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The role of water vapour in the atmosphere

In brief

Greenhouse gases play a major role in determining the balance between the amount of radiation entering the Earth's surface, and the amount of radiation leaving the Earth's surface. The greenhouse effect keeps the Earth's surface about 33 °C warmer than it would otherwise be.
The main greenhouse gases are water vapour, carbon dioxide (CO₂), methane, nitrous oxide, ozone and a suite of synthetic gases, mainly chlorofluorocarbons (CFCs).
There is far more water vapour in the atmosphere than other greenhouse gases, but the other gases are still very important. While human activity does not directly affect the amount of atmospheric water vapour, human activity has significantly increased the concentrations of the other greenhouse gases, such as carbon dioxide (CO₂), which cause the Earth's surface to warm and result in increased water vapour levels and further warming—a positive 'feedback'.
This water vapour feedback is complex, but is calculated to increase the radiative impact (and hence warming) of increased greenhouse gases by about 50 per cent. All climate models incorporate this water vapour feedback in their climate simulations.

In detail

Water vapour as a greenhouse gas

The Earth's surface temperature depends on the balance between incoming and outgoing radiation. Greenhouse gases absorb and re-radiate much of the infrared radiation released by the Earth's surface. They produce a natural greenhouse effect, maintaining the temperature of the Earth's surface some 33 °C warmer than it would otherwise be.

The main greenhouse gases are water vapour, CO₂, methane, nitrous oxide, ozone and the synthetic greenhouse gases, largely chlorofluorocarbons (CFCs). Apart from the synthetics, all of these gases occur naturally. Together, they make up less than one per cent of the atmosphere, which is comprised mainly of nitrogen and oxygen. There is far more water vapour in the atmosphere than any other greenhouse gas. By mass, it accounts for approximately 0.3 per cent of the atmosphere compared to about 0.06 per cent for CO₂.

Human activity does not directly affect the amount of atmospheric water vapour. However, as the atmosphere warms, it can hold more water vapour. The extra water vapour amplifies global warming. This is the so-called positive water vapour feedback, which, when combined with warming in the upper troposphere, can amplify the response to other greenhouse gases by around 50 per cent (IPCC, 2007, Box 8.1, p. 863).

All climate models incorporate this water vapour feedback in their climate simulations. The only way that water vapour levels in the atmosphere can be controlled is through control of the long-lived greenhouse gases—CO₂, methane, nitrous oxide and the synthetics.

Human contributions to warming

Between 1750 and 2005, methane concentrations rose by nearly 150 per cent and nitrous oxide by 18 per cent (IPCC, 2007). Over the last 800 000 years, the amount of CO₂ in the atmosphere has varied naturally between approximately 172 and 300 parts per million (ppm). However, the CO₂ concentration in 2008 was 383 ppm, 38 per cent higher than in 1750.

There is very high confidence that the globally averaged net effect of human activities since 1750 (the beginning of the Industrial Revolution, or more precisely the 'fossil-fuel' era) has been one of warming, with a net radiative forcing of +1.6 Wm-2 (Figure 1). This is mainly due to increases in CO₂, methane and nitrous oxide, amplified by water vapour increases, which overwhelm the net cooling effect of increases in aerosols (mainly sulphate, organic carbon, black carbon, nitrate and dust). The warming contribution from solar variations is 1/20^th (five per cent) of the contribution from greenhouse gases (IPCC, 2007).

Figure 1: Changes in radiative forcing (RF) due to different anthropogenic factors and natural solar irradiance from 1750 to 2005. Volcanic aerosols contribute an additional natural forcing but are not included in the figure due to their episodic nature. The assessed level of scientific uncertainty (LOSU) is given in the right column.

Figure 1: Changes in radiative forcing (RF) due to different anthropogenic factors and natural solar irradiance from 1750 to 2005. Volcanic aerosols contribute an additional natural forcing but are not included in the figure due to their episodic nature. The assessed level of scientific uncertainty (LOSU) is given in the right column. Source: IPCC (2007 Fig SPM2).

Most of the observed increase in global average temperatures since the mid-20th century is very likely (at least 90 per cent likely) due to the observed increase in anthropogenic greenhouse gas concentrations (IPCC, 2007). It is very likely that greenhouse gases caused more global warming over the last 50 years than did changes in solar irradiance.

Radiative forcing and climate sensitivity

As concentrations of any gas increase, there will be a point where so much radiation is already being absorbed that the scope for additional absorption, at its characteristic wavelengths, is reduced (Enting, 2008). The radiative response to increases in water vapour and CO₂ follows a logarithmic relationship, while the radiative response for methane and nitrous oxide follows a power (square root) relationship, and radiative response to the synthetics increases linearly with concentration. The logarithmic relationship between CO₂ concentration and radiative forcing is shown in Figure 2: each doubling of CO₂ gives the same increase in radiative forcing. Currently, CO₂ is not absorbing all it can (the absorption wavelengths around 15 micrometres are not saturated), so there is scope for more warming over a wide range of future increases in CO₂ concentrations.

Figure 2: Dependence of radiative forcing on concentration of CO2 (ppm), using the IPCC (2001) logarithmic approximation (Table 6.2). Each doubling of CO2 concentration adds a fixed amount of radiative forcing. The CO2 concentration in 2008 was 383 ppm.

Figure 2: Dependence of radiative forcing on concentration of CO₂ (ppm), using the IPCC (2001) logarithmic approximation (Table 6.2). Each doubling of CO₂ concentration adds a fixed amount of radiative forcing. The CO₂ concentration in 2008 was 383 ppm. Source: Enting (2008).

The equilibrium climate sensitivity is a measure of the climate system response to sustained radiative forcing. It is defined as the equilibrium global average surface warming following a doubling of CO₂ concentration. A doubling of atmospheric CO₂ would result in a climate sensitivity in the range of 2 to 4.5 °C with a best estimate of about 3 °C, and is very unlikely to be less than 1.5 °C (IPCC, 2007). Values substantially higher than 4.5 °C cannot be excluded however agreement of models with observations is not as good for those values.

References

Enting, I.G. (2008). Twisted: The distorted mathematics of greenhouse denial. Australian Mathematical Sciences Institute, Melbourne, p. 152.

IPCC (2001). Climate Change 2001: The Scientific Basis. Contribution of the Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Houghton, J., Ding, Y., Griggs, D., Noguer, M., van der Linden, P. and Xiaosu, D. (eds). Cambridge University Press, p. 944. www.ipcc.ch

IPCC (2007). Climate Change 2007: The Physical Scientific Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (eds), Cambridge University Press, p. 996. www.ipcc.ch