Water vapour is by far the most abundant greenhouse gas, and scientists estimate it accounts for about half of the total heat-trapping ability of the greenhouse effect. But water vapour is also a very short-lived greenhouse gas, being removed from the atmosphere when it condenses into clouds and falls to the ground as rain. So, unlike other greenhouse gases such as carbon dioxide, water vapour doesn’t accumulate in the atmosphere, because its quantity is directly controlled by the prevailing weather conditions. This means that water vapour can neither strengthen nor weaken the greenhouse effect by itself, though it acts as an amplifier – or positive feedback – following changes in other greenhouse gases.

As well as being present in the atmosphere as a gas, water vapour also condenses into liquid water on tiny particles of dust or ice – forming water droplets and clouds. Clouds are an important absorber of outgoing infrared energy. Scientists estimate that clouds account for about a quarter of the total heat trapped by the greenhouse effect. But clouds also reflect incoming sunlight, so they have both warming and cooling effects on the climate, depending on the type of cloud. Low-level clouds tend to reflect more energy than they trap, resulting in an overall cooling effect. In contrast, high-level wispy clouds tend to trap more energy than they reflect.

Each carbon dioxide (CO₂) molecule absorbs infrared energy more strongly than each water vapour molecule. But there’s less CO₂ than water vapour in the atmosphere, so CO₂ makes a smaller overall contribution to the greenhouse effect. Scientists estimate that CO₂ accounts for about a fifth of the total heat trapped by the greenhouse effect. CO₂ is absorbed by oceans and plants, but doesn’t get caught up in rain and weather systems. So CO₂ can accumulate in the atmosphere over years and decades, meaning its quantity can increase and decrease independently of the climate, strengthening or weakening the greenhouse effect. On average, CO₂ can stay in the atmosphere for over 100 years.

Apart from small quantities of water vapour and carbon dioxide, there are also even smaller quantities of other greenhouse gases in the atmosphere, including methane, ozone and nitrous oxide. Methane and nitrous oxide molecules are strong absorbers of infrared energy. Nitrous oxide can stay in the atmosphere for over 100 years – similar to the lifetime of carbon dioxide – while methane has a shorter lifetime of about ten years. Ozone also acts as a greenhouse gas, but it absorbs ultraviolet sunlight – mostly in the stratosphere – as well as infrared heat energy. Together, these and tiny quantities of other gases account for about 5% of the total heat-trapping ability the greenhouse effect.

The contribution of greenhouse gases and clouds to the greenhouse effect depends on how strongly they absorb infrared energy, their quantity and how long they remain in the atmosphere. In addition, just as visible light comes in all the colours of the rainbow, infrared energy comes in different ‘shades’ – frequencies. Different components (greenhouse gases and clouds) absorb different frequencies. But at some frequencies all the available infrared energy is absorbed by just some of the components, so the rest of the gases or clouds are redundant. These different factors often have competing effects, which scientists calculate when estimating the overall contribution of each component of the greenhouse effect.

Water vapour responds rapidly to weather conditions such as temperature and air pressure, and can condense into clouds and rain. In contrast, other greenhouse gases such as carbon dioxide and methane don’t respond directly to weather conditions and can stay in the atmosphere for years, decades or longer. These ‘long-lived’ greenhouse gases are also well mixed in the lower atmosphere, meaning that they’re spread out more or less evenly around the globe. In contrast, water vapour concentrations in the lower atmosphere vary widely – from less than 1% in dry areas to as much as 4% in humid areas.

Sometimes the best place to look for answers is where you’d least expect them. Ingo would agree – he tries to find out how Earth’s atmosphere works by studying other planets. ‘I use spacecraft observations to measure atmospheric gas properties at other planets,’ he says. Ingo also uses supercomputers to model extraterrestrial atmospheres. Understanding the behaviour of gases on other planets helps scientists to understand what goes on in the Earth’s atmosphere. ‘The most exciting discoveries I’ve been involved in relate to the atmosphere of Saturn’s largest moon, Titan, a body resembling Earth in many ways, but yet so different.’

• Koshland Science Museum: Greenhouse Effect
• IPCC FAQ: What is the Greenhouse Effect