Climate Change Science Overview
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- Earth's climate is changing
- Natural causes alone cannot explain recent changes
- Human causes can explain these changes
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. 
Climate changes will continue into the future. The more greenhouse gases we emit, the larger future climate changes will be.
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.  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:
- Changing temperature and precipitation patterns  
- Increases in ocean temperatures, sea level, and acidity
- Melting of glaciers and sea ice 
- Changes in the frequency, intensity, and duration of extreme weather events
- Shifts in ecosystem characteristics, like the length of the growing season, timing of flower blooms, and migration of birds
- Increasing threats to human health
Learn more about the indicators of climate change.
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.  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
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. 
Human causes can explain these changes
Most of the warming of the past half century has been caused by human emissions of greenhouse gases.  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.  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.
 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.
 IPCC (2013). Climate Change 2013: The Physical Science Basis . 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.