Other meanings of Radiative forcing
Climate science
Radiative forcing is the change in net irradiance caused by an external perturbation of Earth’s climate system. It is expressed in watts per square metre (W m−2) and describes whether a disturbance tends to add energy to or remove energy from the climate system. Positive forcing generally produces warming, while negative forcing generally produces cooling. The concept is used to compare influences such as greenhouse gases, aerosols, land-use change, volcanic eruptions, and variations in solar output. In modern assessments, scientists often use effective radiative forcing (ERF), which includes rapid atmospheric and surface adjustments that occur before global-mean surface temperature responds.1
Radiative forcing measures an energy imbalance imposed on the climate system. In its classical form, it is the change in net downward minus upward irradiance at the top of the atmosphere, or at a specified atmospheric level, after a defined perturbation has been introduced. A positive value means that more energy is retained than before; a negative value means that more energy is lost. The reference period is commonly 1750, chosen as an approximation of preindustrial conditions.1
Forcing is not the same as temperature change. The eventual warming or cooling depends on climate feedbacks involving water vapour, clouds, snow, ice, and the carbon cycle. A forcing can therefore be compared across causes even though each cause produces a different geographical and seasonal pattern of response. The term also excludes the climate response itself: increased evaporation or melting sea ice is generally a feedback, not the original forcing.
Radiative forcing is estimated with observations, laboratory spectroscopy, satellite measurements, and climate models. Atmospheric concentrations are converted into changes in absorption and emission of radiation, while aerosol properties, land reflectivity, clouds, and solar variability are represented in radiation calculations. The result is usually reported as a global annual mean, although forcing can vary sharply by latitude, altitude, season, and surface type.2
The distinction between instantaneous forcing, stratospherically adjusted forcing, and effective radiative forcing reflects different stages of atmospheric response. ERF allows rapid adjustments such as changes in tropospheric temperature, water vapour, clouds, and convection, and is often a better predictor of near-term surface-temperature response than an unadjusted flux change. Uncertainty ranges are essential because aerosol-cloud interactions and land-surface effects remain difficult to constrain.
Human activities have produced a strongly positive net radiative forcing, dominated by long-lived greenhouse gases. The Intergovernmental Panel on Climate Change estimated total anthropogenic effective radiative forcing in 2019 at about 2.72 W m−2 relative to 1750, with carbon dioxide the largest individual contributor.1 Methane, nitrous oxide, halogenated gases, ozone changes, and land-use effects also contribute.
Aerosols generally exert a negative forcing by scattering sunlight and modifying clouds, partly offsetting greenhouse-gas warming, although some absorbing particles produce positive effects. Major volcanic eruptions can cause short-lived negative forcing when sulfur dioxide forms reflective stratospheric sulfate aerosols. Changes in solar irradiance create a smaller but real natural forcing over the approximately 11-year solar cycle and across longer intervals. The uneven spatial pattern of these influences matters for regional climate, even when their global average is modest.
Radiative forcing is a bookkeeping framework, not a single physical location or a direct measure of observed warming. The same global-mean forcing can arise from very different distributions of heating: volcanic aerosols concentrated in the stratosphere, soot deposited on snow, and carbon dioxide mixed through the atmosphere do not produce identical climate responses. For that reason, forcing efficacy and effective radiative forcing are useful when comparing agents whose temperature responses differ from the response to carbon dioxide.
Cloud adjustments are among the most consequential sources of uncertainty, especially for aerosol forcing. Contrails and aviation-induced cirrus are also climate-forcing agents, but their estimates are less certain than those for well-mixed greenhouse gases.3 Historical forcing reconstructions further show that natural variability can temporarily obscure the relationship between a persistent forcing and global surface temperature, particularly after volcanic eruptions.
Forcing values depend on the chosen baseline, altitude or boundary, adjustment method, spatial averaging, and assessment period; quoted estimates should therefore be read with their stated definitions and uncertainty ranges.
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