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Climate Change Indicators

Frequent Questions about Climate Change Indicators

EPA partners with more than 50 data contributors from various government agencies, academic institutions, and other U.S and international organizations to compile a key set of indicators related to the causes and effects of climate change. Explore the sections below to learn more about this resource.

Indicator Basics

EPA’s Approach to Developing Indicators

Indicator Basics

What is an indicator?

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. EPA's indicators are designed to help readers understand observed long-term trends related to the causes and effects of climate change. In other words, they provide important evidence of "what climate change looks like." 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.

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Why does EPA compile and publish climate change indicators?

EPA compiles these indicators with the primary goal of informing our understanding of climate change. They are designed so that the public, scientists, analysts, decision-makers, educators, and others can use climate change indicators as a basis 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.

EPA publishes these indicators to make them accessible and available to use in a variety of ways. These indicators characterize observed changes from long-term records related to the causes and effects of climate change; the significance of these changes; and their possible consequences for people, the environment, and society. The indicators presented here do not cover all possible measures of the causes and effects of climate change, nor do they capture all possible climate change indicators found in the full body of scientific assessment literature. Instead, these indicators represent a wide-ranging set of high-quality, long-term data that show observed changes in the Earth’s climate system and several climate-relevant impacts. Together, these indicators present compelling evidence that climate change is happening now in the United States and globally.

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How do the indicators relate to climate change?

All of the indicators on this website relate to either the causes or the 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, EPA's indicators do not attempt to identify either the extent to which a certain indicator is driving climate change, or 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. For example, see the U.S. Global Change Research Program's National Climate Assessment.

Some indicators are directly linked to human activities that cause climate change, such as Global Greenhouse Gas Emissions. Changes depicted by other indicators, like U.S. and Global Temperature, have been confidently linked with the increase in greenhouse gases caused by human activity. Some of the trends in other indicators, such as Wildfires, are consistent with what one would expect in a warming climate but may also be influenced by limitations in the historical data or other factors in addition to climate change. A few indicators, like West Nile Virus, though known to be influenced by climate change, do not yet show any significant trend. In some cases, this could be because the period for which data are available or the geographic scale in which they are presented is limited.

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Where do the indicator data come from?

EPA partners with more than 50 data contributors from various U.S. and international government agencies, academic institutions, and other organizations to compile and communicate key indicators related to the causes and effects of climate change. EPA's indicators consist of peer-reviewed, publicly available data. In addition to being published on EPA's website, these data sets have been published in the scientific literature and in government and academic reports. Details on each underlying data set as well as where to find it are provided in the technical documentation.

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What geographic areas and time periods do the indicators cover?

Trends relevant to climate change are best viewed at broad geographic scales and over long time periods. EPA's indicators are based on historical records that go back in time as far as possible without sacrificing data quality, so each indicator's time scale varies.

Most of EPA's indicators focus on the United States. However, 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. EPA attempts to present multiple scales (national, regional, or location-specific) in cases where the underlying data allow such scaling. The geographic extent and timeframe of each indicator largely depend on data availability and the nature of what is being measured.

EPA also includes features such as “Community Connection” and “A Closer Look” (e.g., Cherry Blossom Bloom Dates in Washington, D.C.) that focus on a particular region or localized area of interest to engage readers in particular areas or topics of interest within the United States. While the features and their underlying data are not national in scale or representative of broad geographic areas, these features are screened, developed, and documented in a manner consistent with other indicators.

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Why do some indicator trends fluctuate over time?

Although the climate is continually changing, not every climate change indicator shows a constant pattern of steady change. The Earth is a complex system, and there are always 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 play out over a period of several years or even decades. Individual years or even individual decades can deviate from the long-term trend. Human factors may also influence a trend—for example, human activities and land management practices can affect wildfire activity from year to year.

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Which greenhouse gases do these indicators track?

EPA’s greenhouse-gas-related indicators focus on most of the major, well-mixed greenhouse gases that contribute to the vast majority of warming of the climate when they are emitted into the atmosphere. These major gases are carbon dioxide, methane, nitrous oxide, and fluorinated gases. Some of these gases are produced almost entirely by human activities, while others come from a combination of natural sources and human activities.

