Radiative Forcing

Radiative forcing is the change in the balance between solar radiation entering the atmosphere and the Earth's radiation going out. On average, a positive radiative forcing tends to warm the surface of the Earth while negative forcing tends to cool the surface.

Radiative forcing is measured in Watts per square meter, which is a measure of energy. For example, an increase in radiative forcing of +1 Watt per square meter is like shining one small holiday tree light bulb over every square meter of the Earth. 

Greenhouse gases have a positive radiative forcing because they absorb and emit heat. Aerosols can have a positive or negative radiative forcing, depending on how they absorb and emit heat and/or reflect light. For example, black carbon aerosols - which have a positive forcing - more effectively absorb and emit heat than sulfates, which have a negative forcing and more effectively reflect light. The following are estimates of the change in radiative forcing in the year 2005 relative to 1750 for different components of the climate (IPCC, 2007):

The radiative forcing contribution (since 1750) from increasing concentrations of well-mixed greenhouse gases (including CO2, CH4, N2O, CFCs, HCFCs, and fluorinated gases) is estimated to be +2.64 Watts per square meter - over half due to increases in CO2 (+1.66 Watts per square meter), strongly contributing to warming relative to other climate components described below.

The radiative forcing contribution from increasing tropospheric ozone, an unevenly distributed greenhouse gas, is estimated to be +0.35 Watts per square meter (on average), resulting in a relatively small warming effect. This forcing varies from region to region depending on the amount of ozone in the troposphere at a particular location. The radiative forcing contribution from the observed depletion of stratospheric ozone is estimated to be -0.05 Watts per square meter, resulting in a relatively small cooling effect.

While aerosols can have either positive or negative contributions to radiative forcing, the net effect of all aerosols added to the atmosphere has likely been negative. The best estimate of aerosols’ direct cooling effect is -0.5 Watts per square meter; the best estimate for their indirect cooling effect (by increasing the reflectivity of clouds) is -0.7 Watts per square meter, with an uncertainty range of -1.8 to -0.3 Watts per square meter. Therefore, the net effect of changes in aerosol radiative forcing has likely resulted in a small to relatively large cooling effect.

Land use change (including urbanization, deforestation, reforestation, desertification, etc) can have significant effects on radiative forcing (and the climate) at the local level by changing the reflectivity of the land surface (or albedo). For example, because farmland is more reflective than forests (which are strong absorbers of heat), replacing forests with farmland would negatively contribute to radiative forcing or have a cooling effect. Averaged over the Earth, the net radiative forcing contribution of land use changes, while uncertain, is estimated to be -0.2 Watts per square meter (IPCC, 2007), resulting in a relatively small cooling effect.

Based on a limited, 25-year record, the effect of changes in the sun's intensity on radiative forcing is estimated to be relatively small, or a contribution of about +0.12 Watts per square meter, resulting in a relatively small warming effect.

NOAA’s Annual Greenhouse Gas Index (AGGI), which tracks changes in radiative forcing from greenhouse gases over time, shows that radiative forcing from greenhouse gases has increased 21.5% since 1990 as of 2006. Much of the increase (63%) has resulted from the contribution of CO2. The contribution to radiative forcing by CH4 and CFCs has been nearly constant or declining, respectively, in recent years.

How Is Radiative Forcing Determined?

For well-mixed greenhouse gases, mathematical equations are used to compute radiative forcing based on changes in their concentration relative to 1750 (or 1990 for NOAA's AGGI) and the known radiative properties of the gases. Confidence in these calculations is high due to reliable current and historic concentration data and well-established physics.

Due to limited measurements and regional variation, changes in tropospheric ozone, aerosols, land use and the sun’s intensity are much more uncertain. In the case of aerosols, uncertainty is increased due to an incomplete understanding of how aerosols interact with clouds and the effects the interactions have on aerosol radiative forcing.

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