forcings

Forcings are external influences that perturb a system’s energy balance. In climate science, radiative forcing refers to the change in the net downward radiative flux at the top of the atmosphere or tropopause caused by a perturbation, measured in watts per square meter (W/m^2). Positive forcing tends to warm the climate system, while negative forcing tends to cool it. Radiative forcing provides a standardized way to compare different drivers of climate change and to separate external drivers from internal variability and feedbacks.

Major forcings include both natural and human-caused sources. Well-mixed greenhouse gases such as carbon dioxide, methane, and nitrous oxide produce positive radiative forcing by absorbing infrared radiation. Aerosols have mixed effects: sulfate and other reflective aerosols typically produce negative forcing by reflecting sunlight, while black carbon can produce positive forcing by absorbing sunlight, with effects that depend on altitude and surface properties. Land-use changes can alter surface albedo, producing either positive or negative forcing. Solar forcing arises from variations in solar irradiance and is generally smaller on decadal timescales. Volcanic eruptions inject aerosol particles into the stratosphere, causing short-term negative forcing by increasing Earth's reflectivity.

Forcings are used in climate models and attribution studies to estimate past and future climate responses. The concept distinguishes instantaneous radiative forcing from effective forcing, which accounts for rapid atmospheric adjustments and feedbacks that occur after the initial perturbation. Uncertainties remain, especially regarding aerosol properties, cloud responses, and regional impacts, which complicate precise quantification of the net forcing and the resulting climate change.