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Climate Change

Climate Change Science Overview

Earth's climate is changing. Multiple lines of evidence show changes in our weather, oceans, ecosystems, and more.

Natural causes alone cannot explain all of these changes. Human activities are contributing to climate change, primarily by releasing billions of tons of carbon dioxide (CO2) and other heat-trapping gases, known as greenhouse gases, into the atmosphere every year. [1]

Climate changes will continue into the future. The more greenhouse gases we emit, the larger future climate changes will be.

Photo of cars on a highway Photograph of clear-cut forest with standing trees in the distance. Photograph of smokestacks and power lines.

Human activities, like driving, manufacturing, electricity generation, and the clearing of forests contribute to greenhouse gas emissions and warm the planet.
Sources: EPA 2010, EPA 2011, USGCRP 2008 (243 pp, 17.19MB, About PDF)

Changes in the climate system affect our health, environment, and economy. We can prepare for some of the impacts of climate change to reduce their effects on our well-being.

Earth's climate is changing

The global average temperature has increased by more than 1.5°F since the late 1800s. [2] Some regions of the world have warmed by more than twice this amount. The buildup of greenhouse gases in our atmosphere and the warming of the planet are responsible for other changes, such as:

Learn more about the indicators of climate change.

Temperatures across the world in the 1880s (left) and the 1980s (right)

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These maps show temperatures across the world in the 1880s (left) and the 1980s (right). Blue colors represent cooler temperatures compared with the average temperatures from 1951 to 1980, and red colors represent warmer temperatures compared to this average. The map on the left shows that it was colder in the 1880s in most places. The map on the right shows it was warmer in the 1980s in most places. Earth's average surface temperature has increased by more than 1.5°F since the 1880s. Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.3°F-0.4°F per decade.
Source: NASA

Climate Change vs. Global Warming

The term climate change is sometimes used interchangeably with the term global warming. However, the terms do not refer entirely to the same thing.

Global warming refers to the recent and ongoing rise in global average temperature near Earth's surface. It is caused mostly by increasing concentrations of greenhouse gases in the atmosphere. Global warming is causing climate patterns to change. However, global warming itself represents only one aspect of climate change.

Climate change refers to any significant change in the measures of climate lasting for an extended period of time. In other words, climate change includes major changes in temperature, precipitation, or wind patterns, among others, that occur over several decades or longer. Climate change can occur at the global, continental, regional, and local levels. Climate change may refer to natural changes in climate, or changes caused by human activities.

Weather vs. Climate

The difference between weather and climate is primarily a matter of time and geography. Weather refers to the conditions of the atmosphere over a short period of time, such as hours or days, and typically for a local area. Climate refers to the behavior of the atmosphere over a longer period of time, and usually for a large area.

Familiar examples of weather characteristics include the daily temperature, humidity, or the amount of precipitation produced by a storm. Weather also includes severe weather conditions such as hurricanes, tornadoes, and blizzards. Because of the dynamic nature of the atmosphere, it is not possible to predict weather conditions in a specific location months or years in advance.

Climate is typically defined based on 30-year averages of weather. [4] Climate represents our expectations for the weather. For example, climate tells us how warm we expect a typical summer to be, how much rainfall would correspond to a wetter-than-average spring, or how frequently we expect a snowy winter to occur. Scientists can compare recent and long-term observations of the climate to detect the influence of greenhouse gases on climate conditions.

Natural causes alone cannot explain recent changes

Line graph with a line that show the observed temperature increases, a blue band that show how the temperature would have changed over the past century due to only natural forces, and a red band that shows the combined effects of natual and human forces. The blue band that shows natural forces starts and ends the 20th century just above 56 degrees Fahrenheit. The actual observed global average temperatures closely follows the model projections that use both human and natural forces - beginning in 1900 at just above 56 degrees Fahrenheit and ending in 2000 around 58 degrees Fahrenheit.

