March 18, 2004

The Vapor Rub

Filed under: Climate Models

Climate models exaggerate global warming because they wrongly characterize the amount of water vapor reaching the upper atmosphere as temperatures rise.

New research shows that global warming projections from climate models are overblown, something that shouldn’t surprise readers of these pages. The overestimates arise from an inaccurate handling of the physical processes responsible for controlling the amount of water vapor that reaches the upper levels of the atmosphere when temperatures rise.

Both water vapor and carbon dioxide are greenhouse gases that can act to warm the air. Warmer air has the capacity to contain more water vapor than cooler air. For that reason, climate models contain a “positive feedback loop” between atmospheric carbon dioxide levels and atmospheric moisture content: As CO2 levels increase in the atmosphere, they lead to warmer conditions, which lead to more water vapor in the air, which leads to even warmer conditions. In the absence of this positive water vapor feedback, enhanced carbon dioxide levels alone result in the climate models’ projecting only about one-half to two-thirds the amount of warming than when the water vapor feedback is included.

But this doesn’t account for the fact that the water vapor feedback itself can be amplified by consequent changes in cloudiness. All in all, according to the United Nations’ Intergovernmental Panel on Climate Change (IPCC), this feedback can multiply the warming effect of carbon dioxide by more than a factor of three—which is one reason they predict such a humongous range of future warming, from 1.4°C to 5.8°C in the next 100 years.

A paucity of studies has attempted to actually measure the strength of the water vapor feedback loop and its response to warmer temperatures. So it has simply been assumed that the climate model handling of the processes involved was more or less pretty close to reality.

Assumed by most people, that is.

As early as 1990, MIT’s Richard Lindzen began to question the assumed strength of the water vapor feedback (in fact, he even questioned whether it was positive). Lindzen contended that the amount of water vapor in the upper levels of the atmosphere (above about 5 km) may actually decline in a warmer world. He argued that warmer surface temperatures would lead to more and stronger convection events (thunderstorms) that would transport less moisture to higher altitudes as a result of an increasing precipitation efficiency (i.e., the storms rain would themselves out). That effect, he theorized, combined with a few other hypothesized impacts, results in a much diminished water vapor feedback loop and thereby a much diminished greenhouse warming.

Now, about 14 years later comes further support (and actual evidence) for Lindzen’s ideas. Climate researchers Ken Minschwaner and Andrew Dessler have published the results of their careful examination of the water vapor feedback in tropical regions, both as contained in climate models and in observations made by instruments aboard orbiting satellites.

First, they developed their own model, one that has much more detail and resolution in the processes that describe vertical moisture transport in the tropics than do full-blown general circulation models (GCMs). Using this modeling tool, they determined that while the water vapor feedback was indeed positive, it was only about half as strong as found in current GCMs. These researchers then took the nearly unprecedented step of actually comparing their results with observations!

Combining satellite measurements of upper atmospheric moisture levels with the sea-surface temperatures below, the authors were able to demonstrate the actual relationship between temperature changes and upper atmospheric humidity changes. What they found was that the observed relationship was very close (or perhaps even a bit weaker) to what their model showed (Figure 1).

SSTs and Humidity

Figure 1. The relationship between water content (specific humidity—y-axis) at an altitude of about 12km and the sea surface temperature below (x-axis). Notice that the observed relationship falls between the expected relationship from “no positive feedback” and the positive feedback found by Minschwaner and Dessler in their model. The relationship inherent in current GCMs is not even close to reality (adapted from Minschwaner and Dessler, 2004).

The final result of Minschwaner and Dessler’s efforts is that it is likely that the actual strength of the positive water vapor feedback, at least in the tropics, is significantly less than is inherent in current GCMs, such as the ones used by the Intergovernmental Panel on Climate Change (IPCC) in their 2001 Third Assessment Report (TAR). A weaker positive water vapor feedback directly results in less warming.

How much less? Well, in the authors’ model of the tropics, the warming is about 25 percent less when the model is run using their calculated feedback strength than when run using the assumptions that closely match current GCM processes. The tropics make up about one-half of the surface area of the earth. If we make the somewhat conservative assumption that the weakened feedback is contained only within the tropics, the result is a global overestimation of the temperature rise resulting from an enhanced greenhouse effect of about 10-15 percent.

Four years ago, WCR’s own Pat Michaels argued that internal adjustments to greenhouse gases that had to be occurring compelled forecasts of future warming to be racheted down to about three-quarters of a degree in the next 50 years, an amount that is more likely to be beneficial rather than harmful. This value is right at the low end of IPCC’s projections. Three years ago, NASA’s James Hansen, not someone you would typically expect to agree with Michaels, came up with exactly the same value. Now we know one reason why.

References:

Intergovernmental Panel for Climate Change (IPCC), 2001. Climate Change 2001: The Scientific Basis, Cambridge Univ. Press, New York.

Hansen, J.E., and M. Sato, 2001. Trends of measured climate forcing agents. Proceedings of the National Academy of Sciences, 98, 14778–14783.

Michaels, P.J. and Balling, Jr., R.C., 2000. The Satanic Gases: Clearing the Air about Global Warming. The Cato Institute, Washington DC, 234pp.

Lindzen, R.S., 1990. Some Coolness Concerning Global Warming. Bulletin of the American Meteorological Society, 71, 288-299.

Minschwaner, K., Dessler, A.E., 2004. Water Vapor Feedback in the Tropical Upper Troposphere: Model Results and Observations. Journal of Climate, 17, 1272-1282.




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