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Earth receives energy from the Sun, mostly in the form of light, some of which is absorbed and warms the planet, causing it to radiate energy in the form of heat, or infrared radiation, and resulting in a balance between incoming and outgoing energy. Various factors affect the amount of sunlight absorbed and the rate at which energy is radiated by the Earth. When these factors remain constant over a period, the energy flows can be expected to settle into equilibrium at a particular average annual temperature, with the same amount of energy going out as coming in. If any of these factors change, it might result in a mismatch between incoming and outgoing energy, leading to an overall increase or decrease in global average temperatures. A general definition of radiative forcing is the degree of change, positive or negative, to this balance, and it is normally expressed in Watts per square meter (W/m2).
In the context of climate change, a more specific definition of radiative forcing — agreed upon by the Intergovernmental Panel on Climate Change (IPCC) — is the extent to which a factor alters the energy balance in the troposphere, the lowest level of the atmosphere, where almost all our weather takes place. According to the IPCC, using 1750 as a baseline date representative of pre-industrial times, the overall radiative forcing value was estimated to be +1.6 W/m2 as of 2007. The factors affecting energy balance may be natural or man-made. Natural factors include variations in the Sun’s energy output and dust in the atmosphere produced by volcanic eruptions. It is, however, the man-made factors that are of most concern: there is widespread agreement that human activities are contributing to positive radiative forcing, leading to an overall global increase in temperatures.
The burning of fossil fuels since the industrial revolution has increased the amounts of certain gases, most notably carbon dioxide (CO2), and aerosols, such as smoke and soot particles, in the atmosphere. The effects of CO2 are well understood. It is essentially transparent to sunlight, but absorbs infrared, so that while it allows the Sun’s energy in, it hinders the outward radiation of heat, resulting in positive radiative forcing. Atmospheric CO2 levels are estimated to have risen from about 270 parts per million (ppm) in pre-industrial times to nearly 390 ppm in 2010.
Aerosol radiative forcing is more difficult to quantify, as different aerosols vary in their transparency, reflectivity and absorption with regard to light and heat. As a general rule, soot and smoke particles will tend to absorb heat and contribute to positive radiative forcing, while more reflective aerosols such as sulfates, which result from the burning of fuels containing sulfur, might have a negative effect. Estimates of aerosol effects are complicated by the fact that they might also reduce the amount of sunlight reaching the surface.