The rate of solar energy striking Earth averages 168 watts per square meter. The rate of energy radiated from Earth's surface averages 390 watts per square meter. Comparing these numbers, one might expect that the planet would cool quickly, yet it does not. Why not?

Key Concepts

Energy Balance

Energy balance refers to the equilibrium between the energy received from the sun and the energy radiated back into space. The Earth absorbs solar energy and, through various processes, re-emits energy as infrared radiation. If the incoming energy exceeds the outgoing energy, the planet warms; if the outgoing energy exceeds the incoming energy, it cools. Understanding this balance is crucial to grasp why Earth maintains a relatively stable temperature despite the apparent discrepancy in energy rates.

Greenhouse Effect

The greenhouse effect is a natural process where certain gases in Earth's atmosphere trap heat, preventing it from escaping into space. This effect is primarily due to water vapor, carbon dioxide, and methane, which absorb and re-radiate infrared radiation emitted by the Earth's surface. This process helps to maintain a stable and habitable climate, counteracting the cooling that would occur if all emitted energy were lost to space.

Heat Capacity

Heat capacity is the amount of heat energy required to change the temperature of a substance. Earth's surface and atmosphere have significant heat capacities, meaning they can absorb and store heat without a rapid increase in temperature. This property allows the planet to moderate temperature changes over time, contributing to a stable climate despite fluctuations in energy input and output.