The Greenhouse Effect

The greenhouse effect is the way Earth's atmosphere traps some of the Sun's heat and keeps the planet warm. Without it, Earth would be a frozen ball with an average temperature of around -18 °C: far too cold for life. Thanks to the greenhouse effect, the average temperature is actually about +15 °C, perfect for liquid water and living things. The effect is named after the way glass greenhouses trap heat from the Sun, although the actual physics is slightly different. The big modern problem is that humans are making the natural effect stronger by adding extra greenhouse gases to the air, which is warming the planet beyond what life is used to.

  • Average Earth temperatureApprox. 15 °CThanks to the greenhouse effect
  • Without greenhouse effectApprox. -18 °CFar too cold for life
  • Main greenhouse gasesCO2, methane, water vapour, N2O
  • Pre-industrial CO2Approx. 280 ppmFor thousands of years
  • Current CO2Approx. 420+ ppmHighest in 3 million+ years
  • Methane strength vs CO2Approx. 30xPer molecule, over 100 years

How the greenhouse effect works

The Sun pours energy at Earth in the form of visible light and ultraviolet. Most of this energy passes straight through the atmosphere and is absorbed by the ground and the oceans, warming them up. The warm ground then radiates that energy back into the atmosphere, but as infrared radiation (heat radiation), which has a longer wavelength.

The trick is that the atmosphere is mostly transparent to visible light (which is why sunlight reaches the ground), but mostly opaque to infrared radiation. Certain gases (especially water vapour, carbon dioxide, methane and nitrous oxide) absorb infrared and re-emit it in random directions, including back towards the ground. Some of that re-emitted heat warms the surface again, and the cycle keeps going. The net result is that the atmosphere holds onto heat that would otherwise leak straight back out into space.

The main greenhouse gases

  • Water vapour: the most abundant greenhouse gas. Levels vary naturally and respond to temperature.
  • Carbon dioxide (CO2): the most important long-lasting greenhouse gas. Released by burning fossil fuels, cutting down forests, and many natural processes.
  • Methane (CH4): about 30 times more powerful than CO2 per molecule but breaks down faster. Comes from cattle, rice paddies, leaking gas pipes, melting permafrost.
  • Nitrous oxide (N2O): from farm fertilisers and some industrial processes. About 300 times more powerful than CO2.
  • Chlorofluorocarbons (CFCs): man-made chemicals that also damage the ozone layer. Mostly banned since the 1980s.

Natural vs man-made

The greenhouse effect itself is completely natural and has been making Earth liveable for billions of years. The problem is the enhancement of the effect by human activity. Since the start of the Industrial Revolution around 1850, humans have burned huge amounts of coal, oil and gas, plus cleared vast areas of forest. CO2 levels in the atmosphere have risen from about 280 parts per million (ppm) in 1800 to over 420 ppm today: the highest level in at least 3 million years. The extra greenhouse gases trap more heat, and the global average temperature has risen by more than 1.2 °C above pre-industrial levels.

Fact The fact that CO2 in the atmosphere traps heat has been known since the 1850s. The Irish physicist John Tyndall measured the heat-trapping effect of different gases in a London laboratory in 1859, and Swedish chemist Svante Arrhenius worked out in 1896 that burning coal would eventually raise global temperatures. Climate change is not a new discovery; we have known the basic physics for over 165 years.

Why other planets prove the effect is real

The greenhouse effect is not just a theory. We can see it working on other planets.

  • Venus: similar size to Earth, but with an atmosphere of 96% CO2 and pressure 90 times higher than Earth's. The greenhouse effect heats the surface to 460 °C, hot enough to melt lead. Without its greenhouse effect, Venus would be much cooler.
  • Mars: similar atmosphere mix to Venus (mostly CO2), but very thin (less than 1% of Earth's pressure). The greenhouse effect is correspondingly weak, and Mars's average surface temperature is approximately -60 °C.

The dramatic differences match exactly what the physics of greenhouse gases predicts. The same physics applies to Earth.

Did you know? The phrase "greenhouse effect" is slightly misleading. A real garden greenhouse traps heat mostly by stopping warm air from rising and blowing away, not by trapping infrared radiation. But the name has stuck for over 150 years and we still use it today.
Deeper dive: what would happen if greenhouse gases doubled?

One of the most important questions in climate science is called the equilibrium climate sensitivity: how much would the planet's average temperature rise if CO2 levels doubled compared to pre-industrial levels?

The current best estimate, based on computer models, geological evidence and direct measurements, is that doubling CO2 would eventually raise global temperatures by approximately 3 °C (give or take a degree). At current emission rates, CO2 levels are likely to reach double pre-industrial levels (560 ppm) by around the middle of this century.

3 °C does not sound like much. But the world today is only about 1.2 °C warmer than pre-industrial levels, and we are already seeing dramatic effects: more intense storms, longer heatwaves, melting Arctic ice, dying coral reefs, rising sea levels. The difference between the last ice age and today is only about 5 °C, but it was enough to bury much of Britain under a kilometre of ice. A 3 °C rise from here would put us close to the warmest temperatures the planet has seen in millions of years.

This is why almost every country in the world signed the 2015 Paris Agreement, committing to limit warming to "well below 2 °C" above pre-industrial levels, and ideally to 1.5 °C. As of 2025, the world is not on track for these targets, but emissions are now starting to fall in many countries, and renewable energy is growing rapidly. The next few decades will largely decide which path humanity ends up on.

For more on the atmosphere, see what is the atmosphere and the ozone layer.