1. The greenhouse effect is a natural phenomenon that warms the Earth’s surface
- In the mid-nineteenth century John Tyndall carried out experiments on the radiative properties of atmospheric gases. He found that gases such as water vapour, carbon dioxide and methane strongly absorb infrared (thermal) radiation, while gases such as oxygen and nitrogen absorb very little .
- Theoretical and observational developments in physics during the twentieth century have shown that this is because different molecules absorb and re-emit infrared radiation of different wavelengths, depending on their molecular structure. For example, we now know that carbon dioxide strongly absorbs radiation with wavelengths of around 15 microns , and re-emits radiation at the same wavelengths.
- The atmosphere is made up of a number of different gases (Table 1, below). The properties and abundances of these gases mean that, overall, the atmosphere is relatively transparent to visible light, which has short wavelengths, but tends to absorb infrared radiation, which has longer wavelengths (Figure 1, below).
Table 1 (above): Average composition of the dry atmosphere below 25 km. Water vapour is additional to these figures, with its concentration varying significantly both spatially and over time.
Figure 1 (a) (above, top): Curves of black-body energy Bλ at wavelength λ for 5750K (approximating to the sun’s temperature) and 245K (approximating to the atmosphere’s mean temperature). The curves have been drawn of equal areas since integrated over the Earth’s surface and all angles the solar and terrestrial fluxes are equal.
Figure 1 (b) (above, bottom): Absorption by atmospheric gases for a clear vertical column of atmosphere. The positions of the absorption bands of the main constituents are marked. Source: Physics of Atmosphere, Houghton, 2002
- Most of the energy radiated from the sun is concentrated in the visible (light) and near-visible parts of the electro-magnetic spectrum. These have short wavelengths and are able to pass through the atmosphere with little absorption. This is illustrated by the left hand side of Figure 1. Overall, about 20% of solar energy that enters the Earth’s atmosphere is reflected back into space by clouds and atmospheric particles and gases, and a further 10% is reflected by the Earth’s surface (Figure 2, below).
- The rest of the sun’s energy (about 70%) is absorbed by clouds and gases in the atmosphere and by land, water and vegetation at the surface. This energy warms the surface and is re-radiated in the form of heat (infrared radiation) . As certain greenhouse gases, including water vapour, absorb infrared radiation, a significant proportion of this re-radiated energy is absorbed by the atmosphere before it escapes into space.
- After gas molecules absorb radiation, they re-emit it in all directions. Some of the infrared radiation absorbed by gases in the atmosphere is therefore re-radiated out towards space and eventually leaves the atmosphere, but some is re-radiated back towards the Earth, warming the surface and lower atmosphere (illustrated by the ‘back radiation’ term in Figure 2). This warming is known as the greenhouse effect and the gases that are responsible for it are known as greenhouse gases. (Further explanation of the greenhouse effect)
Figure 2 (above): Estimate of the Earth’s annual and global average energy balance - all units are W/m-2 (watts per square metre). About half the incoming solar radiation is absorbed by the Earth’s surface. This energy is transferred to the atmosphere by warming the air in contact with the surface (thermals), by evaporation from the Earth’s surface and plant transpiration, and by long-wave radiation that is partially absorbed by clouds and greenhouse gases. The atmosphere in turn re-radiates long-wave energy back to Earth (the ‘greenhouse effect’) as well as out to space. (Larger version of Figure 2 (PPT, 197 Kb) )
- Without the greenhouse effect, the average temperature at the Earth’s surface would be about 33°C colder  (-18°C rather than 15°C)  . Other planets also have greenhouse effects: Mars has a relatively weak effect (its average surface temperature is about 3°C warmer than it would be without it), while Venus has a dense atmosphere rich in carbon dioxide, resulting in a strong greenhouse effect that keeps it relatively hot (its average surface temperature is about 470°C warmer than it would be without it).
- On Earth, water vapour is responsible for up to about 70% of the natural greenhouse effect . Carbon dioxide (CO2), even though its concentration in the atmosphere is quite low, is responsible for a significant proportion of the remaining greenhouse effect. This is because it is an efficient absorber of radiation with wavelengths that are emitted strongly by the Earth (i.e. wavelengths near the peak of the Earth’s ‘emission spectrum’, as shown by the right hand side in Figure 1) and because CO2 acts within the regions of the spectrum where water vapour effects are small.
- It is relatively straightforward to calculate the temperature change that will arise from a given change in the concentration of greenhouse gases in the climate system (assuming no feedbacks) because this ‘radiative’ warming is determined by basic physics. A doubling of the concentration of CO2 in the atmosphere would cause a temperature rise of approximately 1°C . However this direct effect is modified by feedback processes in the atmosphere, as explained in Section 6.
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- Tyndall, J. 1863 On radiation through the Earth's atmosphere. Philosophical Magazine, Series 4, Volume 25, Issue 170, pp200 – 206
- A micron is a unit of length equal to one millionth of a metre
- This transformation occurs because the wavelengths of energy radiated by a physical object are related to its temperature. The positions of the curves on the spectrum in Figure 1(a) reflect this effect, showing that radiation emitted by the Earth and its atmosphere tends to have longer wavelengths than radiation emitted by the Sun.
- Assuming no change to the planetary albedo
- National Research Council (1982), cited in Mitchell (1989)
- John Tyndall realised this in the mid-nineteenth century, when he concluded that without water vapour the Earth's surface would be "held fast in the iron grip of frost".
- IPCC First Assessment Report - WG1, p. xxxvi (1990).
- See Mitchell, J. (1989) for an explanation.