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Water vapour is the most important greenhouse gas in the atmosphere and accounts for about 60% * of the total greenhouse effect. Until recently, Scientists assumed that the water vapour content of the atmosphere was more or less constant and that water vapour didn't contribute to the enhanced, human induced, greenhouse effect. However, as the Earth continues to warm, the amount of water vapour in the air will rise and water may cause greater warming in the future.
* some references suggest that tropospheric water (including the long wave absorption by clouds) is responsible for 80% of the greenhouse effect.
Probably the greatest uncertainties in future projections of climate arise because we don't know the impact of changing water vapour levels in the air and because we are unsure how clouds affect the amount of solar radiation reaching the surface of the Earth.
The capacity of air to hold water vapour increases with temperature. Warmer air can take up more water. The saturation curve shows how much water air of a certain temperature can hold before condensation begins (i.e. a relative humidity of 100%). In the atmosphere, air is usually unsaturated. The relative humidity RH varies greatly and is given by the following equation:
p = given partial vapour pressure of water
psat = saturated water vapour pressure, this depends strongly on the air temperature.
p class" espere_opis_pod_obrazkiem">1. The water vapour saturation curve tells us the maximum amount of water the air can hold at a particular temperature (a relative humidity of 100%).
Generally, increasing air temperatures lead to increasing evaporation and more water vapour in the air. While ideal thermodynamic conditions are nearly fullfilled for evaporation processes in the tropospheric boundary layer (up to 1 to 2 km in altitude), conditions in the free troposphere above are much more complicated. It is here that the greenhouse potential of water vapour is likely to increase in the future.
The consequence of increasing humidity is an increase in cloud cover. Clouds interact in two ways with radiation. On the one hand, they act like greenhouse gases, absorbing infra-red radiation from the Earth and trapping the heat in the lower atmosphere. On the other hand, they increase the Earth's albedo by reflecting incoming solar radiation back into space before it reaches the Earth, effectively cooling the planet. It strongly depends on the type of cloud which effect is strongest but it is assumed that, on average, the cooling effect dominates.
p class" espere_opis_pod_obrazkiem">2. Clouds partially reflect solar radiation from the sun back into space (shown in yellow), but also absorb infra-red radiation from the surface of the Earth (shown in red) and return it to the Earth in the same way a greenhouse gas does. Picture source: Karlsruher Wolkenatlas (c) Bernhard Mühr
So global warming is likely to both increase the water vapour content of the air and increase cloud formation. These two effects are likely to have opposite feedback effects. One probably leads to further warming, the other to cooling, although the role of high clouds is still unsure.
p class" espere_opis_pod_obrazkiem">3. Feedbacks in the water cycle.
Global warming leads to higher water vapour concentrations and more water vapour leads to further warming - a positive feedback effect leading to higher and higher temperatures. However increasing water vapour concentrations also leads to more clouds. An increase in clouds probably leads to a cooling since clouds can reflect incoming solar radiation back into space. However, some computer models suggest that high clouds may have a positive feedback effect and may encourage further warming. Our knowledge about clouds is not yet sufficient to explain exactly how they affect climate.
Large differences in the amount of cloud which forms and the amount of water vapour in the atmosphere are seen on regional scales. These scales are often smaller than the grid size used in climate models, so climate models don't take into account every cloud which appears in the atmosphere.
The affect of water vapour and clouds on climate is also uncertain because many models also don't take into account:
- large scale dynamics
- sub-grid scale dynamics
- differences in cloud droplet size
- the cloud temperature
and many others.
The atmosphere is very dynamic and nowhere near a state of equilibrium.
Latitute also affects how moisture is transported through the atmosphere:
- convection (vertical transport) is most important in the tropics, where high cumulonimbus clouds are formed
- large scale wave motions occur at mid and higher latitudes, where stratiform clouds dominate
- advection (horizontal transport) is the dominant process in the dry areas between the tropics and the mid-latitudes.
p class" espere_opis_pod_obrazkiem">4. Water vapour as part of a dynamic atmosphere. Source EU
5. Global relative humidity distribution [%] in the free troposphere between 600 and 250 hPa. Attention: Read this map carefully! The free troposphere over the South Pole is more saturated than the free troposphere over the tropics relative to the possible maximum saturation, but it contains by far less water per cubic meter, i.e. in absolute values. Source: IPCC TAR Fig. 7-1.
Estimations of the water vapour and cloud feedback effects
Current estimations say that if we increase CO2 concentrations in the air to 560 ppm (double the preindustrial level of 280 ppm), the average temperature of the Earth will increase by between 1.5 and 4.5 °C. This wide range in estimates is mainly because the overall effect of clouds on our climate is still so uncertain.
A doubling of CO2 without any other change would lead to an additional radiative forcing of 3.5 to 4 W m-2, equivalent to a temperature increase of 1.2 °C. This value doubles to between 7 and 8 W m-2 if we take into account the positive feedback effect of water vapour alone. We now need to add in the impact of clouds, these may lead to a slight cooling of the planet (regarded as most likely) or to an additional warming. The uncertainty in this term is estimated to be between -3 and +3 W m-2 leading to an overall radiative forcing of about 4 to 11 W m-2 for a doubling of CO2 or a temperature increase between 1.5 and 4.5 °C.
Comparing this range in possible temperatures to the 1.2 °C temperature rise which is estimated to come from the CO2 rise alone, you get an idea just how important it is to understand the impact of water vapour on our climate.
6. Radiative forcing estimate for a doubling of CO2, with and without water vapour and cloud feedback effects. Data from IPCC TAR Chapter 7.
Find out more about the interaction of clouds and radiation and the impact on climate in:
Clouds and Particles - Basics - Unit 3 - Clouds and Climate
About this page:
author: Dr. Elmar Uherek - Max Planck Institute for Chemistry, Mainz, Germany
1. scientific reviewing: Dr. Susanne Nawrath- Potsdam Institute for Climate Impact Research - 2003-06-10
2. scientific reviewing: Dr. Benedikt Steil - Max Planck Institute for Chemistry, Mainz - 2004-05-16
educational proofreading: Michael Seesing - Uni Duisburg, Germany - 2003-07-02
last published: 2004-05-17