Katrina-level storm surges have more than doubled due to global warming

Since 1923, there has been a ‘Katrina’ magnitude storm surge every 20 years, according to a storm surge index developed by Aslak Grinsted, an assistant professor at the Centre for Ice and Climate at the Niels Bohr Institute. The index uses data from six tide gauges along the U.S. coast from Texas to New Jersey from 1923 - 2011, and is part of a statistical model that links global temperatures to the risk of Katrina-level storm surges. Because of global warming, Katrina-magnitude storm surge events have now more than doubled in frequency since the late 1800s, Grinsted and colleagues argue, in research published in March 2013 in the scientific journal, Proceedings of the National Academy of Science (PNAS). Their statistical model found that an increase of 0.4°C in global temperatures was sufficient to double the odds of Katrina-magnitude storm surges. Since global temperatures have risen 0.6°C since the late 1800s, "we have already crossed the threshold where more than half of all ‘Katrinas’ are due to global warming,” said Grinsted in a press release. Projecting into the future, the model predicts that if the global climate warms as expected by 2°C before the end of the century, Katrina-level storm surge events will become ten times more common, and a Katrina-level surge will occur, on average, every 2 years, instead of every 20 years. 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. Since this is a simple statistical model, I am hopeful that the relationship Grinsted at al. found might break down as the climate warms, due to unexpected changes in hurricane tracks, wind shear, etc. However, this high-end consequence of global warming is quite possible, and is something coastal planners should should consider, particularly since the U.S. population living along the coast is expected to grow from 123 million in 2010 to 134 million people by 2020, according to a NOAA report issued on March 25. We need to retreat from barrier islands highly vulnerable to storm surge, and invest in significantly improved shoreline protection measures in the coming decades.


Figure 1. High water marks on East Ship Island, Mississippi, after Hurricane Katrina in 2005. Katrina brought the highest storm surge ever recorded on the U.S. coast, 27.8' at Pass Christian, MS. Left image: Bark stripped off a tree with salt-burned pine trees in the background (note the 25 ft (7.65 m) long survey rod for scale). Right: Massive beach and over wash erosion illustrated by damaged and snapped pine trees along the beach. Arrows show the the high water mark left by the storm surge. Image credit: Fritz et al., 2007, "Hurricane Katrina storm surge distribution and field observations on the Mississippi Barrier Islands" (PDF File), Estuarine, Coastal, and Shelf Science (2007), doi:10.1016/j.ecss.2007.03.015.


Figure 2. Number of Katrina magnitude surge events per decade for the past and future computed using gridded global temperatures and a statistical model relating global temperatures to storm surges. Confidence intervals of 5% and 16% are shown in the lighter blue colors. Image credit: Niels Bohr Institute at the University of Copenhagen.

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

Grinsted, A., J. C. Moore, and S. Jevrejeva, 2012, "Projected Atlantic hurricane surge threat from rising temperatures" PNAS March 18, 2013 201209980, doi: 10.1073/pnas.1209980110

Jeff Masters