interactive Map

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This map shows how the number of acres burned in each state as a proportion of that stateâ€™s total land area has changed over time, based on a simple comparison between the first half of the available years (1984â€“1999) and the second half (2000â€“2014). Click each state to reveal the data. For reference, there are 640 acres in a square mile; therefore, a change of 6.4 acres per square mile would mean that burned area increased by 1 percent of a stateâ€™s total land area. A few states did not have any fires that were large enough to be included in this analysis.

Data source: MTBS, 201619
Web update: AugustÂ 2016

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Figure 1. Wildfire Frequency in the United States, 1983â€“2015

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Figure 2. Wildfire Extent in the United States, 1983â€“2015

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Figure 3. Damage Caused by Wildfires in the United States, 1984â€“2014

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Figure 4. Average Annual Burned Acreage by State, 1984â€“2014

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Figure 5. Change in Annual Burned Acreage by State between 1984â€“1999 and 2000â€“2014

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			Key Points

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			Background

About the

			Indicator

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			Indicator Notes

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			Data Sources

Technical

			Documentation

Key Points

Since 1983, the National Interagency Fire Center has documented an average of 72,000 wildfires per year (see Figure 1). Compiled data from the Forest Service suggest that the actual total may be even higher for the first few years of nationwide data collection that can be compared. The data do not show an obvious trend during this time.

The extent of area burned byÂ wildfires each yearÂ appears to have increased since the 1980s. According to National Interagency Fire Center data, of the 10 years with the largest acreage burned, nine have occurred since 2000, including the peak year in 2015 (see Figure 2). This period coincides with many of the warmest years on record nationwide (see the U.S. and Global Temperature indicator).

The late 1990s were a period of transition in certain climate cycles that tend to shift every few decades.10Â This shiftâ€”combined with other ongoing changes in temperature, drought, and snowmeltâ€”may have contributed to warmer, drier conditions that have fueled wildfires in parts of the western United States.11,12

Of the total area burned each year from 1984 to 2014, the proportion of burned land suffering severe damage has ranged from 5 to 21 percent (see Figure 3).

Land area burned by wildfires varies by state. Fires burn more land in the western United States than in the East, and parts of the West and Southwest show the largest increase in burned acreage between the first half of the record (1984â€“1999) and the second half (2000â€“2014) (see Figures 4 and 5).

Background

Together, forests, shrubland, and grassland cover more than half of the land area in the United States.1 These ecosystems are important resources, both environmentally and economically. Although wildfires occur naturally and play a long-term role in the health of these ecosystems, climate change threatens to increase the frequency, extent, and severity of fires through increased temperatures and drought (see the U.S. and Global Temperature and Drought indicators). Earlier spring melting and reduced snowpack (see the Snowpack indicator) result in decreased water availability during hot summer conditions, which in turn contributes to an increased wildfire risk, allowing fires to start more easily and burn hotter. An increase in the length of the fire season has been observed in some areas.2 In addition to climate change, other factorsâ€”like the spread of insects, land use, fuel availability, and management practices, including fire suppressionâ€”play an important role in wildfire frequency and intensity. All of these factors influencing wildfires vary greatly by region and over time, as do variations in precipitation, wind, temperature, vegetation types, and landscape conditions. Therefore, understanding changes in fire characteristics requires long-term records, a regional perspective, and consideration of many factors.3

Wildfires have the potential to harm property, livelihoods, and human health, particularly as population centers expand into wild land areas. The recreation and timber industries depend on healthy forests, and wildfire smoke has been directly linked to poor air quality and illness, even in communities far downwind.4,5 Fire-related threats are increasing, especially as more people live in and around forests, grasslands, and other natural areas.6 The United States spends more than $1 billion every year to fight wildfires and spent more than $2 billion in 2015.7 These efforts have resulted in the deaths of hundreds of firefighters since 1910.8

Beyond the human impact, wildfires also affect the Earthâ€™s climate. Forests in particular store large amounts of carbon. When they burn, they release carbon dioxide into the atmosphere, which in turn contributes to climate change.

About the Indicator

This indicator defines wildfires as â€œunplanned, unwanted wildland fire[s]â€? in forests, shrubland, and grassland, where â€œthe objective is to put the fire out.â€?9Â This indicator tracks three aspects of wildfires over time: the total number of fires (frequency), the total land area burned (extent), and the degree of damage that fires cause to the landscape (severity). The total area and total number of fires are tracked by the National Interagency Fire Center, which compiles reports from local, state, and federal agencies that are involved in fighting wildfires. The U.S. Forest Service tracked similar data using a different reporting system until 1997. Those data have been added to this indicator for comparison. Wildfire severity is measured by comparing the â€œgreennessâ€? of satellite images taken before and after a fire to classify how severely the land has been burned. Burn severity provides an indication of the ecological damage and how long the effects of wildfires are likely to last.

Although some nationwide fire data have been collected since the early 1900s, this indicator starts in 1983 (Figures 1 and 2) and 1984 (Figures 3, 4, and 5), when nationwide data collection became more complete and standardized.

Indicator Notes

Many environmental impacts associated with climate change can affect wildfire frequency, extent, or severity, including changes in temperature, precipitation, and drought. Human activities and land management practices also affect wildfire activity, and preferred practices in wildfire management have evolved over time, from older policies that favored complete wildfire prevention to more recent policies of wildfire suppression and controlled burns. While this indicator is limited to â€œwildlandâ€? fires, it includes fires that encroach onâ€”or perhaps started inâ€”developed areas. Increased development in previously wild lands could also influence trends in wildfire frequency and extent. The total number of fires may also vary due to reporting irregularities, as fires that split or merge together across jurisdictional lines may be counted differently.

Along with the influence of ongoing climate change, wildfire patterns can be influenced by natural climate cycles that tend to shift every few decades. Thus, the approximately 30 years of data shown here may not be enough to draw conclusions about long-term trends. While a longer record would be ideal, data from before 1983 are not consistent enough nationally to be included in this indicator.

