H2O In The Atmosphere
Most of the atmospheric H2O is in the troposphere and it is also where practically all weather takes place, 'although there is some slow exchange between the troposphere and the stratosphere. Formation of clouds is low in the stratosphere as well as the increase of the water content due to air traffic. In the following therefore the troposphere is mostly of interest when dealing with H2O as water vapor and as clouds and for the solution of the climate change problems.
The curve below shows the amount of H2O as gram per cubic meter (equal to thousands of metric tons per cubic kilometer) that saturated air and clouds are holding at various temperatures and at normal pressure.
From the size of the Earth globe with an average radius of 6350 kilometers and an average height of the troposphere of 15 kilometers above the ground we can calculate the volume of the troposphere to be about 7500 000 000 cubic kilometers. It has been calculated by US geological Survey that the present mean global concentration of water in the atmosphere corresponds to a content of abt. 12 900 billion tons of water in the troposphere. This is a vast amount! With the assumed residence time of about 10 days in the atmosphere (troposphere) this means that about 1290 billion tons of this water is precipitated and coming down as rain, snow etc. as an average every day. The temperature and the air pressure decrease from the Earth´s surface up to the tropopause (a thin boundary layer between the troposphere and the stratosphere) so the amount of water vapor that can exist in the atmosphere decreases strongly with the height.
It seems to be a general understanding among climatologists and politicians that a temperature increase of the troposphere of only 2 degrees centigrade above the present average temperature level is the maximum that can be accepted. If we investigate the curve above and calculate how much the amount of saturated water vapor would increase at a temperature increase of 2 degrees we will find that it would be around 13 % in the 10 – 20 degree centigrade span and also not so far from the same amount in the other nearby temperature spans. If we now simply suppose that the saturated air has an average temperature of about 15 degrees centigrade and allow the temperature to increase about two more degrees centigrade to about 17 degrees centigrade the amount of water the air can hold will increase another 13 %. Even the additional 1680 billion tons of H2O in the atmosphere at a two degrees centigrade temperature increase is a vast amount, particularly if it comes down as rain! At the residence time of 10 days in the atmosphere an additional average amount of about 170 billion tons of H2O could then precipitate daily. That is also a vast amount – more than 7 times the amount the Amazon River discharges to the Atlantic Ocean daily! The global warming due to the emissions of greenhouse gases thus leads to considerably higher amounts of water vapor in the troposphere that can at various places give considerably more precipitation: more snow, heavier rain falls, floods and hurricanes. Also a tendency of larger variations between drier and wetter areas can occur and has been noticed.
There is another effect whose importance might be disputed and that is the formation of so called hydrated complexes. H2O can form dimers and various clusters with itself and complexes with other molecules. Interpretations of results from recent research has led to the conclusion that hydrated complexes likely play an important role in the absorption of solar radiation and have an impact on Earth´s energy balance and climate. So little is yet understood about this property of water that it is considered to be one of the unsolved problems of chemistry.
It is thus conceivable that a general temperature increase of only 2 degrees centigrade of the atmosphere will lead to severe climate changes in terms of increased precipitation and floods.