Read here. IPCC Climategate science predicts that as CO2 increase in atmosphere, the resulting warming will increase the atmosphere's water vapor levels, which will cause more warming (a positive feedback).
Unfortunately for the IPCC, that major tenet of the AGW hypothesis has not worked so well, as the below atmospheric humidity chart from www.climate4you.com reveals. (click on image to enlarge)
Now a new study discovers why the water vapor levels have not increased as predicted. Lammertsma et al. determine that as CO2 levels rise, vegetation responds in two ways: one, by absorbing more CO2 for food production, and two, releasing less water vapor. The scientists calculate that with this vegetation response, a doubling of atmospheric CO2 to 800ppm levels will cut in half the amount of atmospheric water vapor - that's called a major, natural, NEGATIVE feedback.
This negative feedback that will have a huge impact on the atmosphere's water vapor content is not included in any climate models that the IPCC, NASA and NOAA utilize. This may be a major reason why these models have continually failed in their predictions. Thus, current models' estimates of climate sensitivity evaporate, or if you prefer, transpire...or, is climate sensitivity kind of a climate model 'vaporware' chartacteristic.
"As carbon dioxide levels have risen during the last 150 years, the density of pores that allow plants to breathe has dwindled by 34 percent, restricting the amount of water vapor the plants release to the atmosphere, report scientists.....“The increase in carbon dioxide by about 100 parts per million has had a profound effect on the number of stomata and, to a lesser extent, the size of the stomata,” ...“Our analysis of that structural change shows there’s been a huge reduction in the release of water to the atmosphere.”...If there are fewer stomata, or the stomata are closed more of the day, gas exchange will be limited.....suggests that a doubling of today’s carbon dioxide levels — from 390 parts per million to 800 ppm — will halve the amount of water lost to the air, concluding in the second paper that “plant adaptation to rising CO2 is currently altering the hydrological cycle and climate..." [Emmy Lammertsma, Hugo de Boer, David Dilcher, Stefan Dekker, Andre Lotter, Friederike Wagner-Cremer, and Martin Wassen 2011: PNAS1 and PNAS2]