Data Sources

The full set of wildfire frequency and burned acreage data in Figures 1 and 2 comes from the National Interagency Fire Center, which compiles wildfire reports sent from local, state, and federal entities that are involved in fighting fires. These data are available online at: www.nifc.gov/fireInfo/fireInfo_statistics.html. Additional data were provided by the U.S. Forest Service based on a different set of records, referred to as Smokey Bear Reports. Burn severity data and state-by-state acreage totals in Figures 3, 4, and 5 come from a multi-agency project called Monitoring Trends in Burn Severity, which maintains a database of wildfire events across the United States. These data are publicly available at: www.mtbs.gov/data/search.html.

Technical Documentation

Download related technical information PDF

References

1 MRLC (Multi-Resolution Land Characteristics) Consortium. 2015. National Land Cover Database 2011 (NLCD 2011) product statistics. www.mrlc.gov/nlcd11_stat.php.

2 Westerling, A.L. 2016. Increasing western U.S. forest wildfire activity: Sensitivity to changes in the timing of spring. Phil. Trans. R. Soc. B. 371:20150178.

3 Stein, S.M., J. Menakis, M.A. Carr, S.J. Comas, S.I. Stewart, H. Cleveland, L. Bramwell, and V.C. Radeloff. 2013. Wildfire, wildlands, and people: Understanding and preparing for wildfire in the wildland-urban interface. Gen. Tech. Rep. RMRS-GTR-299. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. www.fs.fed.us/openspace/fote/wildfire-report.html.

4 Johnston, F.H., S.B. Henderson, Y. Chen, J.T. Randerson, M. Marlier, R.S. DeFries, P. Kinney, D. Bowman, and M. Brauer. 2012. Estimated global mortality attributable to smoke from landscape fires. Environ. Health Persp. 120(5):695â€“701. www.ncbi.nlm.nih.gov/pmc/articles/PMC3346787.

5 Fann, N., T. Brennan, P. Dolwick, J.L. Gamble, V. Ilacqua, L. Kolb, C.G. Nolte, T.L. Spero, and L. Ziska. 2016. Chapter 3: Air quality impacts. 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.

6 National Association of State Foresters. 2009. Quadrennial fire review. www.nifc.gov/policies/pol_ref_QFR.html.

7 NIFC (National Interagency Fire Center). 2016. Historical wildland fire information: Federal firefighting costs: Suppression only (1985â€“2015). www.nifc.gov/fireInfo/fireInfo_documents/SuppCosts.pdf.

8 NIFC (National Interagency Fire Center). 2015. Wildland fire fatalities by year (1910â€“2014). www.nifc.gov/safety/safety_documents/Fatalities-by-Year.pdf.

9 NWCG (National Wildfire Coordinating Group). 2015. Glossary of wildland fire terminology. Updated October 2015. www.nwcg.gov/glossary-of-wildland-fire-terminology.

10 For example, see: Peterson, W.T., and F.B. Schwing. 2003. A new climate regime in northeast Pacific ecosystems. Geophys. Res. Lett. 30(17).

11 Kitzberger, T., P.M. Brown, E.K. Heyerdahl, T.W. Swetnam, and T.T. Veblen. 2007. Contingent Pacificâ€“Atlantic Ocean influence on multicentury wildfire synchrony over western North America. P. Natl. Acad. Sci. USA 104(2):543â€“548.

12 Westerling, A.L. 2016. Increasing western U.S. forest wildfire activity: Sensitivity to changes in the timing of spring. Phil. Trans. R. Soc. B. 371:20150178.

13 NIFC (National Interagency Fire Center). 2016. Total wildland fires and acres (1960â€“2015). Accessed March 2016. www.nifc.gov/fireInfo/fireInfo_stats_totalFires.html.

14 USDA (U.S. Department of Agriculture) Forest Service. 2014. 1991â€“1997 wildland fire statistics. Prepared by USDA Forest Service, State and Private Forestry, Fire and Aviation Management staff, and supplemented with historical records provided by Forest Service staff, April 2014.

15 NIFC (National Interagency Fire Center). 2016. Total wildland fires and acres (1960â€“2015). Accessed March 2016. www.nifc.gov/fireInfo/fireInfo_stats_totalFires.html.

16 Short, K.C. 2015. Sources and implications of bias and uncertainty in a century of U.S. wildfire activity data. Int. J. Wildland Fire 24(7):883â€“891.

17 MTBS (Monitoring Trends in Burn Severity). 2016. MTBS data summaries. www.mtbs.gov/data/search.html.

18 MTBS (Monitoring Trends in Burn Severity). 2016. MTBS data summaries. www.mtbs.gov/data/search.html.

19Â MTBS (Monitoring Trends in Burn Severity). 2016. MTBS data summaries. www.mtbs.gov/data/search.html.

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Learn about other indicators in this section

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Climate Indicators

Climate Change Indicators

Greenhouse Gases

Weather and Climate

Oceans

Snow and Ice

Health and Society

Ecosystems

Wildfires

Streamflow

Stream Temperature

Tribal Connection: Trends in Stream Temperature in the Snake River

Great Lakes Water Levels and Temperatures

Bird Wintering Ranges

Marine Species Distribution

Leaf and Bloom Dates

Community Connection: Cherry Blossom Bloom Dates in Washington, D.C.

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Tribal Connection: Trends in Stream Temperature in the Snake River

This feature tracks the summer water temperature of the Snake River.

Figure 1. Average August Temperature in the Snake River, 1960–2015

This graph shows average August water temperatures at a site along the Snake River in eastern Washington.

