Earth’s Energy Balance, Insolation, and Albedo

Earth’s energy balance is the balance between the solar energy the planet absorbs and the heat energy it radiates back to space; insolation is the incoming solar radiation reaching a surface, and albedo is the fraction of that radiation that a surface reflects. In brief: Earth receives an average of about 340 W/m² at the top of the atmosphere and reflects about 30% of it (a planetary albedo of about 0.30). Without the greenhouse effect, the average surface temperature would be about −18 °C, but the actual mean of about 15 °C is higher because greenhouse gases absorb and re-emit heat.

The balance between incoming and outgoing energy controls the climate, and any lasting change in it changes the temperature. This guide explains insolation and why it varies, what happens to the energy that reaches Earth, the role of albedo and feedbacks, compares the albedo of surfaces in a table, works through calculations, outlines the history and corrects common mistakes.

What is insolation and why does it vary?

The Sun delivers about 1,361 W/m² to a surface at right angles to the rays at Earth’s distance, a value called the solar constant. Earth is a sphere, so its disc intercepts the radiation (area πR²) while its total surface area is 4πR², which is four times larger. The average over the whole planet is therefore 1,361 ÷ 4 = about 340 W/m².

The amount at any place varies with several factors. The angle of the Sun matters most: near the equator, the rays arrive nearly vertically and are concentrated on a small area, and near the poles, the same energy is spread over a larger area and passes through a longer path in the atmosphere, which absorbs and scatters more of it. Day length changes with the seasons because Earth’s axis is tilted by about 23.4°, so summer hemispheres receive more hours of sunlight. Cloud cover and atmospheric transparency and distance from the Sun also play a role, the latter much more weakly (Earth is about 3% closer to the Sun in early January).

Where does the incoming energy go?

The energy budget can be shown as 100 units of incoming solar energy. About 30 units are reflected back to space: about 20 by clouds, 6 by the air and its particles and 4 by the surface. About 19 units are absorbed by the atmosphere (by ozone, water vapor, clouds and dust), and about 51 units are absorbed by the land and oceans. The warmed surface radiates heat as long-wave (infrared) energy. Greenhouse gases such as water vapor and carbon dioxide absorb much of it and radiate it in all directions, including back to the surface, so the surface is warmer than it would be otherwise. The energy is eventually lost to space, and in a stable climate, outgoing energy equals the absorbed energy, which is the radiative balance. The surface also loses energy by evaporation of water (latent heat) and by direct heating of the air (sensible heat).

What is albedo?

Albedo is the reflected fraction of incoming radiation, given as a number between 0 (perfect absorber) and 1 (perfect reflector), or as a percentage. Fresh snow reflects 80–90% of sunlight, while dark oceans and forests reflect only about 6–15%. Light surfaces keep cool, and dark surfaces warm up. Albedo changes can amplify climate change through feedbacks. In the ice-albedo feedback, warming melts snow and ice and exposes darker land or water, which absorbs more energy, which causes more warming, which is a positive feedback. Changes in cloud cover, vegetation and aerosols (tiny particles from volcanoes and pollution) also alter albedo. Large volcanic eruptions, such as Mount Pinatubo in 1991, put reflective sulfate particles into the stratosphere that cooled the planet by about 0.5 °C for around a year or two.

How does heat move from the tropics to the poles?

The tropics receive more energy than they lose, while the poles lose more than they receive. If there were no transport of heat, the tropics would become steadily hotter and the poles colder. The atmosphere and the oceans move heat poleward: warm air and water flow away from the equator, and cold flow in return. Winds, storms and ocean currents such as the Gulf Stream, together with the evaporation and condensation of water, carry out this redistribution. It is the driving force of the global circulation of the atmosphere and oceans.

Albedo of different surfaces

Worked example: temperature and albedo

1. Average insolation: 1,361 ÷ 4 = 340 W/m² (precisely 340.25).
2. Absorbed energy: with an albedo of 0.30, Earth absorbs (1 − 0.30) × 340.25 = 238 W/m².
3. Effective temperature: in balance, the outgoing radiation equals 238 W/m². By the Stefan-Boltzmann law, radiation = σT⁴, with σ = 5.67 × 10⁻⁸ W/m²K⁴. So T = (238.2 ÷ 5.67 × 10⁻⁸)^1/4 = (4.20 × 10⁹)^1/4 = 255 K, or 255 − 273 = −18 °C.
4. Greenhouse effect: the actual mean surface temperature is about 288 K (15 °C). The difference is 288 − 255 = 33 °C, which is the warming caused by the natural greenhouse effect.
5. Effect of a change in albedo: if the albedo rises from 0.30 to 0.31, the absorbed energy falls to 0.69 × 340.25 = 234.8 W/m², a drop of 3.4 W/m². The effective temperature becomes 255 × (0.69 ÷ 0.70)^1/4 = 253.7 K, a fall of about 0.9 °C in this simple model (without feedbacks).
6. Sun angle: at the spring equinox, at noon, the intensity on a horizontal surface at latitude 60° is proportional to cos 60° = 0.5, so it is half that at the equator (cos 0° = 1).

How did we learn about the energy balance?

In the 1820s, Joseph Fourier realized that the atmosphere affects the temperature of Earth, and John Tyndall showed in the 1850s that water vapor and carbon dioxide absorb heat. Svante Arrhenius calculated in 1896 how changes in carbon dioxide would change the temperature. In the 20th century, Milutin Milanković explained how changes in Earth’s orbit and tilt alter the distribution of sunlight and the timing of ice ages. Since the 1970s, satellites have measured the incoming and outgoing radiation directly, and the CERES instruments now monitor Earth’s energy balance. They show that Earth currently absorbs more energy than it emits, an imbalance of the order of 1 W/m², which is the cause of global warming.

Which mistakes are common?

Mistake 1: Using the solar constant as the average insolation

The average over the whole sphere is one quarter of the solar constant, about 340 W/m², because of the geometry of a sphere.

Mistake 2: Saying that the tilt of the axis causes the seasons through distance from the Sun

Seasons result from the tilt, which changes the angle of the Sun and the length of the day. Earth is actually closest to the Sun in January.

Mistake 3: Thinking that the greenhouse effect is only a human-made problem

The natural greenhouse effect warms Earth by about 33 °C and makes it habitable. The concern is the increase caused by additional greenhouse gases.

Mistake 4: Ignoring albedo feedback

A small change in warming can be amplified when ice and snow melt and the darker surface absorbs more energy.