Damaging Katrina-level storm surges are twice as likely in warm years

Perhaps the most stunning images in the wake of Hurricane Sandy were the sight of the roller coaster from the Casino Pier in Seaside Heights, New Jersey lying in the Atlantic Ocean. The images reminded us that hurricane storm surges are capable of causing tremendous destruction along the coast, and one of the main concerns on how global warming might affect hurricanes is the potential for stronger hurricanes to create larger storm surges. We expect that global warming should make the strongest hurricanes stronger, since hurricanes are heat engines that take heat energy out of the ocean and converts it to wind energy. These stronger winds will be capable of piling up higher storm surges. However, it is controversial whether or not we have observed an increase in the strongest hurricanes, since hurricane winds are hard to observe. Our long-term hurricane data base is generally too low in quality and covers too short a period of time to make very good estimates of how climate change may be affecting hurricane winds. However, a new 2012 paper, "Homogeneous record of Atlantic hurricane surge threat since 1923" by Grinsted et al., looked at storm surge data from six tide gauges along the U.S. coast from Texas to New Jersey, and concluded that the number of moderately large hurricane and tropical storm surge events has increased since 1923. Moderately large storm surge events are on pace to nearly double by the year 2100, compared to 20th century levels. Moreover, 1-in-9 year to 1-in-30 year Katrina-level storm surge events are twice as likely to occur in warm years compared to cool years, and thus global warming may be able to dramatically increase the frequency of highly damaging extreme storm surge events. Since sea level is steadily rising due to global warming, these future storm surges will also be riding in on top of an elevated ocean surface, and will thus be able to do even greater damage than in the past. Expect to see many more shocking storm surge damage photos in the coming decades, unless we wise up, retreat from areas highly vulnerable to storm surge, and invest in increased shoreline protection measures.


Figure 1. The Casino Pier in Seaside Heights, N.J. taken during a search and rescue mission by 1-150 Assault Helicopter Battalion, New Jersey Army National Guard on Oct. 30, 2012. Image credit: U.S. Air Force photo by Master Sgt. Mark C. Olsen.


Figure 2. Top: Observed long-term frequency of moderately large storm surge events from hurricanes and tropical storms measured at six tide gauges along the U.S. East Coast (inset map). The thick line is a 5-year moving average. These type of surge events occurred an average of 5.4 times/year between 1923 - 2008, and are on pace to increase to 9.5 events per year by 2100. Bottom: Departure of Earth's annual mean surface temperature from average, shaded to show warmer and colder than median temperatures. Large storm surge events increase in probability during warmer than average years. Image credit: Grinsted et al. 2012, "A homogeneous record of Atlantic hurricane surge threat since 1923."

Using storm surge to evaluate damage normalization studies
Damage from landfalling storms can be used to estimate if hurricanes are growing stronger with time, but damage estimates must first be corrected to account for changes in wealth and population over time. A 2008 study by Pielke et al. found that although hurricane damages had been doubling every ten years in recent decades, there were no increases in normalized hurricane damages in the U.S. from 1900 - 2005. They used census and economic data to adjust for how increases in populations and wealth may have affected hurricane damages over time. However, Grinsted et al. (2012) questioned whether or not this was done correctly. They found that storm surge heights of U.S. hurricanes and tropical storms correlated very well with metrics that looked at storm intensity, when looking at many decades of data to see long-term trends. However, the researchers found that while short-term trends in normalized hurricane damage estimated by Pielke et al. (2008) did correlate well historical storm surges, these normalized damages had poor correlation with the storm surge record, when looking at decades-long time scales. This implies that the corrections were biased. Dr. Stephan Lewandowsky of the University of Western Australia makes the case that efforts such as the one done by Pielke et al. (2008) to normalize disaster losses are probably biased too low, since they only look at factors that tend to increase disaster losses with time, but ignore factors that tend to decrease disaster losses. These ignored factors include improvements in building codes, better weather forecasts allowing more preparation time, and improved fire-fighting ability. He writes, "Most normalization research to date has not accounted for those variables because they are extremely difficult to quantify. (And most researchers have been at pains to point that out; e.g., Neumayer & Barthel, 2011, pp. 23-24.) In effect, normalization research to date largely rests on the oddly inconsistent pair of assumptions that (a) we have built up enormous wealth during the 20th century but (b) did so without any technological advance whatsoever." Grinsted et al. (2012) suggest that it may be possible to use their storm surge data to correct biased hurricane damage estimates, though. Take home message: studies showing no increase in normalized damage from storms have high uncertainty, and it is possible that higher economic damages due to stronger hurricanes are indeed occurring.

References
Grinsted, A., J. C. Moore, and S. Jevrejeva, 2012, "A homogeneous record of Atlantic hurricane surge threat since 1923," PNAS 2012, doi:10.1073/pnas.1209542109

Pielke et al., 2008, "Normalized Hurricane Damage in the United States: 1900–2005", Natural Hazards Review, Volume 9, Issue 1, pp. 29-42.

Links
In this remarkable home video, 15-year-old Christofer Sochacki captures the evening high tide on the day Superstorm Sandy struck Union Beach, New Jersey. The later part of the video shows how high waves on top of a 8-foot storm surge can lead to a punishing assault on beach-front structures.

Jeff Masters