Data source: USGS, 20167
Web update: August 2016

Figure 1. Average August Temperature in the Snake River, 1960–2015

Climate change has challenged and will continue to challenge some of the traditional ways of life that have sustained indigenous peoples for thousands of years. In the Pacific Northwest, warming river and stream temperatures will threaten ecosystems and species, including salmon populations.1 Salmon play a particularly important role in the diet, culture, religion, and economy of Native Americans in this region.2

Salmon are sensitive to water temperature at many stages of their lives. They spend much of their adult lives in the ocean, then migrate inland to spawn. Salmon need cold water to migrate and for their young to hatch and grow successfully. Warmer water can negatively affect fish, making it more difficult for them to swim upstream. It can also make fish more susceptible to disease.3 River and stream temperatures in the Pacific Northwest are influenced by many factors, but are expected to rise as average air temperatures increase (see the U.S. and Global Temperature indicator).4,5,6

The graph shows average August water temperatures at a site in the Snake River, in eastern Washington near Nez Perce tribal lands. Several species of salmon use the Snake River to migrate and spawn. Between 1960 and 2015, water temperature has increased by 1.4°F.

References

1. Melillo, J.M., T.C. Richmond, and G.W. Yohe (eds.). 2014. Climate change impacts in the United States: The third National Climate Assessment. U.S. Global Change Research Program. http://nca2014.globalchange.gov.

2. Dittmer, K. 2013. Changing streamflow on Columbia basin tribal lands—Climate change and salmon. Climatic Change 120(3):627–641.

3. U.S. EPA. 2001. Issue paper 5: Summary of technical literature examining the physiological effects of temperature on salmonids. EPA-910-D-01-005.

4. Caissie, D. 2006. The thermal regime of rivers: a review. Freshwater Biology 51:1389–1406.

5. Van Vliet, M.T.H., F. Ludwig, and P. Kabat. 2013. Global streamflow and thermal habitats of freshwater fishes under climate change. Climatic Change 121:739–754.

6. Isaak, D.J., S. Wollrab, D. Horan, and G. Chandler. 2012. Climate change effects on stream and river temperatures across the Northwest U.S. from 1980–2009 and implications for salmonid fishes. Climatic Change 113:499–524.

7. USGS (U.S. Geological Survey). 2016. Analysis of data from the National Water Information System. Accessed March 2016.

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Climate Indicators

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Streamflow

Stream Temperature

Tribal Connection: Trends in Stream Temperature in the Snake River

Great Lakes Water Levels and Temperatures

Bird Wintering Ranges

Marine Species Distribution

Leaf and Bloom Dates

Community Connection: Cherry Blossom Bloom Dates in Washington, D.C.

Frequent Questions


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A Closer Look:

Rising global average temperature is associated with widespread changes in weather patterns. Scientific studies indicate that extreme weather events such as heat waves and large storms are likely to become more frequent or more intense with human-induced climate change. This chapter focuses on observed changes in temperature, precipitation, storms, floods, and droughts.

Why does it matter?

Long-term changes in climate can directly or indirectly affect many aspects of society in potentially disruptive ways. For example, warmer average temperatures could increase air conditioning costs and affect the spread of diseases like Lyme disease, but could also improve conditions for growing some crops. More extreme variations in weather are also a threat to society. More frequent and intense extreme heat events can increase illnesses and deaths, especially among vulnerable populations, and damage some crops. WhileÂ increased precipitation can replenish water supplies and support agriculture, intense storms can damage property, cause loss of life and population displacement, and temporarily disrupt essential services such as transportation, telecommunications, energy, and water supplies.

Summary of Key Points

U.S. and Global Temperature. Average temperatures have risen across the contiguous 48 states since 1901, with an increased rate of warming over the past 30 years. Eight of the top 10 warmest years on record have occurred since 1998. Average global temperatures show a similar trend, and all of the top 10 warmest years on record worldwide have occurred since 1998. Within the United States, temperatures in parts of the North, the West, and Alaska have increased the most.Â

High and Low Temperatures. Many extreme temperature conditions are becoming more common. Since the 1970s, unusually hot summer days (highs) have become more common over the last few decades in the United States. Unusually hot summer nights (lows) have become more common at an even faster rate. This trend indicates less â€œcooling offâ€? at night. Although the United States has experienced many winters with unusually low temperatures, unusually cold winter temperatures have become less commonâ€”particularly very cold nights (lows). Record-setting daily high temperatures have become more common than record lows. The decade from 2000 to 2009 had twice as many record highs as record lows.

U.S. and Global Precipitation. Total annual precipitation has increased over land areas in the United States and worldwide. Since 1901, precipitation has increased at an average rate of 0.08 inches per decade over land areas worldwide. However, shifting weather patterns have caused certain areas, such as the Southwest, to experience less precipitation than usual.

Heavy Precipitation. In recent years, a higher percentage of precipitation in the United States has come in the form of intense single-day events. The prevalence of extreme single-day precipitation events remained fairly steady between 1910 and the 1980s but has risen substantially since then. Nationwide, nine of the top 10 years for extreme one-day precipitation events have occurred since 1990. The occurrence of abnormally high annual precipitation totals (as defined by the National Oceanic and Atmospheric Administration) has also increased.

Tropical Cyclone Activity. Tropical storm activity in the Atlantic Ocean, the Caribbean, and the Gulf of Mexico has increased during the past 20 years. Storm intensity is closely related to variations in sea surface temperature in the tropical Atlantic. However, changes in observation methods over time make it difficult to know for sure whether a longer-term increase in storm activity has occurred. Records collected since the late 1800s suggest that the actual number of hurricanes per year has not increased.

River Flooding. Increases and decreases in the frequency and magnitude of river flood events vary by region. Floods have generally become larger across parts of the Northeast and Midwest and smaller in the West, southern Appalachia, and northern Michigan. Large floods have become more frequent across the Northeast, Pacific Northwest, and parts of the northern Great Plains, and less frequent in the Southwest and the Rockies.

Drought. 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. However, specific trends vary by region. A more detailed index developed recently shows that 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. However, this index has not been in use for long enough to compare with historical drought patterns.Â

A Closer Look: Temperature and Drought in the Southwest. The southwestern United States is particularly sensitive to changes in temperature and thus vulnerable to drought, as even a small decrease in water availability in this already arid region can stress natural systems and further threaten water supplies.

Weather and Climate

Weather is the state of the atmosphere at any given time and place. Most of the weather that affects people, agriculture, and ecosystems takes place in the lower layer of the atmosphere. Familiar aspects of weather include temperature, precipitation, clouds, and wind that people experience throughout the course of a day. Severe weather conditions include hurricanes, tornadoes, blizzards, and droughts.

