March 5, 2009

Feedbacks and Climate Sensitivity

A week or two ago, Andrew Dessler and Steven Sherwood published a “Perspectives” (largely opinion) piece in Science magazine that argued that the water vapor feedback was unassailably strong and positive. This means that the warming from the increase in greenhouse gas concentrations leads to more water vapor in the atmosphere which leads to even more warming (water vapor is a strong greenhouse gas itself). This positive feedback results in roughly twice as much warming as would occur from anthropogenic greenhouse gas increases alone.

Dessler and Sherwood concluded:

There remain many uncertainties in our simulations of the climate, but evidence for the water vapor feedback—and the large future climate warming it implies—is now strong.

This conclusion has drawn a lot of attention within the community of researchers investigating the behavior of water vapor and the role of water feedback in climate change—and most of it has been highly critical.

Response and discussion has broken out across the web, MasterResource lays out the issue from both sides, while ClimateAudit has an active discussion including contributions from scientists who have directly investigated the issue themselves—including the thoughts of Garth Paltridge who, along with two other colleagues just published a paper showing that evidence for the positive feedback was lacking in one of the standard datasets of long-term atmospheric observations (a finding not mentioned by Dessler and Sherwood).

Another researcher, Roy Spencer, in fact, finds the feedback from water in the atmosphere to be, in net, negative as the effect from changes in cloud characteristics and patterns more than counteracts the effect from added water vapor.

This issue is critical, for the strength of the feedbacks will ultimately determine just how much climate change we experience. High, positive feedbacks lead to estimates of 3-4ºC of temperature change from a doubling of atmospheric carbon dioxide concentrations, while weak or even negative feedbacks lead to warming estimates of only 1-2ºC. Obviously, there is a world of difference between these numbers.

Many members of the community are making their voices heard that the confidence with which Dessler and Sherwood spoke of concerning the strength of the water vapor feedback is not mutually shared.

Another attempt to paint the “science as being settled” is being strongly rebutted by working scientists.


Dessler, A.E., and S. C. Sherwood, 2009. A matter of humidity. Science, 323, 1020-1021.

Paltridge, G., et al., 2009. Trends in middle- and upper-level tropospheric humidity fron NCEP reanalysis data. Theoretical and Applied Climatology, DOI 10.1007/s00704-009-0117-x.

Spencer, R., and W.D. Braswell. 2008. Potential biases in feedback diagnosis from observations data: a simple model demonstration. Journal of Climate, 21, 5624-5628.

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