Many of these major greenhouse gases remain in the atmosphere for tens to thousands 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 emissions worldwide from the past and present. Due to this global mixing, concentrations of these gases are fairly similar no matter where in the world they are measured.

Other substances that have much shorter atmospheric lifetimes (i.e., less than a year) 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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How do indicators relate to human health?

Indicators provide key information on changes that are occurring in our environment, like increasing temperatures or more severe extreme weather events, which can threaten people's health. Tracking changes in climate impacts and exposures improves understanding of changes in health risk, even if the actual health outcome is difficult to quantify. For example, while only some indicators exist for human health outcomes related to climate change, like West Nile Virus, Lyme Disease, Heat-Related Deaths, and Cold-Related Deaths, other indicators—like Ragweed Pollen Season, Heat Waves, or Wildfires—give insight into the changing risks to human health. There are many factors other than climate that affect human health; these are discussed in more detail in EPA's overview of the connections between climate change and human health.

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How do I use this resource? 

EPA's climate change indicators are grouped into six categories:

Each indicator can be accessed through the links above or directly from the home page under "Discover Climate Change Indicators." Within each indicator, you will see:

  • Graphics and maps: One or more graphics depicting changes over time. Beneath each graphic, readers will find an explanatory caption, a data source citation, the date when the most recent data update was published on EPA’s website, and one or more of the following buttons:
    • spreadsheet icon  A downloadable .csv file, which can be opened in programs such as Excel. This file provides all the numbers depicted in the graphic.
    • Downloadable Image Icon  A downloadable high-resolution version of the figure, including the title and a full data source citation. This version may be helpful for those who wish to share or reuse a figure (see recommended citation).
    • KMZ Icon  A link to an interactive map with clickable features.
    • image of binoculars  A link to the Climate Indicators Explorer.
  • Key points: Key points about what the indicator shows.
  • Background: Background on how the indicator relates to climate change.
  • About the indicator: A description of the how the indicator was developed.
  • About the data: Information about the data used to produce the indicator, including data sources and notes about factors that may influence how one interprets the data.
  • Technical documentation: A link to comprehensive documentation that describes the data sources and analytical methods for the indicator.

EPA has added several interactive features, including links to interactive maps KMZ Icon; clickable maps and graphs embedded directly in a few indicators; and the Climate Indicators Explorer, an interactive data viewer. 

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How do I cite this resource?

EPA recommends the following citation for this resource and for individual indicators:

U.S. Environmental Protection Agency. Climate Change Indicators in the United States. Accessed [Month Year]. www.epa.gov/climate-indicators.

U.S. Environmental Protection Agency. Climate Change Indicators in the United States. [Name of Indicator]. Accessed [Month Year]. [URL of Indicator].

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Where can I find downloads of past indicator reports and content?

Visit EPA's downloads page for PDF versions of previous indicator reports, along with related fact sheets and brochures. EPA has published several printed summary editions of Climate Change Indicators in the United States, starting in 2010, then in 2012, 2014, and most recently, 2016.

The online resource is updated periodically as new data become available, and thus may differ from the most recent reports and content.

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How do EPA's climate indicators relate to the USGCRP, the National Climate Assessment, and other federal indicator efforts?

EPA's indicators are an example of a well-established, nationally relevant set of indicators representative of data from several of the federal agencies participating in the U.S. Global Change Research Program (USGCRP). EPA works closely with several of these agencies (e.g., Centers for Disease Control and Prevention, National Aeronautics and Space Administration, National Oceanic and Atmospheric Administration, U.S. Geological Survey, and U.S. Department of Agriculture) and serves as a co-chair of USGCRP’s Interagency Indicators Workgroup. EPA’s climate indicators are leveraged as scientific input to the federal interagency effort. EPA will continue to work in partnership with coordinating bodies such as the USGCRP and with other agencies, organizations, and individuals to collect and communicate useful data and to inform policies and programs based on this knowledge. For example, EPA collaborated with the U.S. Forest Service to develop a StoryMap showing indicators that help the user explore the causes and effects of climate change. 

