Recent cover of Bloomberg Businessweek (Bloomberg)

When it comes to climate change and its effect on Superstorm Sandy, it seems many of the loudest voices fall into two camps: 1) the all-in camp: climate change is making the weather more extreme, and Sandy is the poster child for that. We can and should blame climate change for the storm’s terrible toll and it’s a sign of things to come. 2) the no effect camp: climate change played no role in Superstorm Sandy and there’s no cause for concern about the future.

The inconvenient truth on climate change and Sandy falls in the middle of these extreme views. It’s a fuzzy, messy truth that contains many shades of gray. But it’s a truth that’s we’re all equipped to understand. There’s no need to dumb down this reality or suppose anyone will take the issue of climate change any less seriously by appreciating the many nuances in the climate change and hurricane discussion.

Considering hurricane history, a careful read of peer reviewed literature on hurricanes and climate change, and the views of a range of scientists, I think 5 statements best characterize the role of climate change on Superstorm Sandy and our state of knowledge

1) Sandy should not be “blamed” on climate change. Climate change does not cause storms and did not cause Superstorm Sandy. Storms form when certain weather ingredients come together. The historic record shows violent storms, some even more severe than Sandy, have struck the Northeast repeatedly..

2) While climate change did not cause Sandy, it may have been a performance enhancer like a steroid, injecting it with somewhat more energy and power.

3) Sea level rise from manmade climate change increased the water level along the Northeast coast 6 to 8 inches and, as a result, somewhat worsened the coastal flooding from Sandy.

4) There is speculation that decreased Arctic sea ice from manmade climate change altered atmospheric steering currents, strengthening the weather system in the North Atlantic that helped to push Sandy ashore in the Northeast. This idea is controversial.

5) Climate change is likely to slowly increase the intensity of hurricanes in the future, but trends in storm frequency are less certain and the number of storms may actually decrease. Sea levels will continue to rise adding to the coastal flood risk.

Now let me share the rationale for these 5 statements. This is a long post (but hopefully interesting), so bear with me...

Blaming Sandy on climate change goes too far

Let’s begin by dispensing with the notion that climate change “caused” Sandy. In the tropics, hurricanes require rising air (from converging winds), heat and moisture to form. These ingredients led to the gen­esis of Sandy just as they have led to the formation of countless storms in the tropics year after year.

The historical record shows numerous examples of the Northeast being hit by hurricanes. In the 1950s- a flurry of strong storms afflicted the region.

In the 1950s, several major hurricanes made landfall along the Mid-Atlantic and Northeast coast. Some of these storms had stronger winds and resulted in locally higher storm surges than Sandy. (NOAA)

Of course, what made Sandy somewhat unique was its interaction with an extratropical low pressure system along the East Coast and its transformation into a “hybrid” storm. But Martin Hoerling, a NOAA meteorologist, disputes the idea climate change had anything to do with this tropical-extratropical joining of the forces, arguing the connnection happened by chance.

Hoerling told Andrew Revkin of the NY Times: In this case, the immediate cause is most likely little more that the coincidental alignment of a tropical storm with an extratropical storm. Both frequent the west Atlantic in October…nothing unusual with that. On rare occasions their timing is such as to result in an interaction which can lead to an extreme event...

MIT hurricane researcher Kerry Emanuel, in a commentary in Foreign Policy, stated trying to establish a causal connection between Sandy and climate change is essentially hopeless.

“Attributing Sandy or any other single event to long-term climate trends is rather like blaming El Niño for a car accident on the Santa Monica Freeway,” Emanuel said.

Science blogger David Appell adds: ...blaming everything on climate change is as misleading as ignoring or denying it completely. More importantly, it’s ineffective, not least because it ruins your credibility.

Houston Chronicle science blogger Eric Berger may have said it best: ...telling people that Sandy was caused by climate change, or that Sandy is the “new normal” as a result of global warming, or that Sandy is “global warming, stupid,” is, well, stupid. The science does not support any of these positions.

But climate change may have juiced Sandy, a little...

It’s well-established that warm ocean water - all other things being equal - energizes hurricanes. The warmer the water, the more intense a hurricane can become with stronger winds and heavier rain.

October 2012 temperature difference from long-term average shows above normal temperatures off the East Coast (NASA)

Texas Tech atmospheric science professor Katherine Hayhoe told NPR how elevated ocean temperatures in the Atlantic might have given Sandy some extra fuel:

“Off the coast where Sandy struck..., October surface ocean temperatures have warmed by two degrees over the last hundred years,” Hayhoe says. “So when any given hurricane comes along, on average there’s warmer water than there would have been otherwise,” she notes. “Which gives it more energy, and gives it more strength.”

Related: Greenhouse gases and global warming activate the steroid era of our atmosphere

Hayhoe stopped short of saying how much global warming intensified Sandy except to say “it’s more than zero”.

