Radiative equilibrium · the principle
If Earth's mean temperature is stable, solar energy received is exactly offset by energy returned to space — not only reflected visible light, but also emitted infrared.
The 240 W/m² absorbed do not leave directly: they warm the planet, which then re-emits infrared until balance is restored. That IR flux to space (~239 W/m²) completes the budget with reflected visible light.
Radiative budget · overview
- 1 340 W/m² enter at the top of the atmosphere.
- 2 ~100 W/m² reflected (grey corridor) · ~240 W/m² absorbed (blue corridor).
- 3 Heating of atmosphere and surface.
- 4 Infrared re-emission · GHG effect.
- 5–6 ~100 + ~239 W/m² leave to space (visible + IR).
- Reflected visible
- Absorbed energy
- IR to space
- Solar input
Balance: 100 + 239 ≈ 340 W/m² · mean surface T° ~14 °C. Numbers ①–⑥ in the diagram follow the steps above.
Rounded values in W/m² (global annual mean). Source: Stephens et al. (2012), IPCC AR6.
From the Sun to Earth
The Sun emits mainly visible and ultraviolet radiation (shortwave). At Earth's distance, the solar constant is about 1,361 W/m² on a surface perpendicular to the rays.
Earth is a sphere: only one section receives full daylight, and night receives none. On average, input at the top of the atmosphere is about 340 W/m² (one quarter of the solar constant).
Absorption in the atmosphere
About 75 W/m² of solar flux is absorbed directly by the atmosphere: ozone (UV), water vapour, clouds and aerosols. This heat contributes to air mixing and the global budget.
Clouds play a dual role: they reflect part of the Sun back to space (cooling) and also absorb a fraction of the radiation.
Land, vegetation and built surfaces
About 165 W/m² of solar radiation reaches the surface (after atmospheric reflection). Continents and vegetation absorb ~40 W/m²; built surfaces (roofs, roads, façades) have highly variable albedos.
Any surface warmed by the Sun then re-emits energy as infrared (longwave), proportional to its temperature (Stefan-Boltzmann law).
Oceans
The ocean covers ~71% of Earth's surface and absorbs about 90% of solar radiation that reaches it (very low albedo, ~0.06). It thus captures the largest share of surface-absorbed solar flux (~125 W/m² of ~165 W/m²).
Do not confuse: the ocean absorbs ~90% of solar radiation that reaches it (very low albedo) — a different order of magnitude from ~91% of the climate energy surplus.
When this budget shifts (~1 W/m² surplus on average over 2006–2025), the ocean captures ~91% of the excess — hence the bombs/s pace and the reference sheet on the « Scale » step.
Albedo · snow and glaciers
Albedo measures the fraction of light returned by a surface (0 = all absorbed, 1 = all reflected). Fresh snow and glaciers have high albedo: they return much of the Sun to space and warm less.
| Surface | Albedo | Effect |
|---|---|---|
| Fresh snow | 0,80 – 0,90 | Sends almost all visible light back to space |
| Glaciers / sea ice | 0,50 – 0,85 | Melting = lower albedo = positive feedback |
| Forest / soils | 0,10 – 0,15 | Mostly absorbs solar radiation |
| Ocean | ~ 0,06 | Absorbs ~90% of the flux reaching it |
| Desert / built-up | 0,15 – 0,35 | Roofs, roads, façades: highly variable |
When ice melts, albedo drops and absorption rises: a warming positive feedback.
Infrared and greenhouse gases
Earth's surface emits about 398 W/m² of infrared radiation. Greenhouse gases and clouds absorb part of it and emit infrared both upward and downward. In the estimate by Wild et al. (2012), downward thermal radiation at the surface is about 342 W/m². This flux describes energy received at the surface; it is not itself a feedback. See the estimate and its components (PDF, p. 2).
An atmospheric window lets ~40 W/m² pass directly to space; the rest of the infrared exits via colder high layers. In total, IR emitted to space (~239 W/m²) plus reflected visible (~100 W/m²) balance the 340 W/m² incoming.
Atmospheric layers and CO₂ footprint
CO₂ is well mixed vertically (~427 ppm in 2025, from surface to stratosphere). It is altitude that changes the radiative effect on temperature.
Layers rich in CO₂ and water vapour behave differently by altitude. In the troposphere (low layers), GHGs trap infrared rising from the surface: the lower atmosphere and ground warm.
In the stratosphere, conversely, CO₂ increases infrared emission to space more than absorption from below (already trapped underneath): this layer cools. An expected GHG strengthening signature, distinct from surface warming.
Neutral budget and reference temperature
At climate equilibrium, incoming energy equals outgoing: ~340 W/m² in ≈ ~100 W/m² reflected + ~239 W/m² infrared emitted. The planet neither warms nor cools on average.
Without natural greenhouse effect, mean temperature would be about −18 °C. With natural GHGs, it sits around ~14 °C: 13.9 °C on average over the 20th century (NOAA), about 0.25 °C less in 1850–1900 (HadCRUT5).
Anthropogenic warming is an imbalance: more incoming than outgoing energy (1.04 W/m² on average over 2006–2025), of which the ocean captures about 91% of the surplus (AR6: land 5%, ice 3%, atmosphere 1%).
And the ocean in all this?
The radiative imbalance (~1.0 to 1.1 W/m² on recent average, Forster et al. 2026) does not show up mainly in the air: about 91% of the energy surplus is absorbed by the ocean. Hence dizzying orders of magnitude, expressed in zettajoules then « bomb » equivalents for readability.
2025 values as 20-year trends (OHC IAP/CAS, EEI Forster et al. 2026) · 62.76 TJ/bomb conversion · detail in Methods.
Interactive monitor · Timeline 1957–2025 · Reference sheet W/m² → bombs/s