WASHINGTON // The climate scientist, Michael Oppenheimer, stood along the Hudson River and watched his research come to life as Hurricane Sandy blew through New York.
Just eight months earlier, the Princeton University professor reported that what used to be once-in-a-century devastating floods in New York would soon happen every three to 20 years. He blamed global warming for pushing up sea levels and changing hurricane patterns.
New York "is now highly vulnerable to extreme hurricane-surge flooding", he wrote.
For more than a dozen years, Mr Oppenheimer and other climate scientists have been warning about the risk for big storms and serious flooding in New York. A 2000 federal report about global warming's effect on the United States warned specifically of that possibility.
Still, they say it is unfair to blame climate change for Sandy and the destruction it left behind. They cautioned that they cannot yet conclusively link a single storm to global warming, and any connection is not as clear and simple as environmental activists might contend.
"The ingredients of this storm seem a little bit cooked by climate change, but the overall storm is difficult to attribute to global warming," said Andrew Weaver, a climate scientist at Canada's University of Victoria.
Some individual parts of Sandy and its wrath seem to be influenced by climate change, several climate scientists said.
First, there's sea-level rise. Water levels around New York are a nearly 30 centimetres higher than they were 100 years ago, said the climate scientist Michael Mann, of Penn State University.
Add to that the temperature of the Atlantic Ocean, which is about 1.1°C warmer on average than a century ago, said Katharine Hayhoe, a climate scientist at Texas Tech University. Warm water fuels hurricanes.
And Sandy zipped north along a warmer-than-normal Gulf Stream that travels from the Caribbean to Ireland, said Jeff Masters, a meteorology director for the private service Weather Underground.
Meteorologists are also noticing more hurricanes late in the season and even after the season. A 2008 study said the Atlantic hurricane season seems to be starting earlier and lasting longer but found no explicit link to global warming. Normally there are 11 named Atlantic storms. The past two years have seen 19 and 18 named storms. This year, with one month to go, there are 19.
After years of disagreement, climate scientists and hurricane experts have concluded that as the climate warms, there will be fewer total hurricanes. But those storms that do develop will be stronger and wetter.
Sandy took an unprecedented sharp left turn into New Jersey. Usually storms keep heading north and turn east harmlessly out to sea. But a strong ridge of high pressure centred over Greenland blocked Sandy from going north or east, according to the National Hurricane Centre.
Jennifer Francis of Rutgers University, an expert in how a warming Arctic affects extreme weather patterns, said recent warming in the Arctic may have played a role in enlarging or prolonging that high pressure area. But she cautioned it is not clear whether the warming really had that influence on Sandy.
While components of Sandy seem connected to global warming, "mostly it's natural, I'd say it's 80, 90 per cent natural", said Gerald North, a climate professor at Texas A&M University.
"These things do happen, like the drought. It's a natural thing."
For his published research, Mr Oppenheimer looked at New York's record flood of 1821. Sandy flooded even higher. This week's damage was augmented by the past century's sea-level rise, which was higher than the world average because of unusual coastal geography and ocean currents. Mr Oppenheimer walked from his Manhattan home to the river Monday evening to watch the storm.
"We sort of knew it could happen, but you know that's different from actually standing there and watching it happen," Mr Oppenheimer said. "You don't really imagine what this looks like until you see it."