The Greenhouse Effect

Without any atmosphere, Earth's average surface temperature would be −18 °C. Instead it sits comfortably at +15 °C — a 33-degree bonus provided by greenhouse gases. Understanding the physics behind this also explains why adding CO₂ raises that temperature further.

Earth's Energy Balance

The Sun provides energy as visible light and near-infrared. About 30% is reflected back to space (Earth's albedo); the remaining 70% is absorbed by the surface and atmosphere, warming them.

To maintain a stable temperature, the planet must radiate exactly as much energy back to space as it absorbs. It does this as infrared (thermal) radiation — the same kind your skin emits. The hotter the surface, the more it radiates, via the Stefan-Boltzmann law:

P = σ · T⁴ (σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴)

The balance point — where emitted infrared equals absorbed sunlight — is the equilibrium temperature. For a bare rock at Earth's distance it is 255 K (−18 °C). The real surface is 33 °C warmer because the atmosphere intercepts infrared before it escapes.

How Gases Absorb Infrared

Not every atmospheric gas traps heat. Nitrogen (N₂) and oxygen (O₂) make up 99% of the atmosphere yet are transparent to infrared. Why? Their molecules are symmetric diatomic molecules — they have no electric dipole moment that oscillates at infrared frequencies.

Greenhouse gases are different. Their molecular bonds can bend and stretch in ways that create oscillating dipoles, which couple to infrared photons:

CO₂ — asymmetric stretching and bending modes absorb strongly at 15 μm (far-IR)
H₂O vapour — a bent, polar molecule with many rotational/vibrational modes; absorbs across a wide IR range
CH₄ — symmetric but asymmetric tetrahedral stretches; absorbs at 3.3 μm and 7.7 μm
N₂O, O₃ — absorb at important "window" wavelengths where CO₂ and H₂O leave gaps

When an infrared photon of the matching wavelength hits a greenhouse gas molecule, the molecule absorbs it and its bonds vibrate. It then re-emits a photon in a random direction — including back down toward the surface, warming it further.

The blanket analogy: The greenhouse effect is often called an "atmospheric blanket." Unlike a real blanket (which blocks convection), it works by absorbing and re-emitting radiation. The result — slowing energy loss — is mathematically equivalent, but the mechanism is entirely in the electromagnetic spectrum.

Why CO₂ Is the Controlling Gas

Water vapour is actually the strongest greenhouse gas in absolute terms — responsible for about 50% of the natural greenhouse effect. So why does climate science focus on CO₂?

Because water vapour is a feedback, not a forcing. The amount of water vapour in the atmosphere is set by temperature — warmer air holds more vapour. You cannot directly add water vapour that stays; it rains out within ~10 days.

CO₂, by contrast, persists in the atmosphere for hundreds of years. When we burn fossil fuels, CO₂ accumulates. This raises temperatures slightly, which allows the air to hold more water vapour, which amplifies the warming. CO₂ is the thermostat knob; water vapour is the amplifier.

Logarithmic effect: Each doubling of CO₂ concentration adds roughly the same amount of warming (about 1 °C directly, before feedbacks). This is because the main absorption bands already saturate — additional CO₂ only absorbs at the edges. Going from 280 ppm to 560 ppm has the same direct radiative effect as going from 560 ppm to 1120 ppm.

Feedbacks That Amplify Warming

The primary CO₂ forcing is roughly +3.7 W/m² per CO₂ doubling. But the final temperature change is much larger, due to feedbacks:

Water vapour feedback (+1.8 W/m²/°C): Warmer air holds more vapour, which absorbs more IR. The single largest feedback.
Ice–albedo feedback (+0.3 W/m²/°C): Melting ice exposes darker ocean or land, which absorbs more sunlight.
Lapse-rate feedback (−0.8 W/m²/°C): A negative feedback — the upper troposphere warms faster, emitting more IR to space.
Cloud feedbacks (range −0.5 to +0.5 W/m²/°C): The most uncertain term. Low clouds cool; high clouds warm.

Adding feedbacks to the direct forcing gives climate sensitivity ≈ 3 °C for each doubling of CO₂ — the central estimate used by the IPCC, with a likely range of 2.5–4 °C.

The Runaway Greenhouse: Venus

Venus is almost the same size as Earth and receives about twice as much sunlight per unit area. Its surface temperature is 465 °C — hot enough to melt lead. The culprit: a dense CO₂ atmosphere (96.5% CO₂) at 92 times Earth's surface pressure.

Venus is thought to have once had liquid water. As the young Sun brightened, oceans evaporated, flooding the atmosphere with water vapour, which amplified warming, which evaporated more ocean — a runaway greenhouse. The water was eventually photodissociated and the hydrogen lost to space.

Earth's margin: Earth is far from the runaway threshold. Even doubling CO₂ increases surface temperature by only ~3 °C. A runaway would require increasing solar luminosity by ~10% — something that won't happen for ~1.5 billion years.

The Numbers Today

Pre-industrial CO₂ (1850): ~280 ppm

CO₂ today (2024): ~424 ppm

Paris 2 °C target: ~450 ppm equivalent

The 51% increase in CO₂ since industrialisation has raised average surface temperatures by about 1.1–1.2 °C. Arctic regions have warmed by 3–4 °C due to amplified ice–albedo feedbacks.

Methane (CH₄) has more than doubled (from ~720 ppb to ~1900 ppb) and is ~80× more potent as a greenhouse gas over 20 years, though it breaks down in ~12 years. Nitrous oxide (N₂O) has increased ~20% and persists for 120 years.

Units matter: CO₂ concentrations are given in parts per million (ppm) by volume. 424 ppm means 424 molecules of CO₂ per million air molecules — still a trace gas, but enough to shift the energy balance by ~2.7 W/m² relative to 1850.

Try It Yourself

Visualise convective heat transport in the atmosphere simulation — thermal currents that carry heat upward are also part of the energy balance:

🌡️ Open Atmosphere Simulation →

Or explore fluid dynamics — the same Navier-Stokes equations describe large-scale atmospheric circulation:

💧 Open Fluid Simulation →