Climate forcing

Climate forcing is an imposed change in Earth’s energy budget that shifts the climate away from its previous state. Incoming solar radiation and outgoing infrared radiation normally balance over long periods; a forcing disturbs that balance, producing eventual changes in temperature and other climate properties.¹ Forcing is commonly expressed in watts per square metre and is evaluated relative to a specified baseline, often a preindustrial period.

Positive forcing increases net energy retention and generally causes warming, whereas negative forcing reduces it and generally causes cooling. The initial imbalance is not the same as the final temperature response: oceans absorb heat, ice and snow change reflectivity, and atmospheric circulation redistributes energy. These responses are climate feedbacks, not usually the original forcing.

Human-produced greenhouse gases are the dominant positive forcing behind modern global warming. Carbon dioxide, methane, nitrous oxide, and halogenated gases absorb outgoing infrared radiation, while changes in land cover can alter surface reflectivity and evapotranspiration.² Human-produced aerosols generally exert a negative forcing by scattering sunlight and modifying clouds, although their effects vary by particle type, location, and atmospheric conditions.

Natural forcings include volcanic eruptions, which can inject reflective sulfate aerosols into the stratosphere and cause temporary global cooling, and variations in solar output. Orbital changes operate over thousands to hundreds of thousands of years and help pace ice-age cycles. Internal variability, such as El Niño, redistributes heat but is usually classified as variability rather than an external forcing.

Radiative forcing is estimated by calculating how a perturbation changes radiative fluxes at the top of the atmosphere or at a specified atmospheric level. The result depends on the chosen definition: instantaneous forcing, stratospherically adjusted forcing, and effective radiative forcing include progressively more rapid atmospheric adjustments.³ For this reason, numerical values from different studies are not always directly interchangeable.

Climate models combine forcing estimates with observations to attribute observed warming. The distinctive vertical and geographic patterns associated with greenhouse gases, aerosols, volcanic events, and solar changes provide evidence that recent warming cannot be explained by solar variability alone.¹ Carbon dioxide forcing is approximately logarithmic with concentration, so each additional equal increment has a smaller direct radiative effect than the preceding one, although warming-related feedbacks amplify the overall response.⁴

Forcing can be geographically uneven even when its global average is modest. Aerosols concentrated over industrial regions can produce strong regional cooling, alter monsoon rainfall, and partially mask greenhouse-gas warming; their short atmospheric lifetimes also make their effects more localized than those of carbon dioxide.

Some forcings operate through unusual pathways. Aviation-induced contrails and their spreading cirrus clouds affect infrared and solar radiation, while black carbon deposited on snow and ice lowers reflectivity and accelerates melting. Land-use change can produce both warming and cooling through albedo, moisture, roughness, and carbon-cycle effects. A forcing may therefore have different sign or strength at regional and global scales. The climate response also continues after a forcing stabilizes because the deep ocean exchanges heat slowly with the atmosphere.

Radiative forcing

A change in net radiative energy flow caused by a perturbation to the climate system, usually measured in watts per square metre.

Effective radiative forcing

A forcing measure that includes rapid adjustments in the atmosphere and land surface before the global-mean temperature response develops.

Climate feedback

A process that amplifies or reduces the climate response to an initial forcing, such as the water-vapor or ice-albedo feedback.

Aerosol

A suspension of fine solid or liquid particles in air; aerosols can scatter sunlight, absorb radiation, or modify clouds.