☀️ Earth's Energy Balance 1,361 W/m² incoming solar radiation +0.9 W/m² current energy imbalance
The solar constant — average solar power hitting the top of Earth's atmosphere; varies ±0.1% over the 11-year sunspot cycle
TSI ÷ 4 (Earth's spherical geometry) × (1 − albedo 0.30) = 238 W/m² absorbed; 100 W/m² reflected back
Energy emitted to space as infrared; must equal absorbed solar for equilibrium. Currently ~0.9 W/m² less than absorbed
Measured by CERES satellites & Argo floats; ~93% of this excess heat is absorbed by oceans. This is the fundamental driver of ongoing warming
Without any greenhouse gases, Earth's surface would be −18°C (0°F). GHGs raise the average to +15°C — a 33°C warming that makes life possible
30% of incoming solar is reflected — mostly by clouds (22%), then ice/snow (5%), then land surfaces (3%). Albedo changes are a major climate feedback
Best estimate: 3°C warming per doubling of CO₂ (~3.7 W/m² forcing); range 2.5–4°C. Every 1 W/m² sustained imbalance eventually yields ~0.5–0.8°C
Satellite & surface spectroscopy confirm this enhanced greenhouse forcing from CO₂, CH₄, N₂O, and other anthropogenic GHGs
The Global Energy Budget — All Flows
How Earth's Energy Budget Works
The Sun delivers ~1,361 W/m² at the top of the atmosphere. Because Earth is a sphere, the cross-sectional area that intercepts sunlight is ¼ of the total surface area, so the average over the whole globe is ~340 W/m². Of this:
The surface then emits this energy as longwave (infrared) radiation. The atmosphere absorbs most of this upwelling IR and re-emits it — sending some back to the surface (the greenhouse effect: ~345 W/m² back-radiation) and some to space. In equilibrium, outgoing longwave radiation (OLR) to space = incoming absorbed solar. Currently, OLR ≈ 239 W/m² while absorption ≈ 240 W/m² — a surplus of ~0.9–1.0 W/m² that is accumulating as heat.
Total Solar Irradiance — 1600 to Present
Anatomy of Incoming Solar Radiation
The solar constant and its variation
TSI averages ~1,361 W/m² but varies by ~1–2 W/m² over the 11-year sunspot cycle (solar maximum = more sunspots = slightly higher TSI). The Maunder Minimum (1645–1715), when sunspots almost vanished, is estimated to have caused TSI ~3–5 W/m² below modern values — contributing to the "Little Ice Age" cooling, though volcanic and ocean circulation factors also played major roles.
Distribution by wavelength
The Sun emits electromagnetic radiation peaking in the visible spectrum (~500 nm). About 44% of TSI is visible light, 49% is near-infrared, and 7% is ultraviolet. UV is absorbed by stratospheric ozone (warming the stratosphere). Near-IR is partially absorbed by water vapour and CO₂ in the atmosphere; visible light mostly reaches the surface.
Absorbed solar radiation (ASR)
The fraction absorbed by Earth = (1 − albedo) × TSI/4 = (1 − 0.30) × 340.25 ≈ 238 W/m². This is the energy the planet must radiate back to space in equilibrium. Changes in albedo have an equal and opposite effect to changes in TSI — which is why cloud feedback and ice-albedo feedback are so important.
Solar Spectrum — Incoming vs. Absorbed vs. Reflected
Outgoing Longwave Radiation vs. Absorbed Solar (1985–2023)
How Earth Loses Energy to Space
Outgoing Longwave Radiation (OLR)
Earth's surface (average ~288 K) emits infrared radiation according to the Stefan-Boltzmann law. But the atmosphere is largely opaque to these wavelengths — only the "atmospheric window" (8–13 µm) allows surface radiation to escape directly. Greenhouse gases absorb the rest. The effective emission level for most IR is ~5–10 km altitude, where the temperature is ~220 K (−53°C), producing OLR of ~239 W/m².
