E1 Planetary Energy Balance

Topic

The planetary energy balance describes the thermodynamic equilibrium between the solar energy Earth absorbs and the infrared energy it emits back into space; as long as absorbed solar energy and emitted infrared energy remain equal, the planet's temperature stays stable, and this planetary energy balance forms the foundation of all climate physics. When absorbed solar energy and emitted infrared energy cease to be equal, an Earth energy imbalance arises; this imbalance forces the climate system to warm or cool until the absorbed solar energy and emitted infrared energy once again match and the planetary energy balance is restored.

The infrared energy a body emits depends on its temperature according to a fundamental physical relationship: the power radiated per unit area increases with the fourth power of the radiating body's absolute temperature, meaning that a small rise in temperature produces a much larger—and non-proportional—increase in radiated power. Applied to Earth within the context of the planetary energy balance, this same relationship allows for the calculation of the effective emission temperature—approximately 255 Kelvin. This is the temperature Earth would have if it emitted into space all the infrared energy needed to match the absorbed solar energy, without any greenhouse effect trapping a portion of that infrared energy along the way.

However, the effective emission temperature is not the actual temperature of Earth's surface; the actual surface temperature is around 288 Kelvin—considerably higher than the effective emission temperature of approximately 255 Kelvin. The difference between the actual surface temperature and the effective emission temperature precisely quantifies the natural greenhouse effect: the greater this difference, the more infrared energy the atmosphere retains before allowing it to escape into space, and the more effective the natural greenhouse effect becomes in keeping Earth's surface warmer than it would otherwise be.

The planetary energy balance is not distributed evenly across Earth's surface: the tropics receive more solar energy than they emit as infrared energy—representing a tropical energy surplus—whereas polar regions emit more infrared energy than they receive as solar energy—representing a polar energy deficit. This tropical energy surplus and polar energy deficit constitute the fundamental driving force behind all atmospheric and oceanic circulation; ultimately, both systems of circulation exist to transport excess tropical energy toward the energy-deficient polar regions, thereby compensating—at least partially—for this latitudinal imbalance in the planetary energy balance.

The current terrestrial energy imbalance—approximately nine-tenths of a watt per square meter—is the clearest indication that Earth's planetary energy balance is currently being forced and is not in equilibrium: the solar energy absorbed by Earth exceeds the infrared energy emitted back into space by that amount. As long as this terrestrial energy imbalance persists, the climate system will continue to warm in its attempt to restore the planetary energy balance mentioned at the beginning of this description.

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