Radiative forcing: definition A change in the net radiation budget at the top of the atmosphere, whether due to a change in the net incoming solar radiation, or a change in the emission of infrared radiation is called Radiative Forcing. Radiative forcing perturbs the balance between incoming and outgoing radiation, and the Earth’s climate adjusts to restore the balance. Positive radiative forcing acts to warm the surface, Negative radiative forcing acts to cool the surface.
Radiative forcing has the units of the Solar constant, thus, Watts/m2. • Examples: • Solar constant declines by 1%. This leads to a radiative • forcing of -3.4 W/m2. • CO2 is doubled with respect to pre-industrial • concentrations; this leads to a radiative forcing of • +4 W/m2 (not taking into consideration feedback effects). • Increased aerosol content in the atmosphere will lead • to a negative radiative forcing.
The Radiative forcing of a greenhouse gas is calculated: For greenhouse gases with trace concentrations (CFC’s, etc.): Radiative forcing = (Current concentration – pre-industrial concentration) Radiative efficiency The radiative efficiency is the radiative forcing that would be derived from the increase in the concentration of the gas by 1 ppbv. It depends on the detailed absorption spectrum of the gas.
For non-trace greenhouse gases (CO2, CH4, etc.): The equation to calculate the current radiative forcing is non-linear, as the absorption bands of these gases tend already to be partially saturated. Example: The radiative forcing of CO2 (Hansen et al. 1998): where
Global Warming Potential Is an index that measures the capability of an increase of 1 kg of a given greenhouse gas to contribute to global warming. This index is measured relative to a reference greenhouse gas, generally CO2. It depends not only on the radiative efficiency of the gas, but also on the lifetime of the gas in the atmosphere. The global Warming Potential (GWP) is calculated over a specific time horizon (TH).