Just how much warming should we expect from rising levels of atmospheric greenhouse gases (GHGs)? The answer largely hinges on how much extra warming might be generated by the initial warming—that is, how strong (and in what direction) are the feedbacks from water vapor and clouds.
By most estimates (including climate model outcomes), these feedbacks are positive and result in about a doubling of the warming that would result from greenhouse gas increases alone. By others, however, the total feedbacks are negative, and imply that the total warming will be less than the warming from greenhouse gas increases alone, and only a fraction of that which is commonly expected.
The ultimate warming experienced across the 21st century will depend on the combination of greenhouse gas emissions and how the climate responds to them. The feedback issue is an essential part of the latter, for it spells the difference between a high climate sensitivity to greenhouse gas doubling (say 3-4ºC) and a much lower one (1-2ºC).
As to where the answer lies, the devil is in the details, and in this instance he is hard at work, as the processes involved are exceedingly complex—difficult to not only to fully understand, but even to adequately measure. In a “Perspectives” piece in a recent issue of Science magazine, Andrew Dessler and Steven Sherwood attempt to put to rest any notion that the feedbacks on global temperatures are anything but positive and significant. Dessler and Sherwood point out that the role of water vapor plays the largest role in the feedback process—higher temperature (from greenhouse gases) lead to more water vapor in the atmosphere which leads to even higher temperatures still (as water vapor, itself, is a strong greenhouse gas).
But actual hard evidence that water vapor is increasing in the atmosphere has been hard to come by. Dessler and Sherwood review the recent literature on the topic, including an important contribution from Dessler et al. published last year, and conclude that there now exists sufficient evidence to conclude that atmospheric water vapor is increasing very much in line with climate model expectations and that this increase produces roughly twice the global temperature rise than does anthropogenic greenhouse gas enhancement alone. Meaning, of course, that all is generally right in model world, or as they put it: “There remain uncertainties in our simulations of the climate, but evidence for the water vapor feedback—and the large future warming it implies—is now strong.”
But apparently Dessler and Sherwood didn’t convince everyone that this is the case. One notable person who was less than impressed that this was the whole story was Roy Spencer, who has himself been working on the feedbacks issue. Spencer points out that water is actually involved in two feedback processes—the first, through water vapor as described by Dessler and Sherwood, and the second, through water droplets, or, more commonly, clouds. Changes in the patterns (horizontal, vertical, and temporal) and characteristics (droplet size, brightness, etc.) of cloud cover play an important role not only in the earth’s climate, but in how the climate responds to changes in the greenhouse effect. And, as you may have guessed from their ephemeral nature, the behavior or clouds is not particularly well-understood, and even less well modeled.
Spencer has been looking into the cloud part of the feedback processes. Over the past several years, during the period when Dessler et al. (2008) finds a positive feedback from increases in water vapor, Spencer, in his investigations, finds that cloud cover changes produce a feedback in the opposite direction. And when he adds these two effects together, he finds that the total feedback from warming-induced changes in water in the atmosphere to be negative (that is, the cloud effect dominates the vapor effect). Granted, Spencer’s investigations are far from complete and even farther from being generally accepted, but they do raise important concerns as to the ability of examinations of short-term behavior to diagnose long-term response (a situation relied on by both Spencer, and Dessler and Sherwood). Spencer concludes that “unless you know both [vapor] and [cloud] feedbacks, you don’t know the sensitivity of the climate system, and so you don’t know how much global warming there will be in the future.” Virtually the opposite sense of things than that put forth by Dessler and Sherwood.
Obviously, the final arbiter will be the earth’s climate itself, as it is the true integrator of all forces imparted upon it. But, still today, we struggle to even accurately observe the finer details of how it is responding to the changes to which it is being continually subjected. And we are further still from understanding the processes involved sufficiently to produce unassailable models of the climate’s behavior, much less future projections of its response response (as evidenced by the recent slowdown in the rate of global temperature increase despite ever-growing greenhouse gas emissions). And so the process of science continues…
[Breaking news: A new peer-reviewed paper has just been published in the journal Theoretical and Applied Climatology, by researchers Garth Paltridge and colleagues which finds that the increase in atmospheric water vapor that, according to Dessler and Sherwood most definitely accompanies the increase in temperature, is absent in one of the primary databases used to study climate behavior—the so-called NCEP reanalysis data. The authors admit that perhaps there are errors contained in this dataset which may explain their results, but as it stands now (and unless some errors are identified) the reanalysis data supports a negative water vapor feedback. Paltridge et al. conclude:
Negative trends in [water vapor] as found 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. In this context, it is important to establish what (if any) aspects of the observed trends survive detailed examination of the impact of past changes of radiosonde instrumentation and protocol within the various international networks.
Lead author Garth Paltridge describes the trials and tribulations of trying to get this result (which runs contrary to climate model expectations) published in an enlightening article over at ClimateAudit, including how at least one of the Dessler and Sherwood authors knew of Paltridge’s soon-to-be-published results and yet made no mention of it in their Science piece. Hmmm, so much for an open discussion of the science on this issue.]
Dessler, A.E., and S. C. Sherwood, 2009. A matter of humidity. Science, 323, 1020-1021.
Dessler, A.E., et al., 2008. Water-vapor climate feedback inferred from climate fluctuations, 2003-2008. Geophysical Research Letters, 35, L20704.
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.