Climate is the long-term average of the weather in a given place. While the weather can change in minutes or hours, a change in climate is something that develops over longer periods of decades to centuries. Climate is defined not only by average temperature and precipitation but also by the type, frequency, duration, and intensity of weather events such as heat waves, cold spells, storms, floods, and droughts.

While the concepts of climate and weather are often confused, it is important to understand the difference. For example, the eastern United States experienced a cold and snowy winter in 2014/2015, but this short-term regional weather phenomenon does not negate the long-term rise in national and global temperatures, sea level, or other climate indicators. It may be helpful to think about the difference between weather and climate with an analogy: weather influences what clothes you wear on a given day, while the climate where you live influences the entire wardrobe you buy.Â

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Climate Change Indicators: Heavy Precipitation

This indicator tracks the frequency of heavy precipitation events in the United States.

Figure 1. Extreme One-Day Precipitation Events in the Contiguous 48 States, 1910–2015

This figure shows the percentage of the land area of the contiguous 48 states where a much greater than normal portion of total annual precipitation has come from extreme single-day precipitation events. The bars represent individual years, while the line is a nine-year weighted average.

Data source: NOAA, 20166
Web update: August 2016

Figure 2. Unusually High Annual Precipitations in the Contiguous 48 States, 1895–2015

This figure shows the percentage of the land area of the contiguous 48 states that experienced much greater than normal precipitation in any given year, which means it scored 2.0 or above on the annual Standardized Precipitation Index. The thicker line shows a nine-year weighted average that smooths out some of the year-to-year fluctuations.

Data source: NOAA, 20167
Web update: August 2016

Figure 1. Extreme One-Day Precipitation Events in the Contiguous 48 States, 1910–2015

Figure 2. Unusually High Annual Precipitations in the Contiguous 48 States, 1895–2015

			Key Points

			Background

About the

			Indicator

			Indicator Notes

			Data Sources

Technical

			Documentation

Key Points

In recent years, a larger percentage of precipitation has come in the form of intense single-day events. Nine of the top 10 years for extreme one-day precipitation events have occurred since 1990 (see Figure 1).

The prevalence of extreme single-day precipitation events remained fairly steady between 1910 and the 1980s, but has risen substantially since then. Over the entire period from 1910 to 2015, the portion of the country experiencing extreme single-day precipitation events increased at a rate of about half a percentage point per decade (see Figure 1).

The percentage of land area experiencing much greater than normal yearly precipitation totals increased between 1895 and 2015. There has been much year-to-year variability, however. In some years there were no abnormally wet areas, while a few others had abnormally high precipitation totals over 10 percent or more of the contiguous 48 states’ land area (see Figure 2). For example, 1941 was extremely wet in the West, while 1982 was very wet nationwide.3

Figures 1 and 2 are both consistent with other studies that have found an increase in heavy precipitation over timeframes ranging from single days to 90-day periods to whole years.4,5 For more information on trends in overall precipitation levels, see the U.S. and Global Precipitation indicator.

Background

Heavy precipitation refers to instances during which the amount of rain or snow experienced in a location substantially exceeds what is normal. What constitutes a period of heavy precipitation varies according to location and season.

Climate change can affect the intensity and frequency of precipitation. Warmer oceans increase the amount of water that evaporates into the air. When more moisture-laden air moves over land or converges into a storm system, it can produce more intense precipitation—for example, heavier rain and snow storms.1 The potential impacts of heavy precipitation include crop damage, soil erosion, and an increase in flood risk due to heavy rains (see the River Flooding indicator)—which in turn can lead to injuries, drownings, and other flooding-related effects on health.2 In addition, runoff from precipitation can impair water quality as pollutants deposited on land wash into water bodies.

Heavy precipitation does not necessarily mean the total amount of precipitation at a location has increased—just that precipitation is occurring in more intense events. However, changes in the intensity of precipitation, when combined with changes in the interval between precipitation events, can also lead to changes in overall precipitation totals.

About the Indicator

Heavy precipitation events can be measured by tracking their frequency, examining their return period (the chance that the event will be equaled or exceeded in a given year), or directly measuring the amount of precipitation in a certain period (for example, inches of rain falling in a 24-hour period).

One way to track heavy precipitation is by calculating what percentage of a particular location’s total precipitation in a given year has come in the form of extreme one-day events—or, in other words, what percentage of precipitation is arriving in short, intense bursts. Figure 1 of this indicator looks at the prevalence of extreme single-day precipitation events over time.

For added insight, this indicator also tracks the occurrence of unusually high total yearly precipitation. It does so by looking at the Standardized Precipitation Index (SPI), which compares actual yearly precipitation totals with the range of precipitation totals that one would typically expect at a specific location, based on historical data. If a location experiences less precipitation than normal during a particular period, it will receive a negative SPI score, while a period with more precipitation than normal will receive a positive score. The more precipitation (compared with normal), the higher the SPI score. The SPI is a useful way to look at precipitation totals because it allows comparison of different locations and different seasons on a standard scale. Figure 2 shows what percentage of the total area of the contiguous 48 states had an annual SPI score of 2.0 or above (well above normal) in any given year.

Indicator Notes

Weather monitoring stations tend to be closer together in the eastern and central states than in the western states. In areas with fewer monitoring stations, heavy precipitation indicators are less likely to reflect local conditions accurately.

Data Sources

The data used for this indicator come from a large national network of weather stations and were provided by the National Oceanic and Atmospheric Administration’s (NOAA’s) National Centers for Environmental Information. Figure 1 is based on Step #4 of NOAA’s U.S. Climate Extremes Index; for data and a description of the index, see: www.ncdc.noaa.gov/extremes/cei. Figure 2 is based on the U.S. SPI, which is shown in a variety of maps available online at: www.ncdc.noaa.gov/sotc/drought. The data used to construct these maps are available at: ftp://ftp.ncdc.noaa.gov/pub/data/cirs/climdiv/.

