A huge storm barrels down on the United States, wreaking havoc with punishing winds, record flooding, heavy snowfall and massive blackouts. Is the main culprit climate change or a freak set of coincidences?
Sandy wiped out homes along the New Jersey shore, submerged parts of New York City, and dumped snow as far south as the Carolinas. At least 50 people were reported killed in the United States, on top of 69 in the Caribbean, while millions of people were left without power. For full coverage, see:
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Is Sandy a result of global warming?
It's unusual for hurricanes to strike at this time of year, says meteorologist Assoc Prof Kevin Walsh.
Some scientists say that the key to Sandy's impact may be an extremely rare clash of weather systems, rather than the warmer temperatures that scientists have identified in other hurricanes and storms.
"It's a hybrid storm, which combines some features of tropical hurricanes with some features of winter storms, that operate on quite different mechanisms," said Kerry Emanuel, a professor of Atmospheric Science at Massachusetts Institute of Technology.
While Emanuel said that there is a clear link between climate change and general trends toward more intense tropical hurricanes, in the case of Sandy more long-term study is required to determine whether climate change played a major role.
Other scientists say climate change likely aggravated whatever unique circumstances produced Sandy. They include the global warming that has caused ocean temperatures and sea levels to rise, contributing to more destructive flooding and other damage.
"Sea level rise makes storm surges worse and will continue to do so in the future," said Stefan Rahmstorf, professor of physics of the oceans at the Potsdam Institute for Climate Impact Research in Potsdam, Germany.
World sea levels have risen by 20 centimetres (8 inches) in the past 100 years, a trend blamed on melting ice and expanding water in the oceans caused by rising temperatures. "Every centimetre adds to damage," Rahmstorf said.
Kevin Trenberth, a senior scientist in the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colorado, said the jury was still out.
"There are clearly changes in the environment that all of these storms are occurring in," he said. As for Sandy, however, a lot of the weather conditions that lined up were due to a "crap shoot." A hybrid storm can be an explosive storm, "what we might call a meteorological bomb," without the influence of climate change.
Hurricane, or winter storm?
Sandy began as a late-season hurricane coming up from the Caribbean in what many experts believe were conditions fuelled by unusually warm water temperatures for this time of year. It then joined forces with a large Arctic weather system, which increased its size and transformed it into a winter storm with far more power than would otherwise have been expected.
The third unusual element was a high pressure system off Canada's east coast that blocked Sandy's escape route. While hurricanes usually turn eastward, the system forced Sandy to make a very sharp left turn and slam into the New Jersey coast.
"Many, many hurricanes have threatened the east coast of the United States over many, many years," said David Nolan, an associate professor of meteorology and physical oceanography at the University of Miami. "Virtually all of them move up the coast and eventually turn to the right and head out to sea."
Nolan said there would likely have been a winter storm forming at about this time. Because Sandy happened to be coming up the coast at exactly the right moment, it gave that storm a head start.
"Instead of starting from nothing, the storm is starting from a circulation as strong as a hurricane," he said.
Scientists also note that world temperatures in September rivalled 2005, the year hurricane Katrina devastated New Orleans, as the warmest in modern records, according to the US National Oceanic and Atmospheric Administration.
And a study in the Proceedings of the National Academy of Sciences this month said that chances of Atlantic hurricanes were higher in warmer years stretching back to 1923.
Warmer temperatures also mean that the atmosphere can hold more moisture, bringing more rain in many areas. A UN report this year predicted that a higher proportion of the world's rain would fall in downpours during the 21st century, making floods more likely.
"The latest research suggests that a warming climate will lead to more extreme weather events such as flooding rains and drought," said Michael Rawlins, who manages the Climate System Research Center at the University of Massachusetts - Amherst.
Rahmstorf said a record thaw of sea ice in the Arctic Ocean in September also might have helped build up high pressure in the North Atlantic that drove Sandy westward.
"I would be very cautious," he said. "But there is reason to suspect that there could be a connection between the record sea ice loss this summer and the path of this storm."
Recent research indicates that greenhouse gases have raised the chances of some events, such as the Texas heatwave of 2011 or a European heatwave in 2003 that killed perhaps 70,000 people. Scientists said it was too early to know if there was a link for Sandy.
All debate aside, US states still reeling from Sandy say they need to take a lesson from the increased threat of monster storms. New York Governor Andrew Cuomo said he has no doubt there are more extreme weather events.
"That's not a political statement; it's a factual statement," Cuomo said after a tour of New York City's ravaged infrastructure. New Yorkers will have to deal with "a new reality" when it comes to weather patterns, he said.