Did humans turn Sandy into a superstorm?

From the darkened living rooms of Manhattan to the wave-battered shores of Lake Michigan, the question is occurring to millions of people: Did the scale and damage from Hurricane Sandy have anything to do with climate change? ¶ Scientists offered an answer that is likely to satisfy no one. They simply do not know if the storm was caused by global warming. ¶ They do know, however, that the storm surge was almost certainly intensified by decades of sea-level rise linked to human emissions of greenhouse gases.

And climate scientists emphasized that Sandy, whatever its causes, should be seen as a foretaste of trouble to come as the seas rise faster, the risks of climate change accumulate and the political system fails to respond.

"We're changing the environment -- it's very clear," said Thomas Knutson, a research meteorologist with the government's Geophysical Fluid Dynamics Laboratory in Princeton, N.J. "We're changing global temperature, we're changing atmospheric moisture, we're changing a lot of things. Humans are running this experiment, and we're not quite sure how it's going to turn out."

'Hasn't done its homework'

By the time Sandy hit the Northeast coast on Monday, upending lives across the Eastern half of the country, it had become a freakish hybrid of a large, late-season hurricane and a winter storm more typical of the middle latitudes. Though by no means unprecedented, that type of hybrid storm is rare enough that scientists have not studied whether it is likely to become more common in a warming climate.

"My profession hasn't done its homework," said Kerry A. Emanuel, a climate scientist at the Massachusetts Institute of Technology. "I think there's going to be a ton of papers that come out of this, but it's going to take a couple of years."

Scientists note that a warmer atmosphere can hold more water vapor, which in principle supplies more energy for storms of all types. The statistics seem to show that certain types of weather extremes, notably heat waves and heavy downpours, are becoming more common.

Experts differ sharply on whether such a rise can already be detected in hurricane statistics. Recent decades seem to show an increase in hurricane strength, but hurricanes tend to rise and fall in a recurring cycle over time, so it is possible that natural variability accounts for the recent trends.

Jeff Masters, a meteorologist and founder of a popular website, Weather Underground, suspects some kind of shift is underway. The number of hurricanes and tropical storms in the past three years has been higher than average, with 19 named storms in both 2010 and 2011 and 19 so far this season, which ends Nov. 30. The National Hurricane Center said there are, on average, 12 named storms each season.

"The climatology seems to have changed," he said. "We're getting these very strange, very large storms."

Hurricanes draw their energy from warm waters in the top layer of the ocean. And several scientists pointed out that parts of the western Atlantic were remarkably warm for late October as Sandy passed over, as much as 5 degrees warmer than normal for this time of year.

'A lot of this is chance'

He said that many of Sandy's odd features, including its large scale, derived from its origin as a merger of two weather systems that converged in the western Atlantic. "My view is that a lot of this is chance," he said. "A hybrid storm is certainly one which is always in the cards."