Earth's Energy Balance.: Numbers are in watts per square meter of Earth's surface, and some may be uncertain by as much as 20%. Source: Trenberth (2004)

The climate of Earth is determined by the balance between energy entering and leaving the atmosphere.  The temperature of the Earth is a simple function of its energy balance: the quantity of energy reaching the planet relative to the quantity exiting the planet. The Earth's energy balance distributes incoming solar radiation in such as way to produce an average annual temperature of nearly 15 °C (59 °F), ideal for life to flourish. Among many other vital functions, the Earth’s energy balance heats the to temperatures far above the minus 270°C (254 °F) of deep space.

The Earth intercepts about 1.7 x 10¹⁷ W of solar energy. By comparison, the capacity of all the electric power generation stations in the world is about 3.7 terawatts (TW), or 3.7 x 10¹² watts. Thus, the Sun delivers nearly 46,000 times more energy than the world’s electric power consumption. About 30% of incoming solar energy is scattered or reflected back to space by the atmosphere; the remainder is is absorbed by the Earth's surface and atmosphere. All heated objects release electromagnetic radiation, especially so if, like, the Earth, they are surrounded by empty space. This energy is referred to as outgoing radiation. An object will continue to warm if the incoming radiation exceeds the outgoing radiation.

If the Earth were a blackbody (a body that absorbs all radiation that falls on it), its equilibrium temperature would be -18 °C. In reality, the Earth is much warmer than that because "greenhouse gases" such as carbon dioxide and water vapor absorb some of the outgoing radiation, thereby warming the atmosphere, analogous to the effects of a greenhouse. Without the greenhouse effect, the Earth would be 33 ºC cooler; that is, the average temperature of the Earth would be about -18 ºC as opposed to 15 ºC. Note that the greenhouse effect does not put the Earth’s energy budget “out of balance.” The heat is held temporarily in the atmosphere, and then eventually is released from the atmosphere as long wavelength infrared energy.

Global surface air temperature 1900-200, with projections to 2100. The various projections for future increase are based on different assumptions about future economic growth, government policies, and technological change. Source: IPCC. The global average air temperature near the Earth's surface rose about 0.74 °C (1.33 °F) during the hundred years ending in 2005. The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations" via the greenhouse effect. Human activity such as land use change and the combustion of fossil fuels release greenhouse gases and affect the amount of energy reflected by the planet. The future rate and magnitude of climate change thus depends in part on economic growth, people's behavior, technological change, government polices and other forces that determine energy use. Understanding the important details of this complex system is one of the great challenges facing the world’s scientific community.

Sources

• Pidwirny, Michael (Lead Author); Dagmar Budikova (Topic Editor). 2008. Earth's energy balance. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth October 18, 2006; Last revised July 10, 2008; Retrieved June 17, 2009].
• Ritter, Michael E. Radiation and energy balance of the earth system, In: The Physical Environment: an Introduction to Physical Geography, Accessed 18 March 2008.
• Trenberth, Kevin E. Earth's Energy Balance. In: Cutler J. Cleveland, Editor(s)-in-Chief, Encyclopedia of Energy, Elsevier, New York, 2004, Pages 859-870.