2009 Paper: Water Vapor Feedback is Negative on CO2

From the paper: "Water vapor feedback in climate models is positive mainly because of their roughly constant relative humidity in the mid- to upper-troposphere as the planet warms. Negative trends in [relative humidity] in the NCEP data would imply that long-term water vapor feedback is negative -that it would reduce rather than amplify the response of the climate system to external forcing such as that from increasing atmospheric CO2."