There are many processes inside and outside of the Earth system which affect climate. On the timescales from billion years to decades, the climate is affected by changes in solar luminosity, composition of the atmosphere, volcanic activity, changes in the position of the continents, and variations of the Earth's orbital parameters. Changes in the internal and external (for the Earth system) factors which directly affect climate are often referred to as 'climate forcing'. Climate forcing can be quantified in term of'radiative forcing', defined as an energy imbalance imposed on the climate system by changes of given factor. For example, a doubling of CO2 concentration causes globally averaged imbalance at the top of the troposphere of about 4 W m~2. Radiative forcing is a convenient measure to compare climate impact of different factors, such as changes in concentration of greenhouse gases and aerosols but not all climate forcings can be easily expressed in term of radiative forcing. For example, variations of the Earth orbital parameters cause large changes in seasonal distribution of incoming solar radiation but their globally averaged direct effect on energy balance of the planet is rather small. However, due to a number of strongly nonlinear climate feedbacks associated with the ice sheets and greenhouse gases, the variations in the Earth's orbital parameters caused significant climate changes over the past several millions of years.
While some of climate forcings, such as the Earth orbital parameters or solar luminosity, are external for the Earth system, changes in atmospheric composition on the timescales of thousand years are internal ones and represent internal climate feedbacks to external forcing. For example, the growth of large ice sheets under varying Earth orbital parameters caused widespread cooling, which, in turn, leads to an enhanced carbon uptake by the ocean and a lowering of atmospheric CO2 concentration. This, in turn, cools climate additionally and facilitates further growth of the ice sheets.
Since the last century human activity became an important factor of climate change. The most important anthropogenic climate forcing is the change in atmospheric composition of the so-called greenhouse gases: CO2, CH4, N2O, and others. Apart from that, burning of fossil fuel and forest leads to an increase of atmospheric concentration of several types of aerosols. It is believed that the net effect of anthropogenic aerosols is cooling and hence aerosols partly compensate the warming effect of greenhouse gases. Changes in land cover (land use), primarily via deforestation of the large part of the continents, also affect climate. The direst effect of deforestation is a cooling, but since deforestation also contributes to an increase of CO2 concentration, the sign of temperature changes related to deforestation depends on the regions. Atmospheric pollution by several chemicals also affects tropospheric and stratospheric concentration of ozone. The former is increasing under the influence of anthropogenic factors while the latter is decreasing which contributes to the development of the so-called 'ozone hole' that represents the direct danger for human health.
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