Technical Documentation

Download related technical information PDF

References

1 Melillo, J.M., T.C. Richmond, and G.W. Yohe (eds.). 2014. Climate change impacts in the United States: The third National Climate Assessment. U.S. Global Change Research Program. http://nca2014.globalchange.gov.

2 Bell, J.E., S.C. Herring, L. Jantarasami, C. Adrianopoli, K. Benedict, K. Conlon, V. Escobar, J. Hess, J. Luvall, C.P. Garcia-Pando, D. Quattrochi, J. Runkle, and C.J. Schreck, III. 2016. Chapter 4: Impacts of extreme events on human health. 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 NOAA (National Oceanic and Atmospheric Administration). 2012. National Climatic Data Center. Personal communication: Analysis by Derek Arndt, April 2012.

4 CCSP (U.S. Climate Change Science Program). 2008. Synthesis and Assessment Product 3.3: Weather and climate extremes in a changing climate. www.globalchange.gov/browse/reports/sap-33-weather-and-climate-extremes-changing-climate.

5 Melillo, J.M., T.C. Richmond, and G.W. Yohe (eds.). 2014. Climate change impacts in the United States: The third National Climate Assessment. U.S. Global Change Research Program. http://nca2014.globalchange.gov/.

6 NOAA (National Oceanic and Atmospheric Administration). 2016. U.S. Climate Extremes Index. Accessed January 2016. www.ncdc.noaa.gov/extremes/cei.

7 NOAA (National Oceanic and Atmospheric Administration). 2016. Standardized Precipitation Index data files. Accessed January 2016. ftp://ftp.ncdc.noaa.gov/pub/data/cirs/climdiv/.

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Learn about other indicators in this section

Climate Indicators

Climate Change Indicators

Greenhouse Gases

Weather and Climate

U.S. and Global Temperature

High and Low Temperatures

U.S. and Global Precipitation

Heavy Precipitation

Tropical Cyclone Activity

River Flooding

Drought

A Closer Look: Temperature and Drought in the Southwest

Oceans

Snow and Ice

Health and Society

Ecosystems

Frequent Questions


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A Closer Look: Land Loss Along the Atlantic Coast

This feature provides a closer look at one consequence of sea level rise along the Atlantic coast: the conversion of land to open water.

Figure 1. Land Loss Along the Atlantic Coast, 1996–2011

This graph shows the net amount of land converted to open water along the Atlantic coast during three time periods: 1996–2001, 1996–2006, and 1996–2011. The results are divided into two regions: the Southeast and the Mid-Atlantic (see locator map). Negative numbers show where land loss is outpaced by the accumulation of new land.

Data source: NOAA, 20132
Web update: May 2014

Figure 2. Land Submergence Along the Atlantic Coast, 1996–2011

This graph shows the net amount of land converted to open water along the Atlantic coast during three time periods: 1996–2001, 1996–2006, and 1996–2011. The results are divided into categories to show the type of land that has been converted to open water.

Data source: NOAA, 20133
Web update: May 2014

Figure 1. Land Loss Along the Atlantic Coast, 1996–2011

Figure 2. Land Submergence Along the Atlantic Coast, 1996–2011

Key Points

Background

Notes

Data Sources

Technical Documentation

Key Points

Roughly 20 square miles of dry land and wetland were converted to open water along the Atlantic coast between 1996 and 2011. (For reference, Manhattan is 33 square miles.) More of this loss occurred in the Southeast than in the Mid-Atlantic (see Figure 1).

The data suggest that at least half of the land lost since 1996 has been tidal wetland. The loss of dry land appears to be larger than the loss of non-tidal wetland (see Figure 2).

Background

Why is the Atlantic coast particularly vulnerable to sea level rise?

Much of the land along the Atlantic coast is flat and close to sea level—including thousands of square miles of marshes and other productive wetlands, plus many low-lying cities. In addition, much of the land along the Atlantic coast is sinking, which magnifies the local effect of sea level rise. The land in North America is actually still adjusting to the loss of ice after the last ice age, which peaked about 20,000 years ago. Back then, thick sheets of ice covered areas of what is now Canada and the northern United States. The weight of all that ice depressed the land beneath it, but caused the land farther south (particularly the Mid-Atlantic region from North Carolina to New York) to bulge upward. After the ice melted and the extra weight was lifted, northern areas began to rise, and the Mid-Atlantic region started to sink. This very slow process continues today.

Rising sea level tends to make headlines during extreme events, like the storm surge that caused billions of dollars in damage during Hurricane Sandy in 2012. Yet rising sea level can also cause permanent changes in the landscape when it inundates (submerges) low-lying land. The Atlantic coast is particularly vulnerable because of low elevations and sinking shorelines.

The loss of coastal land can affect a large number of people, as nearly 10 million Americans live in a coastal floodplain.1 Coastal ecosystems are also at risk. These environments provide habitat for many kinds of plants and animals, as well as services that ensure people’s well-being, ranging from food production to recreation. Coastal wetlands provide valuable nursery, feeding, breeding, staging, and resting areas for many fish, shellfish, mammals, and birds, and they can buffer coastal areas against storm and wave damage.

As sea level rises, dry land can turn into wetland or open water. Existing wetlands can be threatened, too, as salt marshes, mangrove forests, and other coastal wetlands are at risk of being converted to open water.

The Sea Level indicator shows that sea level is rising overall in connection with climate change, but the rate of change varies by region, as do the effects. To provide a useful regional perspective, this feature examines the amount of land lost to sea level rise along the Atlantic coast from Florida to New York. It is based on satellite data that have been collected and analyzed at five-year intervals since 1996. Figure 1 divides the Atlantic coast into two regions for comparison, while Figure 2 shows the different types of land that have been lost.

Notes

Measurements of the change in coastal land depend on land cover and elevation data, which have significant limits in terms of accuracy and precision. Some results are field-checked for accuracy. The coastal land cover data are routinely compiled by the National Oceanic and Atmospheric Administration’s Coastal Change Analysis Program, and they represent the federal government’s most comprehensive set of data on land use and land cover in the coastal zone.

