Which unit is used for radiative forcing in climate science?

Radiative forcing is measured in watts per square metre. It describes a change in the balance between incoming and outgoing energy in the climate system, commonly written as W/m².

The measure is a flux: watts indicate energy transferred per unit time, while dividing by square metres expresses that flow over an area. Scientists use it to compare influences such as rising greenhouse-gas concentrations, aerosols, volcanic eruptions and changes in solar output.

A positive forcing generally contributes to warming, while a negative forcing contributes to cooling. Carbon dioxide produces positive forcing because it absorbs outgoing infrared radiation, although clouds, water vapour, ice and other feedbacks affect the eventual temperature response.

A common mix-up is to give a concentration unit such as parts per million. Ppm measures how much of a gas is present; watts per square metre measures its energy effect. Radiative forcing is also not the same as temperature change: the forcing helps drive warming, but climate sensitivity determines how much temperatures respond.