Radiative forcing

Radiative forcing or climate forcing is the difference between insolation (sunlight) absorbed by the Earth and energy radiated back to space. The influences that cause changes to the Earth's climate system altering Earth's radiative equilibrium, forcing temperatures to rise or fall, are called climate forcings. Positive radiative forcing means Earth receives more incoming energy from sunlight than it radiates to space. This net gain of energy will cause warming. Conversely, negative radiative forcing means that Earth loses more energy to space than it receives from the sun, which produces cooling.'Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. In this report radiative forcing values are for changes relative to preindustrial conditions defined at 1750 and are expressed in Watts per square meter (W/m2).' Radiative forcing or climate forcing is the difference between insolation (sunlight) absorbed by the Earth and energy radiated back to space. The influences that cause changes to the Earth's climate system altering Earth's radiative equilibrium, forcing temperatures to rise or fall, are called climate forcings. Positive radiative forcing means Earth receives more incoming energy from sunlight than it radiates to space. This net gain of energy will cause warming. Conversely, negative radiative forcing means that Earth loses more energy to space than it receives from the sun, which produces cooling. Typically, radiative forcing is quantified at the tropopause or at the top of the atmosphere (often accounting for rapid adjustments in temperature) in units of watts per square meter of the Earth's surface. Positive forcing (incoming energy exceeding outgoing energy) warms the system, while negative forcing (outgoing energy exceeding incoming energy) cools it. Causes of radiative forcing include changes in insolation and the concentrations of radiatively active gases, commonly known as greenhouse gases, and aerosols. Almost all of the energy that affects Earth's climate is received as radiant energy from the Sun. The planet and its atmosphere absorb and reflect some of the energy, while long-wave energy is radiated back into space. The balance between absorbed and radiated energy determines the average global temperature. Because the atmosphere absorbs some of the re-radiated long-wave energy, the planet is warmer than it would be in the absence of the atmosphere: see greenhouse effect. The radiation balance is altered by such factors as the intensity of solar energy, reflectivity of clouds or gases, absorption by various greenhouse gases or surfaces and heat emission by various materials. Any such alteration is a radiative forcing, and changes the balance. This happens continuously as sunlight hits the surface, clouds and aerosols form, the concentrations of atmospheric gases vary and seasons alter the groundcover. The Intergovernmental Panel on Climate Change (IPCC) AR4 report defines radiative forcings as: In simple terms, radiative forcing is '...the rate of energy change per unit area of the globe as measured at the top of the atmosphere.' In the context of climate change, the term 'forcing' is restricted to changes in the radiation balance of the surface-troposphere system imposed by external factors, with no changes in stratospheric dynamics, no surface and tropospheric feedbacks in operation (i.e., no secondary effects induced because of changes in tropospheric motions or its thermodynamic state), and no dynamically induced changes in the amount and distribution of atmospheric water (vapour, liquid, and solid forms). Radiative forcing can be used to estimate a subsequent change in steady-state (often denoted 'equilibrium') surface temperature (ΔTs) arising from that forcing via the equation: where λ is commonly denoted the climate sensitivity parameter, usually with units K/(W/m2), and ΔF is the radiative forcing in W/m2. A typical value of λ, 0.8 K/(W/m2), gives an increase in global temperature of about 1.6 K above the 1750 reference temperature due to the increase in CO2 over that time (278 to 405 ppm, for a forcing of 2.0 W/m2), and predicts a further warming of 1.4 K above present temperatures if the CO2 mixing ratio in the atmosphere were to become double its pre-industrial value; both of these calculations assume no other forcings. Radiative forcing (measured in watts per square meter) can be estimated in different ways for different components. For solar irradiance (i.e. 'solar forcing'), the radiative forcing is simply the change in the average amount of solar energy absorbed per square meter of the Earth's area. Approximating the Earth as a sphere, the Earth's cross-sectional area exposed to the Sun ( π r 2 { extstyle pi r^{2}} ) is equal to 1/4 of the surface area of the Earth ( 4 π r 2 { extstyle 4pi r^{2}} ), the solar input per unit area is one quarter the change in solar intensity. Since some radiation is reflected, this must be multiplied by the fraction of incident sunlight that is absorbed, F = ( 1 − R ) { extstyle F=(1-R)} , where R is the reflectivity (albedo) of the Earth —approximately 0.3, so F is approximately equal to 0.7. Thus, the solar forcing is the change in the solar intensity divided by 4 and multiplied by 0.7.