Sea level rise is not the only factor that contributes to the loss of coastal land. In addition to the natural sinking of the shoreline in some areas, such as the Mid-Atlantic, coastal land loss has been made worse by human activities such as navigation and flood control structures that block wetland migration or the movement of sediment; withdrawal of ground water, oil, or natural gas in some regions; and boat traffic that accelerates wetland erosion.4 Natural processes unrelated to current sea level rise can also cause shores to erode.

Data Sources

This feature is based on land cover data from the Coastal Change Analysis Program, which is coordinated by the National Oceanic and Atmospheric Administration. For more information about this program, visit: https://coast.noaa.gov/digitalcoast/?redirect=301ocm.

Technical Documentation

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References

1. Crowell, M., K. Coulton, C. Johnson, J. Westcott, D. Bellomo, S. Edelman, and E. Hirsch. 2010. An estimate of the U.S. population living in 100-year coastal flood hazard areas. J. Coastal Res. 26(2):201–211.

2. NOAA (National Oceanic and Atmospheric Administration). 2013. Coastal Change Analysis Program. Accessed December 2013. https://coast.noaa.gov/dataregistry/search/collection/info/ccapregional?redirect=301ocm.

3. NOAA (National Oceanic and Atmospheric Administration). 2013. Coastal Change Analysis Program. Accessed December 2013. https://coast.noaa.gov/dataregistry/search/collection/info/ccapregional?redirect=301ocm.

4. Titus, J.G., E.K. Anderson, D.R. Cahoon, S. Gill, R.E. Thieler, and J.S. Williams. 2009. Coastal sensitivity to sea-level rise: A focus on the Mid-Atlantic region. U.S. Climate Change Science Program and the Subcommittee on Global Change Research. https://downloads.globalchange.gov/sap/sap4-1/sap4-1-final-report-all.pdf.

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Climate Change Indicators: Streamflow

This indicator describes trends in the amount of water carried by streams across the United States, as well as the timing of runoff associated with snowmelt.

Figure 1. Seven-Day Low Streamflows in the United States, 1940–2014

This map shows percentage changes in the minimum annual streamflow for rivers and streams across the country, based on the long-term rate of change from 1940 to 2014. Minimum streamflow is based on the consecutive seven-day period with the lowest average flow during a given year.

Data source: USGS, 20161
Web update: August 2016

Figure 2. Three-Day High Streamflows in the United States, 1940–2014

This map shows percentage changes in the maximum annual streamflow for rivers and streams across the country, based on the long-term rate of change from 1940 to 2014. Maximum streamflow is based on the consecutive three-day period with the highest average flow during a given year.

Data source: USGS, 20162
Web update: August 2016

Figure 3. Annual Average Streamflow in the United States, 1940–2014

This map shows percentage changes in the annual average streamflow for rivers and streams across the country, based on the long-term rate of change from 1940 to 2014. This map is based on daily streamflow measurements, averaged over the entire year.

Data source: USGS, 20163
Web update: August 2016

Figure 4. Timing of Winter-Spring Runoff in the United States, 1940–2014

This map shows changes in the timing of annual high winter-spring flow carried by rivers and streams from 1940 to 2014. This analysis focuses on parts of the country where streamflow is strongly influenced by snowmelt. Trends are based on the winter-spring center of volume, which is the date when half of the total January 1–July 31 streamflow (in the West) or half of the total January 1–May 31 streamflow (in the East) has passed by each streamflow gauge, reflecting the timing of spring snowmelt.

Data source: USGS, 20164
Web update: August 2016

Figure 1. Seven-Day Low Streamflows in the United States, 1940–2014

Figure 2. Three-Day High Streamflows in the United States, 1940–2014

Figure 3. Annual Average Streamflow in the United States, 1940–2014

Figure 4. Timing of Winter-Spring Runoff in the United States, 1940–2014

			Key Points

			Background

About the

			Indicator

			Indicator Notes

			Data Sources

Technical

			Documentation

Key Points

During the past 75 years, seven-day low flows have generally increased in the Northeast and Midwest (in other words, on the days of lowest flows, streams in these areas are carrying more water than before). Low flows have generally decreased in parts of the Southeast and the Pacific Northwest (that is, streams are carrying less water than before). Overall, more sites have experienced increases than decreases (see Figure 1).

Three-day high-flow trends vary from region to region across the country. For example, high flows have generally increased or changed little in the Northeast since 1940, whereas high flows have increased in some West Coast streams and decreased in others. Overall, more sites have experienced increases than decreases (see Figure 2).

Annual average streamflow has increased at many sites in the Northeast and Midwest, while other regions have seen few substantial changes. Overall, sites show more increases than decreases (see Figure 3).

In parts of the country with substantial snowmelt, winter-spring runoff is happening at least five days earlier than in the mid-20th century at most gauges. The largest changes have occurred in the Pacific Northwest and Northeast (see Figure 4).

Background

Streamflow is a measure of the rate at which water is carried by rivers and streams, and it represents a critical resource for people and the environment. Changes in streamflow can directly influence the supply of drinking water and the amount of water available for irrigating crops, generating electricity, and other needs. In addition, many plants and animals depend on streamflow for habitat and survival.

Streamflow naturally varies over the course of a year. For example, rivers and streams in many parts of the country have their highest flow when snow melts in the spring and their lowest flow in late summer. The amount of streamflow is important because very high flows can cause erosion and damaging floods, while very low flows can diminish water quality, harm fish, and reduce the amount of water available for people to use. The timing of high flow is important because it affects the ability of reservoir managers to store water to meet needs later in the year. In addition, some plants and animals (such as fish that migrate) depend on a particular pattern of streamflow as part of their life cycles.

Climate change can affect streamflow in several ways. Changes in the amount of spring snowpack (see the Snowpack indicator) and air temperatures that influence melting can alter the size and timing of high spring streamflows. More precipitation is expected to cause higher average streamflow in some places, while heavier storms (see the Heavy Precipitation indicator) could lead to larger peak flows. More frequent or severe droughts, however, could reduce streamflow in certain areas.