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Models that account only for the effects of natural processes are not able to explain the warming observed over the past century. Models that also account for the greenhouse gases emitted by humans are able to explain this warming.
Source: U.S. National Climate Assessment (2014)

Line graph with three trend lines that represent concentration of carbon dioxide, methane, and nitrous oxide over time. The three gasses follow a very similar pattern that starts low and is steady from year zero until sometime around 1900 where the concentration of all three gasses start increasing dramatically. In the first 1900 years carbon dioxide and nitrous oxide remain at levels around 280 parts per million and 250 parts per billion, respectively. By 2000, carbon dioxide measures around 380 parts per million and nitrous oxide measures around 320 parts per billion. Similarly methane remains at approximately 700 parts per billion until around 1800. By 2000, methane concentrations measure close to 2000 parts per billion.

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This graph shows the increase in greenhouse gas (GHG) concentrations in the atmosphere over the last 2,000 years. Increases in concentrations of these gases since 1750 are due to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion molecules of air.
Source: USGCRP 2009 (196 pp, 13.1MB, About PDF)

Natural processes such as changes in the sun's energy, shifts in ocean currents, and others affect Earth's climate. However, they do not explain the warming that we have observed over the last half-century. [1]

Human causes can explain these changes

Most of the warming of the past half century has been caused by human emissions of greenhouse gases. [1] Greenhouse gases come from a variety of human activities, including: burning fossil fuels for heat and energy, clearing forests, fertilizing crops, storing waste in landfills, raising livestock, and producing some kinds of industrial products.

Greenhouse gas emissions are not the only way that people can change the climate. Activities such as agriculture or road construction can change the reflectivity of Earth's surface, leading to local warming or cooling. This effect is observed in urban centers, which are often warmer than surrounding, less populated areas. Emissions of small particles, known as aerosols, into the air can also lead to reflection or absorption of the sun's energy.

Learn more about past and present climate trends and their causes.

Climate will continue to change, but the extent of the change will depend on how much, and how quickly, we can reduce greenhouse gas emissions.

During the 21st century, global warming is projected to continue and climate changes are likely to intensify. Scientists have used climate models to project different aspects of future climate, including temperature, precipitation, snow and ice, ocean level, and ocean acidity. Depending on future emissions of greenhouse gases and how the climate responds, average global temperatures are projected to increase worldwide by 0.5°F to 8.6°F by 2100, with a likely increase of at least 2.0°F for all scenarios except the one representing the most aggressive mitigation of greenhouse gas emissions. [2] Learn more about the projections of future climate change.

Climate change impacts our health, environment, and economy

Climate change affects our environment and natural resources, and impacts our way of life in many ways. For example:

  • Warmer temperatures increase the frequency, intensity, and duration of heat waves, which can pose health risks, particularly for young children and the elderly.
  • Climate change can also impact human health by worsening air and water quality, increasing the spread of certain diseases, and altering the frequency or intensity of extreme weather events.
  • Rising sea levels threaten coastal communities and ecosystems.
  • Changes in the patterns and amount of rainfall, as well as changes in the timing and amount of stream flow, can affect water supplies and water quality and the production of hydroelectricity.
  • Changing ecosystems influence geographic ranges of many plant and animal species and the timing of their lifecycle events, such as migration and reproduction.
  • Increases in the frequency and intensity of extreme weather events, such as heat waves, droughts, and floods, can increase losses to property, cause costly disruptions to society, and reduce the availability and affordability of insurance.

We can prepare for some of the likely climate change impacts to reduce their effect on ecosystems and human well-being. Making such preparations is known as adaptation. Examples of adaptation include strengthening water conservation programs, upgrading stormwater systems, developing early warning systems for extreme heat events, and preparing for stronger storms through better emergency preparation and response strategies.

Learn more about how climate change impacts are expected to affect different U.S. regions and sectors and how we can prepare.

Temperature increases and other climate changes may directly impact our food and water supply, ecosystems, coasts, and human health. The bars in the bottom box indicate what temperatures and impacts are expected under the high and low emissions scenarios, which are determined by our actions.

This slideshow describes some of the projected climate change impacts to key sectors, among other topics. Click the image above to play the slideshow.


[1] Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program.

[2] IPCC (2013). Climate Change 2013: The Physical Science Basis Exit EPA Disclaimer . Contribution of Working Groups I, II and III to the Fourth Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

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