Climate can be defined as how weather varies over multiple decades and longer periods. It includes averages, variations, and extremes in such quantities as temperature, precipitation, and cloudiness. Earth’s climate history contains evidence of variations in climate over a wide range of time scales. To understand how humans are changing the climate, we must understand the various factors that influence the climate system.
Basic physics tells us that energy is conserved. Consequently, Earth’s average temperature is controlled by the balance between incoming and outgoing energy at the top of the atmosphere and by the amount of greenhouse gases in the atmosphere. The temperature of the atmosphere is controlled by the balance between incoming energy, which arrives from the Sun primarily in the form of visible light, and outgoing energy that Earth emits back to space, mainly as infrared radiation (heat). The difference in temperature between the atmosphere and the surface is significantly influenced by the amount of greenhouse gases in the atmosphere – more greenhouse gases make it harder for heat to escape from the surface to space, and so more greenhouse gases lead to a warmer surface.
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