Radiative forcing is the change in the net energy balance at the top of the atmosphere — the difference between incoming solar radiation and outgoing thermal radiation — caused by changes in atmospheric composition or other external factors. It is measured in watts per square meter (W/m²) and provides a standardized way to compare the climate impact of different factors: greenhouse gases, aerosols, changes in solar output, and land use changes.
Positive radiative forcing warms the planet (more energy in than out); negative forcing cools it. As of the IPCC Sixth Assessment Report (2021), total anthropogenic radiative forcing is approximately +2.72 W/m² relative to pre-industrial levels, dominated by CO₂ (+2.16 W/m²) with significant contributions from methane, nitrous oxide, and halocarbons. Aerosols provide partially offsetting negative forcing (-1.3 W/m²), which means that reducing aerosol pollution without reducing greenhouse gases would produce rapid additional warming.
Radiative forcing translates directly into temperature change through climate sensitivity — the equilibrium temperature response to a doubling of CO₂. Current estimates place climate sensitivity between 2.5°C and 4°C, with 3°C as the central estimate. The lag between forcing and temperature response (due to ocean thermal inertia) means that current forcing commits the planet to warming that has not yet been fully realized.