The Earth's climate system is powered by the sun. Our planet intercepts 340 W m-2 of solar radiation averaged over the surface of the globe. About 100 W m-2 is reflected to space, and the remainder-about 240 W m-2-heats the planet. On a global average, the Earth maintains a radiative balance between this solar heating and the cooling from terrestrial infrared radiation that escapes to space. When a particular human activity alters greenhouse gases, particles, or land albedo, such activity results in radiative imbalance. Such an imbalance cannot be maintained for long, and the climate system-primarily the temperature and clouds of the lower atmosphere-adjusts to restore radiative balance. We calculate the global, annual average of radiative imbalance (W m-2) to the atmosphere-land-ocean system caused by anthropogenic perturbations and designate that change radiative forcing.
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