CLIMATE AND CLIMATE CHANGE | Greenhouse Effect
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2015 Elsevier Ltd. All rights reserved. The Earth's climate is determined by a balance between the rate of radiation energy from Earth to space and by the rate of its receipt and absorption from the Sun. The radiation rate to space increases rapidly with temperature, so that if the rate of heating exceeds that of cooling to space, the planetary temperature will rise leading to increased radiation emitted until the balance is restored. If there is a deficit, the reverse will happen. When greenhouse gases such as CO2, CH4, and H2O are in the air, they cause the effective altitude of emission to space to be higher in the atmosphere where it is colder. This effect reduces the radiation to space for a given surface temperature. This reduction of emitted radiation means the planetary temperature will increase until balance is reestablished. This article addresses the question of the response of the surface temperature field to a doubling of CO2, for the case where no feedback mechanisms are operating. The value found is 1.0 0.1C, a value often quoted in the literature. Various feedback mechanisms can multiply or diminish the response. These include positive feedback mechanisms such as water vapor, ice-albedo; negative feedback mechanisms include lapse rate. Cloud feedback is not yet fully understood, but the balance of evidence suggests that it is positive.
