The role of water vapor feedback in unperturbed climate variability and global warming

To understand the role of water vapor feedback in unperturbed surface tempe rature variability, a version of the Geophysical Fluid Dynamics Laboratory coupled ocean-atmosphere model is integrated for 1000 yr in two configurati ons, one with water vapor feedback and one without. For all spatial scales, the model with water vapor feedback has more low-frequency (timescale grea ter than or equal to 2 yr) surface temperature variability than the one wit hout. Thus water vapor feedback is positive in the context of the model's u nperturbed variability. In addition, water vapor feedback is more effective the longer the timescale of the surface temperature anomaly and the larger its spatial scale. To understand the role of water vapor feedback in global warming, two 500-y r integrations were also performed in which CO2 was doubled in bath model c onfigurations. The final surface global warming in the model with water vap or feedback is 3.38 degrees C, while in the one without it is only 1.05 deg rees C. However, the model's water vapor feedback has a larger impact on su rface warming in response to a doubling of CO2 than it does on internally g enerated, low-frequency, global-mean surface temperature anomalies. Water v apor feedback's strength therefore depends on the type of temperature anoma ly it affects. The authors found that the degree to which a surface tempera ture anomaly penetrates into the troposphere is a critical factor in determ ining the effectiveness of its associated water vapor feedback. The more th e anomaly penetrates, the stronger the feedback. It is also shown that the apparent impact of water vapor feedback is altered by other feedback mechan isms, such as albedo and cloud feedback. The sensitivity of the results to this fact is examined. Finally, the authors compare the local and global-mean surface temperature time series from both unperturbed variability experiments to the observed r ecord. The experiment without water vapor feedback does not have enough glo bal-scale variability to reproduce the magnitude of the variability in the observed global-mean record, whether or not one removes the warming trend o bserved over the past century. In contrast, the amount of variability in th e experiment with water vapor feedback is comparable to that of the global mean record, provided the observed warming trend is removed. Thus, the auth ors are unable to simulate the observed levels of variability without water vapor feedback.