The change in net irradiance at the top of the atmosphere due to an external factor like CO2, altering the energy balance.Radiative forcing is a measure of how much an external factor—such as increased carbon dioxide, changes in solar output, or volcanic aerosols—disturbs the Earth's energy balance. It is defined as the change in net irradiance (incoming sunlight minus outgoing heat radiation) at the top of the atmosphere, measured in watts per square meter. A positive forcing warms the planet, while a negative forcing cools it. This concept is central to climate science because it provides a common yardstick for comparing the warming or cooling influence of different natural and human-made agents.
The term was formalized in the 1970s and 1980s, with key contributions from scientists like Syukuro Manabe and James Hansen, who used it to quantify the climate impact of greenhouse gases in early climate models.
When a factor like CO2 increases, it absorbs more outgoing infrared radiation that would otherwise escape to space. This reduces the net outgoing energy at the top of the atmosphere, creating an energy surplus—a positive radiative forcing. The climate system then responds by warming until a new equilibrium is reached, with the surface and atmosphere radiating enough extra heat to balance the books again. The forcing value is typically calculated for a specific change (e.g., doubling CO2) using radiative transfer models, isolating the immediate effect before any climate feedbacks like ice-albedo or water vapor changes kick in.
Radiative forcing is the standard metric in IPCC reports for comparing the climate impact of different greenhouse gases, aerosols, and land-use changes, informing policy targets like the Paris Agreement.
The radiative forcing from all human-added greenhouse gases since pre-industrial times is about 3 watts per square meter—roughly the energy of one small LED bulb per square meter of Earth's surface.
