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Water Vapor’s Counterintuitive Effect on Climate

Climatologists are only beginning to model the role that water vapor plays in atmospheric circulation. But the early results are surprising.

Water vapor is by far the most important greenhouse gas in the atmosphere, but it also plays a crucial but far less-well-understood role in atmospheric dynamics, say Tapio Schneider from Caltech and a couple of pals.

These guys want to change that by studying the impact on the global atmospheric dynamics of the release of energy as water vapor condenses and how these dynamics might change as the global temperature increases. What they find is both counterintuitive and important.

In general, water cools the atmosphere when it evaporates and heats it up as it condenses. But on a large scale, the heating effects are far more important for atmospheric dynamics because there is some 250 times more vapor than liquid in the atmosphere.

What Schneider & Co. have done is model the effects this can have on the atmosphere as the temperature changes.

“We view past and possible future climates as parts of a climatic continuum that is governed by universal, albeit largely unknown, macroscopic laws. Our goal is to constrain the forms that such macroscopic laws may take,” the researchers say.

Their main result is surprising and counterintuitive. Take tropical Hadley circulation caused by the evaporation of water at the equator and its subsequent condensation in the subtropics. The conventional view is that if tropical Hadley circulation was weak when the climate was colder and is stronger now, it will be stronger still in an even hotter climate (a so-called monotonic change).

But Schneider and friends find something completely different.

“Contrary to widely held beliefs, atmospheric circulation statistics can change non-monotonically with global-mean surface temperature, in part because of dynamic effects of water vapor. “

That’s extraordinary. What they’re saying is that as the temperature increases, aspects of atmospheric circulation could vary in one way and then back again.

For example, they calculate that the strength of the tropical Hadley circulation can be lower than it is now both in much warmer climates and in much colder ones, too (a so-called nonmonotonic change).

Clearly, more work is needed, but this preliminary study indicates that nonmonotonic changes may come to be much more significant parts of climate change models in future.

Ref: arxiv.org/abs/0908.4410 : Water Vapor and the Dynamics of Climate Change

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