The advocates of AGW do not normally talk about the fact that an increase in atmospheric carbon dioxide alone will not cause a significant increase in Earth's temperature. (Water vapor represents about 95% of the Greenhouse gas effect, with carbon dioxide accounting for 3 or 4%.) The AGW true believers but don't want to confuse the folks, but the increase in temperature predicted by the GCM's is the result of an increase in water vapor in the air resulting from a slight amount of heating due to an increase in carbon dioxide. It is obviously hard to determine the historical record of water vapor in the atmosphere. But, someone has now done a study of the recent history, and concluded that there has been no change. That indicates that the AGW theory is not valid. Here is a reference I got from American Thinker.

The new paper by Smith et al, suggests that there has been no global increase in water vapour content, and undermines the IPCC foundation stone of an enhancement of the increased warming effect of CO2 via increased atmospheric water vapour:

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L06705, doi:10.1029/2005GL025393, 2006 Variations in annual global precipitation (1979-2004), based on the Global Precipitation Climatology Project 2.5° analysis Thomas M. Smith et al

Abstract The Global Precipitation Climatology Project (GPCP) has produced a combined satellite and in situ global precipitation estimate, beginning 1979. The annual average GPCP estimates are here analyzed over 1979-2004 to evaluate the large-scale variability over the period. Data inhomogeneities are evaluated and found to not be responsible for the major variations, including systematic changes over the period. Most variations are associated with El Niño/Southern Oscillation (ENSO) episodes. There are also tropical trend-like changes over the period, correlated with interdecadal warming of the tropical SSTs and uncorrelated with ENSO. Trends have spatial variations with both positive and negative values, with a global-average near zero.