As the Northeast and Middle Atlantic contemplate the approach of Hurricane Sandy, the question arises once again whether climate change is at work.

The same question comes up every time a whopper storm threatens the U.S. And the answer this time, sorry to say, is as mushy as ever. Yes, damaging hurricanes are more frequent in warmer years. And, yes, this year's weather so far (January to September) has been the warmest on record in the U.S.

But the storm making its way up the Atlantic coast is not a typical hurricane, and no one knows if this kind of "Frankenstorm" is a product of global warming.

Here's what climate scientists do know, according to hurricane expert Kerry Emanuel of the Massachusetts Institute of Technology: In years when the sea-surface temperatures are relatively high, the big storms are more plentiful and more powerful. This is evident from historical records of powerful tropical storms in the western Atlantic dating back to 1890.

Earlier this month, the Proceedings of the National Academy of Sciences published evidence that reinforces the connection. Aslak Grinsted, a climate scientist at the Niels Bohr Institute at the University of Copenhagen, examined daily tide records from monitoring stations along the Eastern Seaboard, kept since 1923, looking for changes in sea level large and sudden enough to indicate a tropical storm making landfall. The shifting levels he found are not perfect indicators of hurricanes -- some may have had other causes -- but they are strong enough to constitute interesting and useful new data, Emanuel says.

The measurements suggest not only that there has been a marked increase in the number of cyclones over the past nine decades, as the climate has warmed, but also that storms the size of Hurricane Katrina, which devastated New Orleans and parts of the Gulf Coast in 2005, have been twice as frequent in warm years as in cold ones.

If Sandy were an ordinary hurricane, then we'd have reason to suspect climate change. But Sandy is turning into a combination event -- part cyclone, drawing energy from evaporating ocean water, and part winter storm, gathering strength from a broad "horizontal temperature gradient" (cold air from the north meets warm air from the south). Such superstorms are rare, and scientists don't know whether a warmer climate helps make them happen.

(Mary Duenwald is a member of the Bloomberg View editorial board.)

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