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.
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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.
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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.
