Earth's energy flows: sun, wind, water, heat and life: Sunlight and the planet's energy budget

1.1 The Sun delivers about 1,361 W/m2, but a spinning globe averages it down to about 340

Just outside the atmosphere, a flat panel facing the Sun receives about 1,361 watts on every square metre. This figure is called the solar constant, although it wobbles by a fraction of a percent with the Sun's activity.

Earth does not present a flat face to the Sun, though. It catches sunlight on a disc whose area is pi times the radius squared, but it spreads that energy over a whole sphere whose area is four times larger. Half of that sphere is in darkness at any moment, and the sunlit half receives light at a slant away from noon. Averaged over day and night and over all latitudes, the top of the atmosphere receives about a quarter of the solar constant: roughly 340 W/m2.

Multiply the solar constant by the area of Earth's disc and the total intercepted flow comes to about 174,000 terawatts (174 petawatts). Humanity's entire use of primary energy in 2024 was 592 exajoules, which averaged over the year is about 18.8 TW. The planet intercepts more than 9,000 times that.

The quality of this energy matters as much as its quantity. The Sun's surface is at about 5,800 K, so it radiates mostly short waves: a little under half as visible light, most of the rest as near-infrared and less than a tenth as ultraviolet. Earth, at an average surface temperature of about 288 K (15 C), radiates long-wave infrared at around 10 micrometres. This difference between incoming short waves and outgoing long waves is what makes the greenhouse effect possible, and it also means sunlight arrives as a high-quality energy that plants and solar cells can use.

The lesson Smil draws is simple. Sunlight is not scarce. The difficulty is that it is spread thinly over the whole planet and arrives only by day, so capturing it takes land, materials and storage.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance'
• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance'
• Jaffe and Taylor, The Physics of Energy (2018) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance'
• Energy Institute, Statistical Review of World Energy 2025 web ↗ · 2024 world primary energy of 592 EJ (about 18.8 TW averaged over the year); https://www.energyinst.org/statistical-review

1.2 About 30% is reflected; the rest must eventually be radiated back to space

Of the 340 W/m2 arriving at the top of the atmosphere, about 100 W/m2, or 30%, is reflected straight back to space. This fraction is Earth's albedo. Clouds do most of the reflecting, with help from ice, snow, deserts and tiny particles in the air. The remaining 240 W/m2 is absorbed, a little under a third of it by gases, dust and clouds in the atmosphere and the rest, roughly 165 to 170 W/m2, by land and ocean surfaces.

That absorbed energy does not stay. Over any long period a planet must send out as much energy as it absorbs, or it would keep warming without limit. Earth does this by radiating infrared from its surface and atmosphere. The surface loses its energy in three main ways: by evaporating water, which carries heat up into the atmosphere as latent heat; by warming the air directly; and by radiating infrared, much of which is absorbed by the atmosphere and sent back down before eventually escaping from higher, colder layers.

A useful way to think about the whole system is as a heat engine. Energy comes in at high quality and goes out at low quality. One photon of visible sunlight carries roughly as much energy as twenty photons of Earth's infrared, because the energy of a photon scales with the temperature of the body emitting it, and 5,800 K is about twenty times 288 K. The planet therefore receives a small number of energetic photons and sends back many weak ones. In thermodynamic terms it imports low entropy and exports high entropy, and that difference is what drives the winds, the water cycle and life itself.

Nothing in this budget is used up. Every joule of sunlight absorbed is eventually returned to space. What changes is the form, and along the way the energy does work.

• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance'
• Smil, The Earth's Biosphere: Evolution, Dynamics, and Change (2002) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance'
• Jaffe and Taylor, The Physics of Energy (2018) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance'

1.3 Greenhouse gases keep Earth habitable, and extra ones now tilt the balance by about 1 W/m2

If Earth had the same albedo but no greenhouse gases, its surface would settle at about minus 18 C, the temperature at which a body radiates 240 W/m2. The actual average is about plus 15 C. The 33 degree difference is the natural greenhouse effect. Water vapour, carbon dioxide, methane, nitrous oxide and ozone let short-wave sunlight through but absorb outgoing long-wave infrared and re-emit part of it downwards. Infrared then escapes to space mainly from higher in the atmosphere, where it is colder, so the surface has to be warmer to push the same amount of energy out.

Adding more CO₂ and methane raises that effective emitting height. For a while, the planet sends out a little less than it absorbs. This gap is Earth's energy imbalance. The IPCC's sixth assessment put it at about 0.8 W/m2 averaged over 2006-2018, and satellite and ocean measurements show it roughly doubled between 2005 and 2019. More than 90% of the trapped energy goes into the oceans; the rest melts ice and warms land and air. The imbalance closes only when the surface has warmed enough to radiate the extra energy away.

A watt per square metre sounds trivial against 340. But multiply it by the planet's 510 million square kilometres and the imbalance is around 500 TW, more than 25 times humanity's total energy use, flowing into the climate system continuously. That is why small changes in the atmosphere's composition have large consequences: the natural flows are so big that a fraction of a percent shift in them outweighs everything our machines do directly.

• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance'
• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance'
• IPCC, Climate Change 2021: The Physical Science Basis, Working Group I contribution to AR6, Chapter 7 · Earth's energy imbalance of about 0.8 W/m2 for 2006-2018, most of it stored in the ocean; https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-7/
• Loeb et al., 'Satellite and ocean data reveal marked increase in Earth's heating rate', Geophysical Research Letters (2021) · imbalance roughly doubled 2005-2019; reported by NOAA Climate.gov (2021), https://www.climate.gov/news-features/feed/joint-nasa-noaa-study-finds-earths-energy-imbalance-has-doubled
• Energy Institute, Statistical Review of World Energy 2025 web ↗ · 2024 world primary energy of 592 EJ (about 18.8 TW averaged over the year); https://www.energyinst.org/statistical-review

See it in the table

Check your understanding

Why does the top of the atmosphere receive on average about a quarter of the solar constant?

Earth's energy imbalance is about 1 W/m2. Roughly how large is that as a global flow?

What does Earth export to space that is different from what it receives?