About the Indicator

The U.S. Geological Survey measures streamflow in rivers and streams across the United States using continuous monitoring devices called stream gauges. This indicator is based on data from 192 stream gauges located in areas where trends are not substantially influenced by dams, reservoir management, wastewater treatment facilities, or land-use change. The indicator also excludes stream gauges with watershed areas that overlap substantially.

This indicator examines four important measures of streamflow conditions that occur during the course of a year. Figure 1 shows trends in the lowest flows each year, which are commonly calculated by averaging the lowest seven consecutive days of streamflow. Figure 2 shows trends in high flow conditions, which are commonly calculated by averaging the highest three consecutive days of streamflow in a year. Three days is an optimal length of time to characterize runoff associated with large storms and peak snowmelt. Figure 3 shows changes in the annual average streamflow, which is calculated by averaging daily flows through the entire year.

Figure 4 shows trends in the timing of winter and spring runoff. This measure is limited to 56 stream gauges in areas where at least 30 percent of annual precipitation falls as snow. Scientists look at the total volume of water that passes by a gauge between January 1 and July 31 for the western United States, and January 1 and May 31 for the eastern United States, then determine the date when exactly half of that water has gone by. This date is called the winter-spring center-of-volume date. A long-term trend toward an earlier date could be caused by earlier spring snowmelt, more precipitation falling as rain instead of snow, or other changes in precipitation patterns.

Indicator Notes

Streamflow measurements were used from gauges in areas where streamflow is minimally affected by human influences such as dams, land development, or changes in land cover. Changes in land cover and land use over time, however, could still influence streamflow trends at some streams. The gauges used for this indicator are not evenly distributed across the country, due to variation in the availability of long-term data.

Data Sources

Streamflow data were collected by the U.S. Geological Survey. These data came from a set of gauges in watersheds with minimal human impacts, which have been classified as reference gauges.5 Daily average streamflow data are stored in the National Water Information System and are publicly available at: http://waterdata.usgs.gov/nwis.

Technical Documentation

Download related technical information PDF

References

1 USGS (U.S. Geological Survey). 2016. Analysis of data from the National Water Information System. Accessed May 2016.

2 USGS (U.S. Geological Survey). 2016. Analysis of data from the National Water Information System. Accessed May 2016.

3 USGS (U.S. Geological Survey). 2016. Analysis of data from the National Water Information System. Accessed May 2016.

4 USGS (U.S. Geological Survey). 2016. Analysis of data from the National Water Information System. Accessed May 2016.

5 Lins, H.F. 2012. USGS Hydro-Climatic Data Network 2009 (HCDN-2009). U.S. Geological Survey Fact Sheet 2012-3047.

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Climate Change Indicators: U.S. Greenhouse Gas Emissions

This indicator describes emissions of greenhouse gases in the United States.

Figure 1. U.S. Greenhouse Gas Emissions by Gas, 1990â€“2014

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This figure shows emissions of carbon dioxide, methane, nitrous oxide, and several fluorinated gases in the United States from 1990 to 2014. For consistency, emissions are expressed in million metric tons of carbon dioxide equivalents.

* HFCs are hydrofluorocarbons, PFCs are perfluorocarbons, SF6 is sulfur hexafluoride, and NF3 is nitrogen trifluoride.

Data source: U.S. EPA, 20164
Web update: AugustÂ 2016

Figure 2. U.S. Greenhouse Gas Emissions and Sinks by Economic Sector, 1990â€“2014â€‹

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This figure shows greenhouse gas emissions and sinks (negative values) by source in the United States from 1990 to 2014. For consistency, emissions are expressed in million metric tons of carbon dioxide equivalents. All electric power emissions are grouped together in the â€œElectricity generationâ€? sector, so other sectors such as â€œResidentialâ€? and â€œCommercialâ€? are only showing non-electric sources, such as burning oil or gas for heating. Totals do not match Figure 1 exactly because the economic sectors shown here do not include emissions from U.S. territories outside the 50 states.

Data source: U.S. EPA, 20165
Web update: August 2016

Figure 3. U.S. Greenhouse Gas Emissions per Capita and per Dollar of GDP, 1990â€“2014

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This figure shows trends in greenhouse gas emissions from 1990 to 2014 per capita (heavy orange line), based on the total U.S. population (thin orange line). It also shows trends in emissions per dollar of real GDP (heavy blue line). Real GDP (thin blue line) is the value of all goods and services produced in the country during a given year, adjusted for inflation. All data are indexed to 1990 as the base year, which is assigned a value of 100. For instance, a real GDP value of 178 in the year 2014 would represent a 78 percent increase since 1990.

Data source: U.S. EPA, 20166
Web update: AugustÂ 2016

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Figure 1. U.S. Greenhouse Gas Emissions by Gas, 1990â€“2014

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Figure 2. U.S. Greenhouse Gas Emissions and Sinks by Economic Sector, 1990â€“2014

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Figure 3. U.S. Greenhouse Gas Emissions per Capita and per Dollar of GDP, 1990â€“2014

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			Key Points

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			Background

About the

			Indicator

Â

			Indicator Notes

Â

			Data Sources

Technical

			Documentation

Key Points

In 2014, U.S. greenhouse gas emissions totaled 6,870 million metric tons (15.1 trillion pounds) of carbon dioxide equivalents. This total represents a 7 percent increase since 1990 but a 7 percent decrease since 2005 (see Figure 1).

For the United States, during the period from 1990 to 2014 (see Figure 1):

Emissions of carbon dioxide, the primary greenhouse gas emitted by human activities, increased by 9 percent.

Methane emissions decreased by 6 percent, as reduced emissions from landfills, coal mines, and natural gas systems more than offset increases in emissions from activities such as livestock production.2

Nitrous oxide emissions, predominantly from agricultural soil management practices such as the use of nitrogen as a fertilizer, decreased by 1 percent.

Emissions of fluorinated gases (hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride), released as a result of commercial, industrial, and household uses, increased by 77 percent.

