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Climate Change

Climate Change Indicators in the United States


EPA has released the third edition of this Climate Change Indicators in the United States report, presenting 30 indicators to help readers understand observed long-term trends related to the causes and effects of climate change. The indicators in the report, also presented on this website, look at the composition of the atmosphere, fundamental measures of climate, and the extent to which several climate-sensitive aspects of the oceans, snow and ice, human health, society, and ecosystems are changing. Together, these indicators present compelling evidence that climate change is happening now in the United States and around the world.

Use the links below to find answers to some common questions about EPA’s climate change indicators.

Why use indicators?

One important way to track and communicate the causes and effects of climate change is through the use of indicators. An indicator represents the state or trend of certain environmental or societal conditions over a given area and a specified period of time. Indicators are designed to help readers understand observed long-term trends related to the causes and effects of climate change. For example, long-term measurements of temperature in the United States and globally are used as an indicator to track and better understand the effects of changes in the Earth’s climate.

This compilation is designed to be useful for the public, scientists, analysts, decision-makers, educators, and others who can employ climate change indicators as a tool for:

  • Effectively communicating relevant climate science information in a sound, transparent, and easy-to-understand way.
  • Assessing trends in environmental quality, factors that influence the environment, and effects on ecosystems and society.
  • Informing science-based decision-making.

How do the indicators relate to climate change?

All of the indicators on this website relate to either the causes or effects of climate change. Some indicators show trends that can be more directly linked to human-induced climate change than others. Collectively, the trends depicted in these indicators provide important evidence of “what climate change looks like.”

Although each indicator has a connection to climate change, this website is not intended to identify the extent to which a certain indicator is driving climate change, nor the relative role of climate change in causing a trend in an observed indicator. Connections between human activities, climate change, and observed indicators are explored in more detail elsewhere in the scientific literature.

Where do the indicator data come from?

EPA’s indicators are compilations of key data sets to be used for communication purposes. They consist of peer-reviewed, publicly-available data from a number of government agencies, academic institutions, and other organizations. In addition to being published here, these data sets have been published in the scientific literature and in other government or academic reports.

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What geographic area do the indicators cover?

Most of EPA’s indicators focus on the United States, but some include global trends to provide context or a basis for comparison, or because they are intrinsically global in nature. To maximize interpretation, EPA attempts to present multiple scales (national, regional, or location-specific) for a given indicator provided the underlying data allow such scaling.

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How were EPA’s indicators chosen and developed?

The indicators in this compilation were evaluated and chosen using a set of general assessment factors and a standard set of criteria that considers usefulness, data quality, and relevance to climate change. See the technical documentation overview for a full description of these general assessment factors and criteria and for a description of the EPA’s process for evaluating indicators.

EPA’s climate change indicators and the accompanying detailed technical documentation have been designed to ensure that the science and underlying peer-reviewed data are presented and documented transparently. EPA also receives feedback from scientists, researchers, and communications experts to help inform the content and new features of EPA’s indicators. The entire set of indicators, including the technical support documentation, is peer-reviewed by independent technical experts. Indicators are based on historical records that go back in time as far as possible without sacrificing data quality.

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What do EPA’s indicators tell us about climate change?

Trends relevant to climate change are best viewed at broad geographic scales and over long time horizons, rather than at localized scales or over a few years or a season. Most of the indicators in this compilation focus on the United States, but some include global trends to provide context or a basis for comparison, or because they are intrinsically global in nature, such as atmospheric concentrations of greenhouse gases, which are influenced by global activities. The geographic extent and timeframe that each indicator represents largely depends on data availability and the nature of what is being measured.

All of the indicators discussed on this website relate to either the causes or effects of climate change. Some indicators are directly linked to human activities that cause climate change, such as Global Greenhouse Gas Emissions. Changes depicted by other indicators, such as U.S. and Global Temperature, have been confidently linked with the increase in greenhouse gases caused by human activity. Some of the trends depicted in other indicators, such as Wildfires, although consistent with what one would expect in a warming climate, cannot yet be firmly attributed to human-induced climate change for various reasons (for example, limitations in the historical data, or other factors in addition to climate change that may influence the trend). A few indicators do not yet show any significant trend over the period for which data are available.

Although the climate is continually changing, not every climate change indicator will show a smooth pattern of steady change. The Earth is a complex system, and there will always be natural variations from one year to the next—for example, a very warm year followed by a colder year. The Earth’s climate also goes through other natural cycles that can play out over a period of several years or even decades. Individual years or even individual decades can deviate from the long-term trend. Thus, EPA’s indicators present trends for as many years as the underlying data allow.

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Which greenhouse gases do these indicators track?

The indicators in the Greenhouse Gases section of this website focus on most of the major, well-mixed greenhouse gases that contribute to the vast majority of warming of the climate. The major greenhouse gases emitted into the atmosphere are carbon dioxide, methane, nitrous oxide, and fluorinated gases. Some of these gases are produced almost entirely by human activities; others come from a combination of natural sources and human activities.

Many of the major greenhouse gases can remain in the atmosphere for tens to hundreds of years after being released. They become globally mixed in the lower part of the atmosphere, called the troposphere (the first several miles above the Earth’s surface), reflecting the combined contributions of emissions sources worldwide from the past and present. Due to this global mixing, concentrations of these gases will be fairly similar no matter where in the world they are measured.

EPA’s indicators also cover some other substances that have much shorter atmospheric lifetimes (i.e., less than a year) but are still relevant to climate change. Important short-lived substances that affect the climate include water vapor, ozone in the troposphere, pollutants that lead to ozone formation, and aerosols (atmospheric particles) such as black carbon and sulfates. Water vapor, tropospheric ozone, and black carbon contribute to warming, while other aerosols produce a cooling effect. In addition to several long-lived greenhouse gases, EPA’s Atmospheric Concentrations of Greenhouse Gases indicator tracks concentrations of ozone in the layers of the Earth’s atmosphere, while Figure 2 of the Climate Forcing indicator shows the influence of a variety of short-lived substances.

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