While the human imprint on hurricanes is almost certainly more than zero, it would also be a stretch to say it’s anything more than small. According to a consensus statement from leading hurricane and climate change researchers published in 2010, scientists have been unable to definitively detect a human contribution in hurricane activity. NOAA hurricane and climate change researcher Tom Knutson - who contributed to the statement - put it this way:

It is premature to conclude that human activities--and particularly greenhouse gas emissions that cause global warming--have already had a detectable impact on Atlantic hurricane activity. That said, human activities may have already caused changes that are not yet detectable due to the small magnitude of the changes or observational limitations, or are not yet properly modeled

Crystal clear: climate change raised the oceans and worsened Sandy’s coastal flooding

Of all the possible connections between climate change and Superstorm Sandy, the increased flood water resulting from sea level rise is probably least controversial. Sea levels have risen around 10 to 15 inches over the last 100 years along the East Coast. About half of that rise is from natural causes (sinking land) while the other half can be reasonably attributed to manmade warming.

So one could say climate change added 6 to 8 inches to Sandy’s storm surge. I’ll share two perspectives on the signficance of those 6 to 8 inches.

Historic sea level rise at the Battery, NY (Permanent Service for Mean Sea Level)

Chip Knappenburger, a climatologist affiliated with the libertarian Cato Institute, downplayed the additional water:

So of the total 17.34 feet of water (above the station datum) recorded at The Battery tide gauge during the height of Sandy, about 0.5 feet of that could probably be linked to anthropogenic global warming. This is not nothing, but the overwhelming majority of the damage done by the storm surge would have happened anyway.

Chris Mooney, a science writer and author of the Republican War on Science, claimed the extra globally warmed water, in fact, had profound consequences: it turns out, eight inches matters a lot. First of all, using Climate Central’s Surging Seas tool, [climate scientist Scott] Mandia estimated that 6,000 more people were impacted for each additional inch of sea level rise. ... “An inch or two could be enough to get over a home’s threshold and down into the basement, or make it into one more subway entrance,” [Climate Central’s Ben] Strauss explains.

The foggy role of Arctic sea ice

As Sandy paralleled the Mid-Atantic coast, well offshore, it suddenly ran into a road block - a strong high pressure system in the North Atlantic - and made a sharp left towards the Jersey shore. Some researchers speculate the strength of this road block was a possible consequence of record low Arctic sea ice - which is linked to climate change.

A comparison of the Arctic sea ice minimum in 1980 vs. 2012 (NASA)

The theory goes like this: Less Arctic sea ice reduces temperature contrasts at high latitudes and the strength of westerly winds that serve as the atmosphere’s steering current. As a result, the atmosphere’s flow slows down and bigger blocks develop.

Rutgers climate researcher Jennifer Francis, who has published research supporting this theory, told the New Jersey Star Ledger she “absolutely” thinks Arctic sea ice played a role in Sandy’s track.

Related: Shrinking Arctic ice and the wicked backlash on our weather

“The block was what scared [Sandy] into the west,” Francis said. “It’s a very, very unusual pattern for a hurricane to take, especially this late ... to take a hard left turn and hit [New Jersey] square in the nose.”

But some prominent climate scientists are skeptical about this theory.

“[W]ith respect to the Arctic connection, I don’t believe it,” University Center for Atmospheric Research (UCAR) scientist Kevin Trenberth told the NY Times’ Andrew Revkin. “...the null hypothesis has to be that this is just “weather” and natural variability.”

Related (with more view points about this theory): Study: Arctic ice loss may be making North America weather more extreme

The future of hurricanes: stronger storms amidst rising seas, but maybe not as often

After a hurricane, you often hear talking heads and even scientists say something to the effect of “Expect more events like this in the future. Climate change is increasing the odds of these kinds of events.” But it’s not that simple.

Yes, scientists project gradual increases in hurricane intensity, with heavier rainfall and stronger peak winds. But, with the exception of the most intense storms, hurricane frequency is projected to decline. The consensus statement from hurricane researchers published in 2010 lays this all out there:

....models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%. Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre.

I should note that the 2007 report of the Intergovernmental Panel on Climate Change stated there is “less confidence” in projections for reductions in the number of hurricanes compared to projections for increases in their intensity.

Not to be left out of the discussion, sea levels will continue to rise as the Earth warms, continuing to raise the maximum water level when hurricanes come ashore and the potential for flooding.


Everything I’ve discussed, including the top 5 points, can be condensed into three even more simple points: 1) we should not blame climate change for Sandy 2) climate change likely made Sandy’s effects marginally worse. 3) The future favors stronger hurricanes and higher seas but perhaps fewer storms overall.

In his commentary in Foreign Policy, Kerry Emanuel strongly makes the point that establishing a causal connection between climate change and storms like Sandy is not needed as a basis for action. The risks inherent in toying with the atmosphere and the possible consequences, Emanuel says, are reason enough to stop “keep kicking the climate can down the road.”

“[T]he argument that there is no risk or that we should do nothing is both scientifically and morally indefensible,” Emanuel writes.

There is a mountain of evidence manmade climate change is real and poses risks to society and the environment, especially if its pace accelerates. Stronger hurricanes are just one of many possible unwelcome consequences. There’s really no need to oversell connections between storms like Sandy and global warming to build a case for responding to these risks.