Climate Change Indicators in the United States: Climate Forcing
www.epa.gov/climatechange/indicators - Updated June 2015
Climate Forcing
This indicator measures the "radiative forcing" or heating effect caused by greenhouse gases in the
atmosphere.
Background
When energy from the sun reaches the Earth, the planet absorbs some of this energy and radiates the
rest back to space as heat. The Earth's surface temperature depends on this balance between incoming
and outgoing energy. Average conditions tend to remain stable unless the Earth experiences a force that
shifts the energy balance. A shift in the energy balance causes the Earth's average temperature to
become warmer or cooler, leading to a variety of other changes in the lower atmosphere, on land, and
in the oceans.
A variety of physical and chemical changes can affect the global energy balance and force changes in the
Earth's climate. Some of these changes are natural, while others are influenced by humans. These
changes are measured by the amount of warming or cooling they can produce, which is called "radiative
forcing." Changes that have a warming effect are called "positive" forcing, while changes that have a
cooling effect are called "negative" forcing. When positive and negative forces are out of balance, the
result is a change in the Earth's average surface temperature.
Changes in greenhouse gas concentrations in the atmosphere affect radiative forcing (see the
Atmospheric Concentrations of Greenhouse Gases indicator). Greenhouse gases absorb energy that
radiates upward from the Earth's surface, re-emitting heat to the lower atmosphere and warming the
Earth's surface. Human activities have led to increased concentrations of greenhouse gases that can
remain in the atmosphere for decades, centuries, or longer, so the corresponding warming effects will
last for a long time.
About the Indicator
Figure 1 of this indicator measures the average total radiative forcing of 20 long-lived greenhouse gases,
including carbon dioxide, methane, and nitrous oxide. The results were calculated by the National
Oceanic and Atmospheric Administration based on measured concentrations of the gases in the
atmosphere, compared with the concentrations that were present around 1750, before the Industrial
Revolution began. Because each gas has a different ability to absorb and emit energy, this indicator
converts the changes in greenhouse gas concentrations into a measure of the total radiative forcing
(warming effect) caused by each gas. Radiative forcing is calculated in watts per square meter, which
represents the size of the energy imbalance in the atmosphere.
The National Oceanic and Atmospheric Administration also translates the total radiative forcing of these
measured gases into an index value called the Annual Greenhouse Gas Index (right side of Figure 1). This
number compares the radiative forcing for a particular year with the radiative forcing in 1990, which is a
common baseline year for global agreements to track and reduce greenhouse gas emissions.
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Climate Change Indicators in the United States: Climate Forcing
www.epa.gov/climatechange/indicators - Updated June 2015
For reference, this indicator also presents an estimate of the total radiative forcing associated with a
variety of human activities from 1750 to the present. Figure 2 shows the influence of:
• Tropospheric ozone, a short-lived greenhouse gas.
• Emissions that indirectly lead to greenhouse gases through chemical reactions in the
atmosphere. For example, methane emissions also lead to an increase in tropospheric ozone.
• Aerosol pollution, which consists of solid and liquid particles suspended in the air that can
reflect incoming sunlight.
• Black carbon (soot), which can make the Earth's surface darker and less reflective when it is
deposited on snow and ice.
• Several other factors, like land use change, that affect radiative forcing.
Key Points
• In 2014, the Annual Greenhouse Gas Index was 1.36, which represents a 36 percent increase in
radiative forcing (a net warming influence) since 1990 (see Figure 1).
• Of the greenhouse gases shown in Figure 1, carbon dioxide accounts for by far the largest share
of radiative forcing since 1990, and its contribution continues to grow at a steady rate. Carbon
dioxide alone would account for a 28 percent increase in radiative forcing since 1990.
• Although the overall Annual Greenhouse Gas Index continues to rise, the rate of increase has
slowed somewhat overtime. This change has occurred in large part because methane
concentrations have increased at a slower rate in recent years and because chlorofluorocarbon
(CFC) concentrations have been declining, as production of CFCs has been phased out globally
due to the harm they cause to the ozone layer (see Figure 1).
• Greenhouse gases produced by human activities have caused an overall warming influence on
the Earth's climate since 1750. The largest contributor to warming has been carbon dioxide,
followed by methane, and black carbon. Although aerosol pollution and certain other activities
have caused cooling, the net result is that human activities on the whole have warmed the Earth
(see Figure 2).
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Climate Change Indicators in the United States: Climate Forcing
www.epa.gov/climatechange/indicators - Updated June 2015
Figure 1. Radiative Forcing Caused by Major Long-Lived Greenhouse
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Climate Change Indicators in the United States: Climate Forcing
www.epa.gov/climatechange/indicators - Updated June 2015
Figure 2. Radiative Forcing Caused by Human Activities Since 1750
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Cooling Warming
Radiative forcing {watts per square meter)
This figure shows the total amount of radiative forcing caused by human activities—including indirect
effects—between 1750 and 2011. Radiative forcing is calculated in watts per square meter, which
represents the size of the energy imbalance in the atmosphere. Each colored bar represents scientists'
best estimate, while the thin black bars indicate the likely range of possibilities. The natural change in the
energy received from the sun over this time period is provided for reference.
Data source: IPCC, 20132
Indicator Notes
The index in Figure 1 does not include short-lived greenhouse gases like tropospheric ozone, reflective
aerosol particles, black carbon (soot), or the indirect influence of methane through its effects on water
vapor and ozone formation. Figure 2 includes these and other indirect influences.
Data Sources
Data for Figure 1 were provided by the National Oceanic and Atmospheric Administration. This figure
and other information are available at: www.esrl.noaa.gov/gmd/aggi. Data for Figure 2 came from the
Intergovernmental Panel on Climate Change (www.ipcc.ch). which publishes assessment reports based
on the best available climate science data.
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Climate Change Indicators in the United States: Climate Forcing
www.epa.gov/climatechange/indicators - Updated June 2015
1 NOAA (National Oceanic and Atmospheric Administration). 2015. The NOAA Annual Greenhouse Gas Index.
Accessed May 2015. www.esrl.noaa.gov/gmd/aggi.
2 IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis.
Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge
University Press, www.ipcc.ch/report/ar5/wgl.
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