I understand that some water vapor escapes into space but whats the amount say per year?
seems to be negligible: from http://en.wikipedia.org/wiki/Atmosp...tmospheric_escape_and_loss_processes_on_Earth …
Because of the temperature regime of Earth, CO2 and H2O are sequestered in the hydrosphere and lithosphere.
Because of the temperature regime of Earth, CO2 and H2O are sequestered in the hydrosphere and lithosphere. H2O vapor is sequestered as liquid H2O in oceans, greatly decreasing the atmospheric density.
Therefore, the dominant “loss” mechanism of Earth's atmosphere is not escape to space, but sequestration.
I think that you would be very hard put to get a figure for the loss. This article ( http://www.scientificamerican.com/article.cfm?id=how-planets-lose-their-atmospheres ) which is referenced from the Wikipedia Atmospheric Escape article ( http://en.wikipedia.org/wiki/Atmospheric_escape ) is quite interesting.
One interesting thing to take away from it is that while there might by hydrogen and oxygen loss it's very unlikely that full molecules of water escape from the atmosphere intact.
If you read to the end you will see that there are estimates that when the Sun becomes 10% brighter in a billion years time then there will start to be greater water loss but it will take another billion years to lose most of the water. So you can imagine that the rate of water loss at the moment is quite low.
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