1.2 The greenhouse effect
How does the Earth’s greenhouse effect work? The greater part of the Earth’s atmosphere is in the lower atmosphere, and when dry it is made up almost entirely of three gases: nitrogen, with 78% (78 parts out of 100) by volume; oxygen (21%); and an inert gas, argon, making up most of the remainder.
These three gases do not interact significantly with heat (i.e. infra-red) radiation from the Sun or the Earth. If these were the only gases in our atmosphere, then the Earth’s mean temperature would remain at −18 °C. The two main naturally occurring greenhouse gases are water vapour and carbon dioxide. They are normally present only in small amounts, but their impact is very significant.
Greenhouse gases in the atmosphere are largely ‘transparent’ to most regions of the spectrum of the incoming solar radiation, such as the visible and ultraviolet, but they intercept the outgoing infra-red radiation from the Earth by absorbing it and then emitting it again in all directions. As Figure 3 illustrates, most energy radiating from the Earth’s surface no longer escapes directly to space. Instead, it is absorbed and re-radiated several times within the atmosphere. Some of this re-radiated energy is sent back to the Earth’s surface, some to the lower layers of the atmosphere; both are warmed. Therefore, the Earth’s surface receives more radiated energy than just the direct solar, so its temperature rises until a new balance is struck when it again emits energy at a rate to match the increased input. The upper atmosphere still radiates energy back to space at an average temperature of −18 °C, from approximately 7 km high where the air is much colder, but now the Earth is much warmer at the surface.
The overall effect is for the atmosphere to act like the panes of glass in a greenhouse, keeping the Earth’s surface much warmer than it would otherwise be, hence the term ‘greenhouse effect’. Glass has similar selective properties of absorption or transmission as the Earth’s atmosphere – it is more transparent to visible and some ultraviolet radiation than infra-red.
Box 1 Greenhouse gases
A greenhouse gas (often referred to as ‘GHG’) is a gas that can absorb infra-red radiation, so contributing to the greenhouse effect. Not all gases can do this. The atmosphere absorbs infra-red radiation only because certain gases in the mixture that makes up air are greenhouse gases. A greenhouse gas molecule can absorb some infra-red radiation when the chemical bonds that hold molecules together act like springs, and like springs, they can vibrate. When the bond absorbs energy from the infra-red radiation, it vibrates more energetically. However, it turns out that infra-red radiation is absorbed only if a molecule contains more than two atoms, or – if it contains only two atoms – it must be a compound so the atoms at each end of the bond are of different elements.
The Earth’s atmosphere is largely made up of five gases: nitrogen (N2), oxygen (O2), argon (Ar), water vapour (H2O) and carbon dioxide (CO2). Which of these could act as a greenhouse gas? Argon exists as single atoms and so has no bonds and can’t absorb infra-red radiation. Oxygen and nitrogen molecules consist of two atoms of the same type, so these molecules can’t absorb infra-red radiation. Molecules of carbon dioxide and water both contain more than two atoms (and also two types of atom) – so these molecules can absorb infra-red radiation through changes in the way they vibrate. Both of these greenhouse gases are products of combustion of fossil fuels such as oil and gas.
There are smaller amounts of other greenhouse gases in the atmosphere. These include methane (CH4), which can arise from many natural processes such as decomposition (rotting) of organic matter, a byproduct of some agriculture including wet rice cultivation, and ruminants (including cows and antelopes) digesting food (belches). It can also arise from natural gas leakage during fossil fuel extraction and distribution. Another example is nitrous oxide (N2O), from vehicle emissions and the decomposition of agricultural fertilisers. Although they currently occur at much lower concentrations than carbon dioxide, these compounds can individually be more powerful absorbers of radiation than carbon dioxide (CO2). For instance, methane may be rated 20 times more potent as a greenhouse gas than carbon dioxide. This is measured in terms of the relative Global Warming Potential (GWP), a somewhat complex quantity as it involves several aspects of the behaviour of the gas in the atmosphere.
OpenLearn - Climate change and renewable energy
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