The Role of Water Vapour in the Climate
Water vapour absorbs outgoing longwave radiation and thus acts as a natural greenhouse gas. Without water vapour, the Earth's mean temperature would be too cold to sustain life as we know it. The Earth's mean temperature has increased over the past century and because of the increasing amounts of carbon dioxide in the atmosphere it is expected to increase further. The concentration of water vapour in the atmosphere can increase by about 6 % per degree Centigrade. As the atmosphere warms, atmospheric water vapour is therefore expected to increase. This will in turn lead to less outgoing longwave radiation from the earth surface while the irradiation by the sun is less affected. This leads to further warming and is known as a positive water vapour feedback.
The Intergovernmental Panel for Climate Change emphasise the importance of water vapour in climate feedbacks in their report Climate Change 2001: The Scientfic Basis. "Water vapour continues to be the single most important feedback accounting for the warming predicted by general circulation models in response to a doubling of carbon dioxide. Water vapour feedback acting alone approximately doubles the warming from what it would be for fixed water vapour."
Because of the crucial role which water vapour plays in the climate, it is important to understand the processes which determine the distribution of water vapour. It is also important to make water vapour observations over a relatively long time period in order to determine whether there is an increasing trend. Studying the role of water vapour in the climate is not simple. Water vapour distribution is highly variable across the globe and also depends on the seasonal cycle. Not only that, due to vertical and horizontal water vapour transport, changes in the course of a day can occur which far exceed any expected climatic changes. Until recently the only atmospheric water vapour observations were in situ measurements made by the humidity sensors on radiosondes. These are usually limited to a few locations and to two observations a day. The availability of satellite and ground based remote sensing techniques in recent years allows us to make water vapour observations at higher spatial and temporal resolution.
Knowledge about changes in water vapour at upper tropospheric and lower stratospheric levels is of great importance because strong alterations in radiative forcing can result from small absolute changes in water vapour at these levels. Increases in lower stratospheric water vapour mixing ratio over the last few decades are likely to have caused a decresase in stratospheric temperature by an amount comparable to that produced by ozone decreases. This cooling has been shown to lead to increased ozone depletion.
Recent work (WAVAS-report of SPARC) suggests an increase in stratospheric H2O in the last decades, but the reasons for this positive trend are not fully understood nor well characterised. The trend is also obscured by large decadal and interannual fluctuations. The magnitude of the trend in stratospheric water vapour cannot totally be accounted for by an increase in methane . This leads to the hypothesis that the remaining increase must originate from increased injection of tropospheric water vapour across the tropical tropopause. Whereas most of the stratosphere will cool as greenhouse gas concentrations increase, the tropical tropopause may become warmer, resulting in an increase of the mean saturation mixing ratio of water vapour and hence an increased transport of water vapour from the troposphere to the stratosphere. But these mechanisms are not well understood.
Contactlast update: Sep. 2009