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 22^nd, 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 30^th. 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.”

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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.

References
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.

Welcome to Warmer America

On August 6th, NOAA's National Climatic Data Center reported that July was the single warmest month in the 117-year temperature history of the continental United States. During July, over half of the continental US experienced temperatures in excess of 2°F above average, an unprecedented area and magnitude of warmth:

July 2012 Temperature anomalies; darkest red is 8°F above average. Source: NOAA

July's record temperature are just the latest in an exceptional year of heat for the continental US. A late June super-heat wave led to 208 locations tying or breaking their all-time highest temperature records-- all of this, despite the fact that June isn't even the warmest month of the year! From Alaska to Arkansas, a vast swath of the country set or tied their highest-ever June temperature during this heat wave:

U.S. Monthly Highest Max Temperature Records set in June 2012. Source: NCDC/NOAA

And this hot summer is a continuation of the status quo for the past several months. This past March was the warmest March in recorded history for the continental US. To date, the year 2012 is more than 4°F warmer than the 20th century average, smashing records of previous warm years:

Evolution of yearly temperature anomalies for the contiguous US (2012 in red). Click for larger. Source: NCDC/NOAA

All told, the past 12 months have been the warmest 12-month period in recorded history for the US. In fact, since May of 2011, every month has been warmer than its climatological average, a fourteen-month stretch of warmer-than-average temperatures.

What are the odds that 14 straight months would be warmer than average? One can assume there is a 50% chance that any given month will be warmer (or cooler) than average. The odds of having 14 such warmer-than-average months in a row, as we've had since May of last year, would thus be equivalent to the odds of flipping a coin heads fourteen times in a row: one in sixteen thousand.*

Of course, 14 straight months of anomalous warmth are more likely if the chance of warmth is better than 50/50. Since the 1980s, scientists have postulated that carbon dioxide-forced global warming would lead to increases in the frequency of heat waves and droughts, in effect "stacking the deck" in favor of warmer conditions. In 1981, climatologist Jim Hansen of the Goddard Institute of Space Studies, writing in Science, hypothesized "the creation of hot, dry conditions in much of the western two-thirds of the United States" as a result of additional atmospheric carbon dioxide.

Thirty-one years later, Hansen's predictions appear substantiated, as the US is currently in the grips of the worst drought since the 1930s Dust Bowl. 62% of the country is experiencing drought conditions, and the US Department of Agriculture is projecting that the 2012/13 corn yield will be the lowest in 17 years due to "extreme heat and dryness" across the Great Plains. As a consequence, global food prices spiked 6% in July, led by a 17% increase in the price of cereals and a 23% surge in the price of corn:

Figure 4: Spot price for December, 2012 Corn Futures (cents/bushel). Note the spike in prices during the late June super-heat wave. Source: CME Group

As with any climatic changes, there are winners and losers. Locally, farmers in North Carolina may gain from the high corn prices (above), with the USDA projecting a 30% increase in NC corn production over last summer despite the drought conditions. Globally, however, the World Bank notes that rising food prices tend to exacerbate political and social conflict, especially in developing countries.

Globally, the hot summer of 2012 in the US contrasts with cooler-than-average conditions in the UK and other locations. But the two do not fully offset: Like every year since 1977, global temperatures in 2012 will be above the 20th century average:

Measured global temperature anomalies, 1900-2011, with future projected temperatures. Click for larger. Sources: NOAA Climate Services, IPCC

By 2030, global temperatures are expected to be 0.9°F (0.5°C) warmer than today, rising to almost 2°F (1.1°C) warmer than today by 2050 (above). With such an increase in background warmth, hot months--and hot years--will undoubtedly become hotter.

Will the warmth continue in the US? As far as this year is concerned, it doesn't matter: the first half of 2012 has been so warm that, even if temperatures in the rest of 2012 are in line with 20th century averages, 2012 will still be the warmest year in US history. And with a developing El Niño historically favoring a warmer late fall in the central and eastern US, it is likely that the second half of 2012 will continue to be warmer than average:

Historical temperature anomalies in November and December during El Niño events. Red colors indicate warmer-than-average conditions. Source: NOAA Climate Prediction Center

In the meantime, go ahead and pencil in 2012 as the warmest year in US history. Just don't expect it to stay in the record books for very long.

* Note from above: This probability assumes that the chances of any given month being warmer or cooler than average is independent of all other months, which isn't necessarily true. In reality, the odds are probably slightly higher than one in 16,000, but difficult to quantify exactly.

In Search of Hurricane History

Friday marks the beginning of the 2012 Atlantic hurricane season. Cue the groans, the crossed fingers and the hope that mad rushes for plywood and batteries will wait for another year. Many of you are probably wondering what's the chance that you will get hit this year.

