Tuesday, November 27, 2012

Searching for Signs of Changing Climate in Storms of Chaos

New York, NY—In the aftermath of a storm that left more than 100 dead, over 8.5 million without power, and caused an estimated 50 billion dollars in damage, scientific experts and coastal residents alike are grappling with the portent of Superstorm Sandy. Was this an incredibly damaging but incredibly rare strike on New York and New Jersey? Or is Sandy a harbinger of things to come, of a climate system driven to extremes by anthropogenic greenhouse gas emissions?

For a scientific community convinced that global climate will undergo dramatic changes in the coming decades, extreme events like Sandy serve as something of a flashpoint. On the one hand, there is always a probability, however low, that an extreme event will occur within a given year. On the other, a warming climate fundamentally alters the probability of such extreme events.

Sandy’s Track and Surge
Conflating the issue is the very unusual track of Sandy. Forming as a depression in the Caribbean Sea on October 22nd, Sandy strengthened into a late-season hurricane while heading northward through Jamaica, Cuba, and the Bahamas. Off the coast of the Carolinas, Sandy interacted with a low-pressure system over the eastern U.S., transitioning into a large and powerful hybrid storm before taking a westward turn into southern New Jersey on October 30th. Historically, most storms in this region, including Irene in 2011, have tracked to the northeast (Figure 1).
Figure 1. Tracks of Hurricane Sandy (red line), Hurricane Irene (orange line), and  all tropical storms and hurricanes from the NOAA HurDat2 historical database that crossed within 3° latitude and 5° longitude of Long Island, New York (blue lines)

Professor Adam Sobel
“There is no storm like it in the historical database,” remarked Adam Sobel, Professor of Applied Physics and Applied Mathematics at Columbia University and an expert on tropical meteorology. “The track was a worst-case scenario for New York City.”

Tropical system strikes on New England are, by themselves, not usual: tropical storm or hurricane landfalls along the coast between New Jersey and Massachusetts occur about every six to ten years (ref 1). But the storm surge from Sandy in New York City was likely larger than any storm in recorded history. A study published this year by Ning Lin, Kerry Emanuel, and colleagues in Nature Climate Change suggests that the storm surge from Sandy was a once in 700 years event (ref 2).

The Attribution Paradox
Any search for a link between Superstorm Sandy and global climate change is hampered by a fundamental issue: Sandy was just one storm. And in the realm of noisy weather patterns, practically anything is possible given enough time, even without climate change.
Professor Kerry Emanuel

“There are two logical possibilities,” said Dr. Emanuel, Professor of Atmospheric Science at MIT. “One is that we were extraordinarily unlucky, and Sandy was a very rare event that we just happened to see this year. The other is that some change in the system has made events like this more probable.

“The big problem is that Sandy was a hybrid event (a combination of a hurricane and a nor’easter), and we haven’t done our homework on hybrid events,” Dr. Emanuel continued. “It’s impossible to know without more work.”

On the link between Sandy and climate change, Dr. Sobel is “skeptical but not dismissive”.

“It’s very hard to connect one event to climate. But we’ve had two storms [Irene and Sandy] in two years and this track that’s never happened before in 150 years. We have to have an open mind when trying to determine the links between climate and severe weather events, even if we don’t yet understand all of them,” said Dr. Sobel.

For Joerg Schaefer, Research Professor at the Lamont-Doherty Earth Observatory, proving the influence of climate change on Sandy is “putting the burden of proof on the wrong shoulders”.

Professor Joerg Schaefer
“Everything behaves as we’d predict in a warmer world,” Dr. Schaefer contends, citing the record loss of Arctic sea ice this past summer as the latest in a litany of global weather extremes over the last decade. “The question needs to be turned around: can we prove that climate change did not affect Sandy?”

The Development Issue
As longtime residents of coastal areas can attest to, hurricane-prone regions have witnessed dramatic increases in population, population density and development over the past decades. All of the above lead to an intensification of the damage incurred from storms, even if the storms are not getting stronger. But add stronger storms on top of this, and the losses mount precariously.

“Over the last 100 years, the vast majority of the increase in hurricane damage is from demographics, not climate”, noted Dr. Emanuel, who studies hurricane losses and climate change. “But the two are multiplicative, not additive.”

“Even 150 years ago before CO2 was rising, the Jersey Shore, Battery Park, and the Rockaways were risky places to develop,” said Dr. Schaefer. “But what climate change does is enhance the risk.”

Future Trajectories
Going forward, Drs. Emanuel, Schaefer and Sobel all agree that extreme events may become more likely due to anthropogenic climate change. The modeling work by Lin, Emanuel and colleagues suggests that the probability of a Sandy-sized storm surge event, a once-in-700 years event today, could increase to better than a once-in-300 years event by 2100, owing to sea level rise and an increase in the strength of hurricanes from climate change.

“The right question to ask is, ‘Do we have a good reason to expect that the probability of an event like Sandy will increase in a warmer climate?’ CO2 is pushing us in one direction, on top of what the variability is, which may make extreme events like this more probable,” said Dr. Sobel.

“There should be a presumption of some unknown degree of climate influence on unusual weather by this point,” continued Dr. Emanuel. “It’s a question of how we should treat risk.”

If there is risk for more frequent or stronger hurricanes, then there is at least one silver lining in Sandy: the hurricane forecasts are better than they have ever been, thanks in no small part to decades of government funding for basic and applied research. “In Katrina, the forecast was very good and in Sandy, the forecast was uncannily good,” noted Dr. Sobel. “These were directly the result of long-term government financed improvements in forecasts and models”.

In the meantime, the question on everyone’s mind is obvious: when will the next Sandy strike? “I don’t know whether we have any better ability to predict that now than we did a year ago,” said Dr. Sobel.

“I don’t want to make predictions, but we’d be smart to start preparing.”

Author’s note: Residents of the coastal communities affected by Sandy in New Jersey and New York are still suffering, even nearly a month after the storm. And winter is coming fast. Please consider donating to the American Red Cross (http://www.redcross.org/charitable-donations), the NYSEA Beach Relief Fund (http://www.nysea.com/nybeachrelief/), Occupy Sandy (http://interoccupy.net/occupysandy/), or the New Jersey Relief Fund (https://sandynjrelieffund.org/index.html) to help those in greatest need.

1. Keim, B.D., R.A. Muller, and G.W. Stone. 2007. Spatiotemporal Patterns and Return Periods of Tropical Storm and Hurricane Strikes from Texas to Maine. Journal of Climate, 20, doi:10.1175/JCLI4187.1.
2. Lin, N., K. Emanuel, M. Oppenheimer, and E. Vanmarcke. 2012. Physically based assessment of hurricane surge threat under climate change. Nature Climate Change, 2, doi:10.1038/nclimate1389.