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Theory vs. Data on Warming Hurricanes |
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Theory vs. the Data (August, Letter to Nature )
Pure theory (such as it is) predicts that a 1° C increase in ocean temperature will lead to a 5% increase in wind speed. The computer models based on this theory predict slightly less. Most climatologists agree that oceans have warmed up about 1/2° C so far from global warming. That should have increased wind speeds about 2.5% which doesn't sound like much. But ...
(1) A 2.5% increase in wind speed leads to about a 10% increase in damage, and
(2) The data indicate the increase may have been as much as 7 times greater.
We will return to this puzzle shortly, but first let's look at the connection between ocean temperature and destructiveness (which is proportional to energy and much more than proportional to wind speed).
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Data Show the Connection between Temperature and Destructiveness.
It has been known for decades that it takes warm ocean water, generally warmer than 81 degrees, to make and drive a hurricane. More recently Dr. Emanuel of MIT illuminated this connection with observations of hurricane intensity increasing and decreasing with ocean surface temperatures as shown in this graph from his letter to Nature, Aug. 2005. Most of the changes in temperature shown here are not from global warming. What this shows is that whatever the cause of changing temperature, it has a big impact on hurricane energy.
The solid line is September ocean temperature in the prime spawning grounds for Atlantic hurricanes, and the dashed line is the total annual energy of Atlantic hurricanes.
This graph sidesteps the global warming issue and directly tests the theory linking temperature to wind speed (or energy). It says the impact of temperature is considerably stronger than predicted by theory. This is not surprising. It's actually amazing that the theory does as well as it does predicting something as difficult as the energy of an entire season of extremely complex storms interacting with each other in an ocean basin.
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The graph above is an update of the one published in Nature in August. It also shows the actual September sea temperatures on the left axis instead of an adjusted temperature. Total hurricanes power (energy) is measured by the same index, now shown on the right axis. Zero on this axis would indicate no hurricanes at all.
Although a strong link can be seen between energy and temperature, this can only be statistically analyzed, and is best visualized by graphing energy against temperature without concern for time. This is what can be seen in the page above. That shows that the connection is both strong—a small change in temperature has a large effect—and statistically significant; we can be very sure the effect is real.
These empirical results contradict the theoretical calculations which predict only a 5% increase in wind speed for each 1° C increase in sea temperature. This contradiction does not depend on any assumption that the temperature changed because of global warming. It's just what happens when temperature changes for any reason.
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http://zfacts.com/p/109.html | 01/18/12 07:18 GMT
Modified: Thu, 03 Apr 2008 18:04:59 GMT
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Amazon
Divine Wind: The History And Science Of Hurricanes
Twenty miles across, with brilliant white walls that soar 10 miles into the sky covered by cascading ice crystals. That's the eye of a hurricane.
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