Radiative forcing is the degree to which the equilibrium between energy input and output varies, expressed in watts per square meter. Human activities, such as burning fossil fuels, have increased atmospheric CO2 levels, resulting in positive radiative forcing and global temperature increases. Aerosol radiative forcing is more difficult to quantify, but soot and smoke particles contribute positively while reflective aerosols could have a negative effect.
The Earth receives energy from the Sun, mainly 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 energy input and output. Various factors affect the amount of sunlight absorbed and the rate at which energy is radiated from the Earth. When these factors remain constant over a period, energy flows can be expected to settle into equilibrium at a particular annual mean temperature, with the same amount of energy out as in. If any of these factors change, it could result in a mismatch between energy inputs and outputs, leading to an overall increase or decrease in global average temperatures. A general definition of radiative forcing is the degree to which this equilibrium varies, positive or negative, and is usually 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 it plays almost all of our time. According to the IPCC, using 1750 as a baseline date representative of pre-industrial times, the overall value of radiative forcing was estimated to be +1.6 W/m2 in 2007. Factors influencing the energy balance can be natural or caused by man. Natural factors include variations in the sun’s energy output and dust in the atmosphere from volcanic eruptions. However, it is anthropogenic factors that are of greatest concern: there is broad consensus that human activities are contributing to positive radiative forcing, leading to global increases in temperatures.
The burning of fossil fuels since the Industrial Revolution has increased the amount of some gases, particularly carbon dioxide (CO2) and aerosols, such as smoke and soot particles, in the atmosphere. The effects of CO2 are well known. It is essentially transparent to sunlight, but absorbs infrared, so that while it allows in solar energy, it hinders outward heat radiation, resulting in positive radiative forcing. Atmospheric CO2 levels are estimated to have increased 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 respect to light and heat. As a general rule, soot and smoke particles will tend to absorb heat and contribute positive radiative forcing, while more reflective aerosols such as sulphates, which result from the burning of sulfur-containing fuels, could have a negative effect. Estimates of the effects of aerosols are complicated by the fact that they could also reduce the amount of sunlight reaching the surface.
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