Many questions but few answers on climate, storm

From the darkened living rooms of lower Manhattan to the wave-battered shores of Lake Michigan, the question is occurring to millions of people at once: Did the enormous scale and damage from Hurricane Sandy have anything to do with climate change?

Hesitantly, climate scientists offered an answer this week that is likely to satisfy no one, themselves included. They simply do not know for sure if the storm was caused or made worse by human-induced global warming.

They do know, however, that the resulting storm surge along the Atlantic coast was almost certainly intensified by decades of sea-level rise linked to human emissions of greenhouse gases.

And they emphasized that Hurricane 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 R. 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."

By the time Hurricane 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.

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.

But how those general principles will influence hurricanes has long been a murky and contentious area of climate science. Most scientists expect that the number of Atlantic hurricanes will actually stay steady or decline in coming decades as the climate warms, but that the proportion of intense, damaging storms is likely to rise.

The 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 under way. The number of hurricanes and tropical storms over the past three years has been higher than average, with 19 named storms in both 2010 and 2011 and 19 so far this hurricane season, which ends Nov. 30. According to the National Hurricane Center there are, on average, 12 named storms each season.

"The climatology seems to have changed," Masters said. "We're getting these very strange, very large storms with very low central pressures that don't have that much wind at the surface."

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

Kevin E. Trenberth, a scientist with the National Center for Atmospheric Research, said that natural variability probably accounted for most of that temperature extreme.

But, he added, human-induced global warming has raised the overall temperature of the ocean surface by about one degree Fahrenheit since the 1970s. So global warming probably contributed a notable fraction of the energy on which the storm thrived – maybe as much as 10 percent, he said.

He said many of Sandy's odd features, including its 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, and it's one we've always worried about."

• Read more articles by Justin Gillis

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