The greenhouse effect, live

Watch sunlight warm the Earth and greenhouse gases trap the heat. Add CO₂ and methane, switch the climate feedbacks on and off, and see where the temperature settles.

What are greenhouse gases, and why does a tiny amount of CO₂ change the temperature of a whole planet? Follow the guided tour, then take the controls.

  • ppm / ppb: parts per million / billion. 424 ppm of CO₂ means 424 CO₂ molecules in every million molecules of air.
  • W/m²: watts per square metre, averaged over the whole Earth, day and night. 1 W/m² over the whole planet is about 25 times all the energy humanity uses.
  • Settled: the temperature at which heat out to space equals sunlight in, so it stops changing.
  • Feedback: a knock-on effect of warming that changes the warming. Warmer air holds more water vapour, which traps more heat.

Things to try

  1. In Explore yourself, press 1750 levels, then Today’s levels (2024). Watch the energy balance: for a while less heat goes out than sunlight comes in. Where does the temperature settle?
  2. Press 1750 levels and switch the feedbacks off. Drag CO₂ to 560 ppm (double 1750). How much warmer than 1750 does it settle? Now switch the feedbacks back on. How many times bigger is the warming?
  3. Methane is counted in parts per billion: today’s 1,940 ppb is under 2 ppm. Press Today’s levels (2024), then bring methane back to 730 ppb, its 1750 level: that removes just 1.2 ppm of gas. Note Heading towards. Now put methane back to 1,940 and lower CO₂ from 424 to 400 ppm instead, removing 24 ppm. Which cools the planet more? What does that say about methane, molecule for molecule?
  4. The world today is about 1.3 °C warmer than in the late 1800s (averaged over the last ten years), less than this model’s settled value for today’s gases. The main reason: the oceans take decades to centuries to warm up, so if the gases stayed at today’s levels, more warming would still be on its way. Air pollution (aerosols) also reflects sunlight and hides some warming. Cleaning up the air saves many lives but unmasks some of that hidden warming. How should health policy handle that trade-off?
  1. A little over 2 °C warmer than 1750 (about 16.2 °C).
  2. About 1.1 °C without feedbacks and 3.0 °C with them: almost three times as much.
  3. Taking methane back to 1750 levels cools the settled temperature by nearly half a degree (to about 15.7 °C); cutting CO₂ by 24 ppm cools it by about a quarter of a degree (to about 15.9 °C), with 20 times more molecules removed. Molecule for molecule, methane traps far more heat.
  4. There is no single right answer. Cutting fossil fuels removes CO₂ and air pollution together: the cleaner air saves lives straight away, and less CO₂ limits the warming over the long run.

Why CO₂, but not nitrogen?

The main greenhouse gases

Gas In the air today Main human sources How long it stays Heat trapped per kg over 100 years (CO₂ = 1)
Carbon dioxide (CO₂) 0.04% (424 ppm) Burning coal, oil and gas; clearing forests Centuries to millennia (part of it) 1
Methane (CH₄) 0.0002% (about 1,940 ppb) Livestock, rice, landfills, gas and coal leaks About 12 years About 27 to 30
Nitrous oxide (N₂O) 0.00003% (about 338 ppb) Farm fertilisers, manure, industry About 110 years About 270
Water vapour (H₂O) Up to a few % near the ground Not directly: it follows the temperature About 10 days A feedback, not a driver

The last column is the “global warming potential” (GWP-100), the factor carbon footprints use to turn emissions of other gases into CO₂-equivalents (CO₂e). It adds up the heat a kilogram traps over 100 years. Methane does most of its trapping in the first decade or two after it is released, so in those early years it warms far more than its GWP suggests, and a century later little of it is left.

Common mix-ups

CO₂ is only 0.04% of the air. How can it matter? Nitrogen, oxygen and argon, 99.9% of dry air, do nothing to infrared, so all the catching is done by clouds and a handful of other gases: water vapour, CO₂, methane and a few more. Small doses can have big effects: a blood alcohol level of 0.08% is enough to impair a driver.

Water vapour is the biggest greenhouse gas. Why worry about CO₂? Water vapour rains out within about ten days, so the amount in the air simply follows the temperature. CO₂ stays for centuries, so it sets the temperature, and water vapour then amplifies the change.

Is this the ozone hole? No. The ozone hole lets more of the Sun’s ultraviolet through, which raises the risk of skin cancer and cataracts. It is a separate problem, slowly healing thanks to the 1987 Montreal Protocol.

Do greenhouse gases form a layer that reflects heat? No. They are mixed all through the air, and they absorb infrared rather than reflect it like a mirror.

Hasn’t CO₂ already caught all the infrared it can? Near the ground, almost. But heat escapes to space from high up, where the air is thin and cold. More CO₂ pushes that escape level higher, into colder air that gives off less heat, so the planet still warms.

Why it matters for health

The greenhouse effect is not the problem in itself: without it the planet would be frozen. The problem is how fast we are strengthening it. A couple of degrees on the global average sounds small, but averages hide the extremes. Extreme heat that used to strike once a decade now strikes almost three times as often, and would strike more than five times as often with 2 °C of global warming (IPCC 2021, Figure SPM.6). For health, that shows up as more frequent and intense heatwaves, changing patterns of infectious disease, and pressure on food and water supplies. And the fossil fuels that add CO₂ also pollute the air, so cutting them brings health benefits straight away. These links are the subject of the Health and Environment module.

This is a teaching model, not a climate model. The planet is one average point with a single layer of air that absorbs a share of the surface’s infrared and sends it back out, half up and half down.

  • Sunlight: 1,361 W/m² at the top of the atmosphere, facing the Sun. Spread over the whole globe, day and night, that is about 340 W/m², and 30% of it is reflected, so about 238 W/m² is absorbed.
  • The share of infrared absorbed is set so that, with 1750 levels of CO₂ (280 ppm) and methane (730 ppb), the surface settles at about 14 °C. That share (76% in 1750) is a setting of this one-layer model. Real air absorbs nearer 90% of the surface’s infrared, but the surface also loses heat by evaporation and by rising warm air, and the air soaks up some sunlight directly. This model leaves both out, so a smaller share stands in for them.
  • Extra CO₂ and methane strengthen the trapping using the simplified formulas of Myhre et al. (1998). Nitrous oxide and the other gases stay at their 1750 levels in every scenario, including today’s.
  • The feedbacks switch stands for all the feedbacks together: extra water vapour (the largest), melting ice and changing clouds. It is calibrated so that doubling CO₂ gives 3 °C of warming once settled, the IPCC’s best estimate. Without it, the model gives the no-feedback response of a little over 1 °C.
  • Not included: aerosols (air pollution) and the oceans’ slow uptake of heat. Time is sped up enormously: the real climate takes decades to centuries to settle.
  • In real air, a molecule that catches infrared almost always passes the energy to its neighbours by bumping into them, warming the air; the warm greenhouse gases then give off infrared in all directions. The animation shortens this to one molecule catching and re-sending a wave.
  • The molecules are not to scale. CO₂ is about 0.04% of the air (about 2,300 nitrogen and oxygen molecules for each CO₂), and water-vapour molecules multiply with warming much faster than in reality (real air holds about 7% more per °C) so that the feedback is visible.

Sources