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The Sun emits shortwave radiation (visible and UV) that heats Earth's surface, while Earth re-emits longwave infrared radiation back to space. At equilibrium, incoming solar energy absorbed equals outgoing longwave radiation emitted. Albedo — the fraction of incoming sunlight reflected — determines how much energy is absorbed; snow and ice have high albedo, oceans and forests have low albedo. Unequal distribution of solar energy by latitude (tropics receive more than poles) is the primary driver of atmospheric and oceanic circulation.
Work through the planetary energy balance equation quantitatively, then explore how changing albedo shifts equilibrium temperature. Compare Earth's effective radiating temperature (~255 K) to actual surface temperature (~288 K) to motivate the greenhouse effect.
The Sun continuously delivers energy to Earth in the form of shortwave radiation — mostly visible light and ultraviolet, peaking around 0.5 μm. From your study of blackbody radiation, you know that the wavelength of peak emission scales inversely with temperature (Wien's law): the Sun at ~5,778 K peaks in visible light, while Earth at ~288 K peaks around 10 μm in the thermal infrared. These two radiation streams — incoming shortwave, outgoing longwave — are the two sides of Earth's energy budget, and they must balance on a global average if the climate is to remain stable.
Not all incoming solar radiation is absorbed. Albedo quantifies the reflective fraction: snow, ice, and clouds are highly reflective (high albedo); dark oceans and forests absorb most incoming light (low albedo). Earth's global average albedo is about 0.30, meaning 30% of incoming solar radiation is immediately reflected back to space before doing any work. The remaining 70% is absorbed — partly by the atmosphere, mostly by the surface — and must eventually be re-emitted as longwave infrared radiation to close the energy budget.
You can estimate Earth's equilibrium temperature from first principles. The solar constant (power per unit area at Earth's orbit) is S ≈ 1361 W/m². Earth intercepts solar radiation over a disk of area πR², but it emits over its full spherical surface of area 4πR² — a factor-of-four difference. Setting absorbed power equal to emitted power: S/4 × (1 − albedo) = σT⁴ gives T ≈ 255 K. This is the effective radiating temperature. The actual surface is ~288 K, 33 K warmer — the natural greenhouse effect of the atmosphere cycling energy back to the surface.
Solar energy is not delivered uniformly. Because Earth is a sphere, tropical latitudes receive nearly perpendicular (concentrated) solar radiation year-round, while polar regions receive oblique (spread-out) radiation. This latitudinal gradient — tropics absorb more energy than poles — is the primary driver of atmospheric and oceanic circulation. The atmosphere and oceans transport heat poleward, trying to erase the temperature gradient. The Coriolis effect and continental geometry complicate this transport, generating the jet streams, ocean gyres, and climate zones we observe.
A critical distinction: the energy balance equation describes a global average at equilibrium. Regionally and seasonally, imbalances are normal and necessary — they drive weather. A region in summer absorbs more solar energy than it emits, warming up; in winter, the opposite. The global balance holds only when you average across all latitudes and all seasons. Changes to any component — albedo (from ice loss or land use change), greenhouse gas concentrations (affecting longwave emission), or the solar constant (from sunspot cycles) — perturb the balance and force the system to find a new equilibrium temperature.