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Global warming refers to the rising average temperature of Earth's atmosphere and oceans, which started to increase in the late 19th century and is projected to keep going up. Since the early 20thcentury, Earth's average surface temperature has increased by about 0.8 °C (1.4 °F), with about two thirds of the increase occurring since 1980.[2] Warming of the climate system is unequivocal, andscientists are more than 90% certain that most of it is caused by increasing concentrations of greenhouse gases produced by human activities such as deforestation and burning fossil fuels

An increasein global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, and a probable expansion of subtropical deserts.

Evidence for warming of the climatesystem includes observed increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level

Orbital cycles vary slowly over tens ofthousands of years and at present are in an overall cooling trend which would be expected to lead towards an ice age, but the 20th century instrumental temperature record shows a sudden rise in globaltemperatures

The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in the atmosphere warm a planet's lower atmosphere and surface. It was proposed by JosephFourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, wasobserved from 1961 until at least 1990

Radiative forcing due to particulates is temporally limited due to wet deposition which causes them to have an atmospheric lifetime of one week. Carbon dioxide hasa lifetime of a century or more, and as such, changes in particulate concentrations will only delay climate changes due to carbon dioxide

In addition to their direct effect by scattering and...