The Greenhouse Effect

To see why certain gases, such as carbon dioxide, methane and water vapour, what are (now) referred to as ‘greenhouse gases’, cause climate change, we need some understanding of the greenhouse effect. The way this effect is often presented is to think of the planet Earth as a greenhouse – the Earth, on this way of thinking, is like a greenhouse, with the glass of the greenhouse representing the planet’s atmosphere and the contents of the greenhouse representing the surface of the Earth.

People who live in cool climates may be familiar with the ways in which vegetables and other plants are grown in greenhouses; a greenhouse being a like a shed but with lots of glass, which warms up when the sun shines. People who live in hot climates will know that cars parked in the sun in summer get very hot inside. The explanation of the phenomenon, which is manifested in lots of other ways besides these two, is this: glass is quite transparent to the high frequency radiant heat energy which comes from the sun and falls on greenhouses, etc., but more opaque to radiant heat reemitted by objects which absorb the sun’s heat in greenhouses, etc. This means that while the sun shines, the interior of greenhouses get hotter because the heat is trapped inside the greenhouse. All hot objects themselves radiate heat – the hotter they get, the more heat they give off - and hence there is a limit to how hot greenhouses can get. After a while, the greenhouse will be in what is called thermal equilibrium with its surroundings – it emits as much heat as it absorbs and therefore does not get any hotter -  and after the Sun goes down, it will start to cool down. The nature of the glass itself will influence how much radiant heat gets in, some radiation is always reflected, and how much gets out. For instance, coating the inside layer of the glass with a very thin layer of certain metals produces heat reflective glass, which reduces the amount of heat radiating from the greenhouse.

Most of our atmosphere is comprised of nitrogen, with the next most abundant gas being oxygen. Neither of these gases absorb any of the radiation from the Sun – this is just a fact about how things are – and hence they are not greenhouse gases. We define a greenhouse gas as a gas that does absorb radiation from the Sun, and consequently as gases that are warmed by the Sun. If nothing in the atmosphere absorbed radiation and hence was warmed by the Sun, then all the radiation from the Sun would fall directly on the Earth’s surface which would heat up during the day and start radiating more and more heat until, of if, thermal equilibrium was reached. Objects on the Earth’s surface would carry on emitting heat all night getting rapidly cooler, until the Sun rose again. Such radical heating and cooling would mean that life (as we know it) could not have evolved, so it is perhaps as well we have greenhouse gases to keep us warm at night and cool during the day.

Anything which gets warmed up becomes hotter and hence will itself radiate more heat than it did before it was warmed up. When carbon dioxide, methane, water vapour and other (much less abundant) gases in the atmosphere get warmed up they radiate heat back into space or down towards the Earth’s surface. They can also exchange heat among themselves, but if they are at the same temperature, there will be no net effect. The greenhouse gases that inhabit the top layers of the atmosphere, the ones that will radiate heat back out to space, will be cooler than the gases at lower levels, the ones that will radiate the heat back down to Earth. If there were no other factors at play then one might suppose that the Earth would be gradually hotter, as the warmer lower layers of greenhouse gases gradually heated up the Earth’s surface. This effect occurred on Venus, which now has an atmosphere composed in part of super-heated steam, all the liquid water on the planet having boiled off, in what is known as a ‘runaway greenhouse effect’. This has not (yet?) happened here because the Earth does not receive as much radiation from the Sun as Venus, being further away, and because of other ways in which the planet can get rid of heat. One is these is the reflection of radiation back into space by clouds. Another is by the mixing of colder upper layers of gas with the warmer lower layers by convection – hot air tends to rise, for example – and by conduction whereby ‘warmer’ gas molecules give up some heat to the colder upper neigbours. The net effect of these exchanges is that the average temperature of the Earth’s surface has been about a constant 15 degree Celsius since the last ice age: on balance, as much of the Sun’s heat has been radiated away into space as has been transmitted back to Earth. But now more and more greenhouse gases are being introduced into the atmosphere, which means, inevitably, that the average temperature of the Earth’s surface will begin to rise and continue to do so until the amount of greenhouse gas becomes constant. This is because all the mechanisms for compensating for the fact that colder gases in the upper reaches of the atmosphere radiate less heat that those at lower levels are already ‘at capacity’. Increasing the amount of greenhouse gases in the atmosphere is therefore like adding heat reflective glass to the greenhouse.