Reflected Solar Radiation (RSR)
~100 W/m² of incoming solar is reflected directly to space before being absorbed. Clouds are the dominant reflector (albedo ~0.5–0.7 for thick clouds). High clouds (cirrus) are semi-transparent to solar but opaque to outgoing IR — a net warming effect. Low clouds (stratus, cumulus) reflect strongly — a net cooling effect. Changes in cloud cover, altitude, and optical thickness are the largest source of uncertainty in climate projections.
The emission temperature puzzle
If Earth had no atmosphere, it would need to emit from the surface at ~255 K (−18°C) to balance 240 W/m² of absorbed solar. The actual surface temperature is ~288 K (+15°C). The 33°C difference is the natural greenhouse effect, maintained by the back-radiation from atmospheric GHGs. Adding more GHGs raises the effective emission altitude (colder atmosphere → less OLR → energy imbalance → warming until a new equilibrium is reached).
Greenhouse Gas Radiative Forcing (1750 → 2023)
How the Greenhouse Effect Works
Molecular absorption — the key physics
Greenhouse gases are transparent to shortwave solar radiation (visible + near-IR) but absorb outgoing longwave (mid-IR) radiation emitted by the warm surface. This absorption excites vibrational and rotational modes of asymmetric molecules (CO₂, H₂O, CH₄, N₂O, O₃). The excited molecule then re-emits in all directions — sending roughly half the energy back toward the surface (downwelling longwave radiation, ~345 W/m²), warming it above the temperature it would reach by solar absorption alone.
The key greenhouse gases and their roles
Earth's Energy Imbalance — Ocean Heat Content Trend
Where the Excess Energy Goes
The oceans — Earth's heat buffer
Approximately 93% of the excess energy accumulating from the EEI goes into the ocean, primarily the upper 2,000 m measured by the Argo float network. Ocean heat content (OHC) has risen unambiguously since the 1970s and has been accelerating since the 2000s. This stored heat will continue to drive sea level rise (thermal expansion) and influence atmospheric circulation for centuries even if GHG concentrations stabilised today.
Ice melt — committed sea level rise
About 3% of the EEI goes into melting land ice (Greenland, Antarctica, mountain glaciers) and sea ice. The distinction matters: melting sea ice does not raise sea level (like ice in a glass of water), but melting land ice does. Current EEI alone commits ~0.3 mm/yr of sea level rise from ice melt, plus ~1.5 mm/yr from thermal expansion — totalling ~2+ mm/yr even at today's CO₂ levels.
Climate Feedback Strengths (W/m²/°C)
Understanding Climate Feedbacks
A feedback is a process where an initial warming (or cooling) changes a physical quantity that, in turn, amplifies or dampens the original change. The sum of all feedbacks, plus the initial forcing, determines the equilibrium climate sensitivity (ECS) — how much global mean temperature rises per doubling of CO₂.
Planck feedback (stabilising)
As Earth warms, it emits more longwave radiation (Stefan-Boltzmann: power ∝ T⁴). This is the primary restoring force that prevents runaway warming and gives ECS its finite value. Without it, any positive forcing would cause indefinite warming. The Planck feedback parameter is ~−3.3 W/m²/°C.
Water vapour feedback (amplifying — strongest positive)
Warming increases evaporation → more water vapour in the atmosphere → stronger greenhouse effect → more warming. This is the single largest positive feedback, roughly doubling the warming from CO₂ alone. The lapse rate feedback (how temperature changes with altitude) partially offsets water vapour, so they are usually quoted together: combined water vapour + lapse rate = +1.0 to +1.3 W/m²/°C.
Ice-albedo feedback (amplifying)
Warming melts snow and ice → darker ocean/land exposed → less solar reflection → more absorption → more warming. This feedback is particularly strong at the poles, driving "Arctic amplification" where the Arctic warms 2–4× faster than the global average.
Cloud feedbacks (uncertain — net slightly positive)
Clouds both cool (reflect solar) and warm (trap outgoing IR). How clouds change with warming is the single largest uncertainty in climate models. IPCC AR6 assesses net cloud feedback as positive (+0.42 W/m²/°C), primarily because high-altitude clouds rise as climate warms (keeping their temperature roughly constant, so they emit as much IR even at higher altitudes) and low marine clouds decrease slightly in coverage.