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EPA's Approach to Developing Indicators

How does EPA identify candidate indicators? 

Identification of candidate indicators is a function of the criteria used to evaluate them as well as the need to transparently document the underlying data and methods. EPA screens and selects each indicator using a standard set of criteria that consider data availability and quality, transparency of the analytical methods, and the indicator’s relevance to climate change. This process ensures that all indicators selected are consistently evaluated, are based on credible data, and can be transparently documented. EPA considers candidate indicators through coordinated outreach, stakeholder engagement, and reviewing the latest scientific literature.

Key considerations for new indicators include: 1) filling gaps in the existing indicator set in an attempt to be more comprehensive; 2) newly available, or in some cases improved, data sources that have been peer-reviewed and are publicly available from government agencies, academic institutions, and other organizations; 3) analytical development of indicators resulting from existing partnerships and collaborative efforts within and external to EPA (e.g., development of streamflow metrics in partnership with the U.S. Geological Survey for the benefit of the partner agencies as well as key programs within EPA’s Office of Water); and 4) indicators that communicate key aspects of climate change and that are understandable to various audiences, including the general public. 

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What criteria does EPA use to evaluate candidate indicators?

EPA chooses indicators that meet a set of 10 criteria that consider data quality, transparency of analytical methods, and relevance to climate change. Based on the availability of these data, some indicators present a single measure or variable while others have multiple measures, reflecting different data sources or different ways to group, characterize, or zoom in on the data.

The criteria EPA uses to select indicators are:

  1. Trends over time: Data are available to show trends over time. Ideally, these data will be long-term, covering enough years to support climatically relevant conclusions. Data collection must be comparable across time and space. Indicator trends have appropriate resolution for the data type.
  2. Actual observations: The data consist of actual measurements (observations) or derivations thereof. These measurements are representative of the target population.
  3. Broad geographic coverage: Indicator data are national in scale or have national significance. The spatial scale is adequately supported with data that are representative of the region/area.
  4. Peer-reviewed data (peer-review status of indicator and quality of underlying source data): Indicator and underlying data are sound. The data are credible, reliable, and have been peer-reviewed and published.
  5. Uncertainty: Information on sources of uncertainty is available. Variability and limitations of the indicator are understood and have been evaluated.
  6. Usefulness: The indicator informs issues of national importance and addresses issues important to human or natural systems. It complements existing indicators.
  7. Connection to climate change: The relationship between the indicator and climate change is supported by published, peer-reviewed science and data. A climate signal is evident among stressors, even if the indicator itself does not yet show a climate signal. The relationship to climate change is easily explained.
  8. Transparent, reproducible, and objective: The data and analysis are scientifically objective, and methods are transparent. Biases, if known, are documented, minimal, or judged to be reasonable.
  9. Understandable to the public: The data provide a straightforward depiction of observations and are understandable to the average reader.
  10. Feasible to construct: The indicator can be constructed or reproduced within a reasonable timeframe. Data sources allow routine updates of the indicator.

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How does EPA screen candidate indicators?

EPA researches, screens, and selects indicators based on an objective, transparent process that considers the scientific integrity of each candidate indicator, the availability of data, and the value of including the candidate indicator. EPA conducts the screening process in two stages. As an initial screen, each candidate indicator is evaluated against five of 10 criteria to assess whether or not it is reasonable to further evaluate and screen the indicator. These “Tier 1” criteria include peer-review status of the data, feasibility to construct, usefulness, understandability to the public, and connection to climate change. Indicators that reasonably meet these criteria are researched further; indicators that do not meet these criteria are eliminated from consideration. Some of the candidate indicators ruled out at this stage include ideas that could be viable indicators in the future (e.g., indicators that do not yet have published data or need further investigation into methods).

Indicators deemed appropriate for additional screening are assessed against the Tier 2 criteria: transparency, reproducibility, and objectivity, broad geographic range, actual observations, trends over time, and uncertainty. Based on the findings from the complete set of 10 criteria, the indicators are again evaluated based on the assessment of the remaining criteria.

