What Caused Hurricane "Frankenstorm" Sandy? (Was It Climate Change?)

^{As Hurricane Sandy made a historic landfall on the New Jersey coast during the night of Oct. 29, the Visible Infrared Imaging Radiometer Suite (VIIRS) on NASA/NOAA's Suomi National Polar-orbiting Partnership (NPP) satellite captured this night-time view of the storm. This image provided by University of Wisconsin-Madison is a composite of several satellite passes over North America taken 16 to18 hours before Sandy's landfall.} 

“Frankenstorm” Sandy has thankfully died down, though millions of northeasterners are still trapped in a real-life Halloween horror flick: no power, no potable water, and no transportation. As we put our lives back together, we face looming questions: What role did anthropogenic climate change play? And what will the sequel be like in terms of hurricane activity?

Sandy’s intensity was likely attributable in part to abnormally high surface temperatures in the western Atlantic Ocean right before the storm—“in places, about five degrees Fahrenheit higher than normal for this time of year,” wrote Justin Gillis in a New York Times blog post.

About one degree of that anomaly can be chalked up to anthropogenic climate change, says Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research, who is referenced in Gillis’s post. “Human-induced global warming has been raising the overall temperature of the surface ocean by about one degree Fahrenheit since the 1970s,” Gillis writes. “So global warming very likely contributed a notable fraction of the energy on which the storm thrived—perhaps as much as 10 percent.”

Part of what forced Sandy into the U.S. rather than east, into the Atlantic, was a so-called “blocking pattern” in the form of a high-pressure system over Greenland. Think of it as football linemen (the blocking pattern) preventing the receiver from running away with the ball (the hurricane). As a result, the storm “merged with a winter system moving in from the west, putting forecasters in the unusual position of having to issue snow advisories for a tropical-hurricane system,” reports Jeff Tollefson for Nature.com. (For a detailed description of how the storm formed, check out Andrew Freedman’s piece on Climate Central.)

Melting sea ice might have enhanced that blocking pattern, Jennifer A. Francis, a research professor at the Institute of Marine and Coastal Sciences at Rutgers University, told Andy Revkin over at Dot Earth. Sea ice hit a record low this year, and increasingly open and warming waters in the Arctic could alter the jet stream’s flow. That, in turn, could intensify blocking patterns like the one that forced Sandy into the northeast.

Still, it’s impossible to pin Sandy squarely on climate change. “Attribution of single extreme events to anthropogenic climate change is challenging,” notes an IPCC report [“Managing the Risks of Extreme Events and?Disasters to Advance Climate Change?Adaptation (SREX)”].

And as Adam Frank writes in a piece for NPR , while climatologists are increasingly confident in linking phenomena like extreme heat and global temperature rise to climate change, for example, they can’t draw the same connections for tropical storms. “We can say with high confidence that the recent heat waves in Texas and Russia, and the one in Europe in 2003, which killed tens of thousands, were not natural events—they were caused by human-induced climate change,” James Hansen wrote in a New York Times op-ed in May. But with tropical cyclones, it’s more complicated. There are uncertainties in such things as North Atlantic hurricane records, for instance—data wasn’t great before satellites (unfortunately, the number of satellites we use for everything from predicting weather to documenting rainforest loss is declining; read why here.).

Still, scientists are making progress. A study published in October in the Proceedings of the National Academy of Sciences that investigated storm surges as a measure of hurricane activity found that intense hurricanes like Katrina were more frequent during warm years.

“As ocean temperatures have risen inexorably higher in the general warming of the planet due to human greenhouse-gas emissions, the scientists concluded, hurricane numbers have moved upward as well,” writes Michael D. Lemonick for Climate Central. “The implication: they’ll keep increasing along with global temperatures unless emissions are cut significantly.”

So, while we can’t pinpoint climate change as Sandy’s driving force, dumping more greenhouse gases in the atmosphere certainly won’t help discourage a terrible sequel.