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Features
Earth receives energy from the sun in the form of solar radiation and also reradiates energy into space. Some of the solar energy that strikes Earth is reflected as well. Earth's orbit around the sun is slightly elliptical, so the total amount of solar energy Earth receives varies slightly, and the total solar radiation also varies over long periods of time (i.e., millennia), so when calculating Earth's heat capacity, it's best to work with an average value for solar energy. The value scientists use is called the Solar Constant and is 1,370 watts per square meter, meaning 1,370 joules per second per square meter of area.
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As the temperature of the Earth increases, the amount of energy it radiates back into space also increases. Other factors, such as the amount of greenhouse gas in the atmosphere, can change the rate at which Earth loses energy through radiation, while factors such as the total land and sea area covered by ice can change the amount of solar energy that is reflected from Earth's surface rather than being absorbed.
Considerations
A 2007 paper from Brookhaven National Laboratory estimated a planetary heat capacity of 16.7 plus or minus 7 watts years per square meters Kelvins, where a watt is a joule per second. There is of course some uncertainty in this estimate (as evident from the margin of error), and climate scientists are working to make their calculations as accurate as possible to reduce uncertainty in climate predictions.