How increasing carbon dioxide affects Earth’s climate
The gas makes up a tiny quantity of our atmosphere but it can have a big impact
Carbon dioxide (CO2) accounts for just 0.04 per cent of Earth’s atmosphere, so how can it have such a big effect on the planet’s climate? Well, firstly, CO2 traps heat (hence it’s known as a greenhouse gas) and secondly, it remains in the atmosphere for centuries.
Earth receives huge quantities of energy from the Sun in the form of sunlight. As this is short-wave radiation, it passes through the atmosphere largely unhindered. Most of that energy is absorbed by the surface, but about 17 per cent is reflected back in the form of heat (long-wave, infrared radiation).
Infrared radiation effectively passes straight through oxygen and nitrogen (which comprise the vast majority of Earth’s atmosphere), thanks to their relatively simple molecular structure. But the molecular structure of CO2 is more complex, so it absorbs the infrared and heats up, then begins to radiate its heat to molecules nearby. The more CO2 molecules there are, the more of that heat is absorbed, the hotter Earth’s atmosphere becomes, and the greater the impact on Earth’s climate.
Methane, another greenhouse gas, has an even more complex molecular structure than CO2 so absorbs even more heat. But methane stays in the atmosphere for little more than a decade. CO2 can hang around for anything from 300 to 1,000 years; hence the accumulation of it has a significant impact on our planet’s heat balance.
For evidence of the effect greenhouse gases can have on a planet’s climate, look no further than Venus. With a similar size and atmosphere to Earth early in its history, it was essentially Earth’s twin until around 700 million years ago, when a runaway greenhouse effect took hold. Today, its atmosphere is 96 per cent CO2 and its surface temperature is around 460°C – hot enough to melt lead.