The distinction between Tier 1 and Tier 2 criteria is not intended to suggest that one group is necessarily more important than the other. Rather, EPA determined that a reasonable approach was to consider which criteria must be met before proceeding further and to narrow the list of indicator candidates before the remaining criteria were applied

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How does EPA develop indicators?

Based on the results of the screening process, the most promising indicators are developed into proposed indicator summaries. EPA consults the published literature, subject matter experts, and online databases to obtain data for each of these indicators. Upon acquiring sound data and technical documentation, EPA prepares a set of possible graphics for each indicator, along with a summary table that describes the proposed metric(s), data sources, limitations, and other relevant information. Summary information is reviewed by EPA technical staff, and then the indicator concepts that meet the screening criteria are formally approved to begin development.

Graphics, summary text, and technical documentation for all proposed new or revised indicators are developed in accordance with the format established for the original 24 indicators in the 2010 indicator report. An additional priority for development is to make sure that each indicator communicates effectively to a non-technical audience without misrepresenting the underlying data and source(s) of information. Regional features (e.g., Community Connections) are developed in the same manner.

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How are EPA's indicators reviewed?

The complete indicator packages (graphics, summary text, and technical documentation) undergo EPA internal review, data provider/collaborator review, and an independent peer review.

Indicators are reviewed at various stages of development by EPA technical staff and various levels of management within the Agency. Organizations and individuals who collected and/or compiled the data (e.g., the National Oceanic and Atmospheric Administration and the U.S. Geological Survey) review the relevant indicators. EPA also enlists technical experts to assist in review of new indicators or revisions.

Each new indicator also undergoes independent peer review by subject-matter experts external to EPA. EPA’s indicator compilation reports (i.e., the 2010, 2012, 2014, and 2016 print editions) have also gone through independent external peer review. EPA follows the procedures in EPA’s Peer Review Handbook (the most recent edition is the fourth, EPA/100/B-15/001) for reports that do not provide influential scientific information. The reviews are managed by a contractor under the direction of a designated EPA peer review leader, who prepares a peer-review plan, the scope of work for the review contract, and the charge for the reviewers. The peer review leader plays no role in producing the indicators or reports.

EPA’s peer-review process also includes a quality-control check by the peer review leader to ensure that the authors took sufficient action and provided an adequate response for every peer review and re-review comment.

Where can I find documentation for each indicator?

EPA's climate change indicators and the accompanying technical documentation are designed to ensure that the science and underlying peer-reviewed data are presented and documented transparently. Once published on EPA’s website, each indicator includes a link to technical documentation that describes the data sources and analytical methods for the indicator and addresses these 13 elements:

  1. Indicator description
  2. Revision history          
  3. Data sources
  4. Data availability
  5. Data collection (methods)
  6. Indicator derivation (calculation steps)
  7. Quality assurance and quality control
  8. Comparability over time and space
  9. Data limitations
  10. Sources of uncertainty (and quantitative estimates, if available)
  11. Sources of variability (and quantitative estimates, if available)
  12. Statistical/trend analysis (if any has been conducted)
  13. References

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Are EPA’s existing indicators ever revised?

Existing indicators are re-evaluated to ensure they are relevant, comprehensive, and sustainable. The process of re-evaluating indicators includes monitoring the availability of newer data, eliciting expert review, and assessing indicators in light of new science. EPA improves existing indicators by adding or replacing metrics or underlying data sources. These revisions involve obtaining new data sets and vetting their scientific validity. For example, a new independent data set and analysis has been made available related to trends in ocean heat content. As a result, EPA updated the Ocean Heat indicator to include this new data series, making the indicator more comprehensive. 

EPA determined that the underlying methods for developing the Plant Hardiness Zone indicator that appeared in the first edition of Climate Change Indicators in the United States (April 2010) had significantly changed, such that updates to the indicator are no longer possible. Therefore, EPA removed this indicator in 2012. EPA will revisit this indicator in the future if USDA releases new editions of USDA’s Plant Hardiness Zone Map that allow users to examine changes over time. For more information about the 2012 map, see: https://planthardiness.ars.usda.gov/PHZMWeb. The original version of this indicator can be found in EPA's 2010 report

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