As residents of Eastern North Carolina know well, hurricanes are not idle threats. According to the National Climatic Data Center, tropical storm and hurricane strikes are the single most common causes of billion-dollar natural disasters in the United States, accounting for nearly $260 billion in damages between 1980 and 2005, or more than half of the combined losses from all U.S. natural disasters. And since 1851, 18 percent of all hurricane strikes on the United States occurred in North Carolina.

Part of the frustration with hurricanes—and one reason why they are so destructive—is that hurricane strikes are anything but predictable. Along the North Carolina coast, the total number of storm that make landfall varies enormously from year to year. For instance, between 1986 and 1995, the N.C. coast was directly struck by only one hurricane (Charley in 1986) and brushed by another (Emily in 1993). However, six hurricanes would make landfall along the coast over the next 10 years (Bertha and Fran in 1996, Bonnie in 1998, Dennis and Floyd in 1999, and Isabel in 2003). Back in 1955, three hurricanes—Connie, Diane, and Ione—struck the NC coast within a six-week span.

Such great variability in the number hurricane landfalls demands an explanation. Intuitively, the number of landfalls reflects the total number of hurricanes: the more hurricanes in a season, the greater chance that any area could receive a direct hit. For North Carolina, the number of hurricane impacts per decade tracks the average number of yearly storms, as the figure below shows:

Average number of yearly Atlantic hurricanes per decade (blue line), and number of hurricanes striking within 150 miles of North Carolina (red bars). Atlantic hurricane counts from Weather Underground, North Carolina Hurricane statistics from State Climate Office of NC.

One way to estimate the chance of a hurricane strike to a certain area is to estimate the total number of storms during a hurricane season. For such estimates, it is crucial to have accurate records of past hurricane strikes. Unfortunately, historical data are limited in length, only going back to the mid-19th century, and their accuracy and coverage are questionable at best.

For Jeff Donnelly, an associate scientist at Woods Hole Oceanographic Institution in Massachusetts, extending the historical record of hurricane strikes is a matter of digging deeper. Just not into library archives or journals; instead, Donnelly searches for evidence of hurricanes in salt marshes on the landward side of barrier islands.

Over the last two decades, scientists like Dr. Donnelly have increasingly turned to clues from the earth for records of past hurricane strikes. This approach, termed paleotempestology (paleo- past, tempest- storms; logy- the study of), relies on a simple principle: Hurricanes tend to move things to where they normally would not be found.

In salt marshes, the material moved is sand. When a hurricane approaches the coast, wave action and storm surge erodes sand from the beach and brings it inland, depositing the sand on top of the muddy sediment in the salt marsh. After the storm surge recedes, the marsh recovers, and muddy sediment is again deposited on top of the sandy layer. The end result, according to Jon Woodruff, assistant professor at University of Massachusetts-Amherst and a former student of Donnelly, is “the perfect dirt layer cake”:

A sediment sample collected from the Florida Panhandle. The dark sediment is mud that is normally deposited in a marsh; the lighter bands of sediment represent layers of sand that are washed into the marsh during hurricanes. Photo credit: Jon Woodruff

Donnelly and Woodruff take cores in the landward salt marshes, and reconstruct past hurricane strikes from the sand layers in these tubes of sediment. Previous work published by Donnelly and colleagues has recovered evidence for major hurricane strikes in the 18th, 19th and early 20th centuries in the salt marshes of New Jersey, Long Island and Rhode Island that coincided with documented hurricane strikes.

But the potential doesn’t stop there. With longer sediment cores and the right location, hurricane strikes can be inferred from times long before the historical record. “Cores help us take a look at history over 5,000 years, and that’s a powerful tool,” Donnelly told Oceanus magazine in 2009. A recent hurricane strike reconstruction from Puerto Rico, published in Nature, suggests that increased hurricane activity in the North Atlantic over the last 5,000 years generally corresponds to weaker El Niño events and a stronger West African monsoon.

Unfortunately, similar hurricane reconstructions have yet to be generated from eastern North Carolina. A 2006 study by Steven Culver and colleagues from East Carolina University examined salt marsh cores from Pea Island. The authors showed that sand deposits in these salt marshes were far too variable and widespread to be explained by hurricane activity and likely related to changes in inlet positions along Pea Island. Tellingly, the North Carolina coast seems to be too energetic to preserve records of past hurricane strikes.

Hurricanes will never cease to be a risk to the residents and economy of eastern North Carolina. But with methods like paleotempestology, scientists are gaining a more complete picture of past hurricane variability and of what factors may contribute to future hurricane variability. While scientists may never be able to predict the chance of a hurricane strike in one location in any given year with certainty, research into past hurricane strikes has certainly underscored the dynamic history of the coasts we call home.

Author's note: Cross posted from Coastal Review Online, a publication of the North Carolina Coastal Federation.