Among the various sectors of the U.S. economy, electricity generation (power plants) accounts for the largest share of emissionsâ€”31 percent of total greenhouse gas emissions since 1990. Transportation is the second-largest sector, accounting for 26 percent of emissions since 1990 (see Figure 2).

Emissions sinks, the opposite of emissions sources, absorb carbon dioxide from the atmosphere. In 2014, 11 percent of U.S. greenhouse gas emissions were offset by netÂ sinks resulting from land use and forestry practices (see Figure 2). One major sink is the net growth of forests, which remove carbon from the atmosphere. Other carbon sinks are associated with how people use the land, including the practice of depositing yard trimmings and food scraps in landfills. While the land use, land-use change, and forestry category represents an overall net sink of carbon dioxide in the United States, this category also includes emission sources resulting from activities such as wildfires, converting land to cropland, and biomass burning.

Emissions increased at about the same rate as the population from 1990 to 2007, which caused emissions per capita to remain fairly level (see Figure 3). Total emissions and emissions per capita declined from 2007 to 2009, due in part to a drop in U.S. economic production during this time. Emissions decreased again from 2010 to 2012, largely due to the growing use of natural gas to generate electricity in place of more carbon-intensive fuels.3

From 1990 to 2014, greenhouse gas emissions per dollar of goods and services produced by the U.S. economy (the gross domestic product or GDP) declined by 40 percent (see Figure 3). This change may reflect a combination of increased energy efficiency and structural changes in the economy.Â

Background

A number of factors influence the quantities of greenhouse gases released into the atmosphere, including economic activity, population, consumption patterns, energy prices, land use, and technology. There are several ways to track these emissions, such as by measuring emissions directly, calculating emissions based on the amount of fuel that people burn, and estimating other activities and their associated emissions. EPA has two key programs that provide data on greenhouse gas emissions in the United States: the Inventory of U.S. Greenhouse Gas Emissions and Sinks and the Greenhouse Gas Reporting Program. See â€œSources of Data on U.S. Greenhouse Gas Emissionsâ€? to learn more about these programs.

About the Indicator

This indicator focuses on emissions of carbon dioxide, methane, nitrous oxide, and several fluorinated gasesâ€”all important greenhouse gases that are influenced by human activities. These particular gases are covered under the United Nations Framework Convention on Climate Change, an international agreement that requires participating countries to develop and periodically submit an inventory of greenhouse gas emissions. Data and analysis for this indicator come from EPAâ€™s annual inventory submission, the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990â€“2014.1Â This indicator is restricted to emissions associated with human activities. It starts in 1990, which is a common baseline year for global agreements to track and reduce greenhouse gas emissions.

Each greenhouse gas has a different lifetime (how long it stays in the atmosphere) and a different ability to trap heat in our atmosphere. To allow different gases to be compared and added together, emissions are converted into carbon dioxide equivalents. This step uses each gasâ€™s 100-year global warming potential, which measures how much a given amount of the gas is estimated to contribute to global warming over a period of 100 years after being emitted. Carbon dioxide is assigned a global warming potential equal to 1. This analysis uses global warming potentials from the Intergovernmental Panel on Climate Changeâ€™s (IPCCâ€™s) Fourth Assessment Report. In that report, methane has a global warming potential of 25, which means a ton of methane emissions contributes 25 times as much warming as a ton of carbon dioxide emissions over 100 years, and that ton of methane emissions is therefore equal to 25 tons of carbon dioxide equivalents. See the table for comparison with global warming potentials from IPCCâ€™s Fifth Assessment Report. For additional perspective, this indicator also shows greenhouse gas emissions in relation to economic output and population.

Indicator Notes

While this indicator includes the major greenhouse gases emitted by human activities, it does not include other greenhouse gases and substances that are not covered under the United Nations Framework Convention on Climate Change but that still affect the Earthâ€™s energy balance and climate (see the Climate Forcing indicator for more details). For example, this indicator excludes ozone-depleting substances such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which have high global warming potentials, as these gases have been or are currently being phased out under an international agreement called the Montreal Protocol. This indicator also excludes black carbon and aerosols, which most greenhouse gas emissions inventories do not cover. There are also many natural greenhouse gas emissions sources; however, this indicatorÂ focuses onÂ emissions that are associated with human activitiesâ€”those that are most responsible for the observed buildup of these gases in our atmosphere.Â Although the land use, land-use change, and forestry emission estimates include emissions and sinks from unmanaged lands due to forest growth, fires, and other factors, this indicator does not include other natural sources such as methane emissions from unmanaged wetlands.

Data Sources

Data for this indicator came from EPAâ€™s Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990â€“2014. This report is available online at: /ghgemissions/us-greenhouse-gas-inventory-report-1990-2014. The calculations in Figure 3 are based on GDP and population data provided by the U.S. Bureau of Economic Analysis and the U.S. Census, respectively.

Technical Documentation

Download related technical information PDF

References

1. U.S. EPA (U.S. Environmental Protection Agency). 2016. Inventory of U.S. greenhouse gas emissions and sinks: 1990â€“2014. EPA 430-R-16-002. www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.

2. U.S. EPA (U.S. Environmental Protection Agency). 2016. Inventory of U.S. greenhouse gas emissions and sinks: 1990â€“2014. EPA 430-R-16-002. www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.

3. U.S. EPA (U.S. Environmental Protection Agency). 2016. Inventory of U.S. greenhouse gas emissions and sinks: 1990â€“2014. EPA 430-R-16-002. www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.

4. U.S. EPA (U.S. Environmental Protection Agency). 2016. Inventory of U.S. greenhouse gas emissions and sinks: 1990â€“2014. EPA 430-R-16-002. www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.

5. U.S. EPA (U.S. Environmental Protection Agency). 2016. Inventory of U.S. greenhouse gas emissions and sinks: 1990â€“2014. EPA 430-R-16-002. www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.

6. U.S. EPA (U.S. Environmental Protection Agency). 2016. Inventory of U.S. greenhouse gas emissions and sinks: 1990â€“2014. EPA 430-R-16-002. www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.

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