The most damaging aspect of a hurricane is the storm surge, as in the case of Hurricane Katrina and Hurricane Sandy. We have already seen that sea-level rise is increasing the chances of a Sandy-level storm surge. In addition, we know that global warming increases the intensity of rainfall from the biggest storms, which further adds to flooding. However, there is also evidence to suggest that the warming itself provides fuel for the biggest storms.
Long-term tropical storm records around the world tend to be problematic because “we do a poor job estimating the intensity of storms that are not surveyed by aircraft,” as Massachusetts Institute of Technology hurricane expert Dr. Kerry Emanuel explained in 2015. He notes that “Currently, only North Atlantic tropical cyclones are routinely reconnoitered by aircraft, and only if they threaten populated regions within a few days.” So the best recent analyses attempt to create a consistent or homogeneous way of comparing hurricanes.
A 2012 study, led by Dr. Aslak Grinsted, created a consistent record of large storm surge events in the Atlantic over the previous nine decades. It found the worst storm surges “can be attributed to landfalling tropical cyclones”—hurricanes cause the biggest storm surges. It also found the worst storm surges “also correspond with the most economically damaging Atlantic cyclones”—hurricanes with the biggest storm surges caused the most destruction. A major finding was that Katrina-sized surges “have been twice as frequent in warm years compared with cold years.”
Why does this happen? There are more active cyclones in warm years than cold years, and “The largest cyclones are most affected by warmer conditions.” This is not a huge surprise given that hurricanes get their energy from warm surface waters. In fact, tropical cyclones and hurricanes are threshold events: if sea surface temperatures are below 80°F (26.5°C), they do not form. One of the ways that hurricanes are weakened is the upwelling of colder, deeper water due to the hurricane’s own violent churning action. However, if the deeper water is also warm—as one would expect in warmer years—it does not weaken the hurricane. In fact, it may continue to intensify.
Typically, for a hurricane to become a Category Four or Five superstorm, it must pass over a pool of relatively deep, warm water. For instance, the National Climatic Data Center 2006 report on Katrina begins its explanation by noting that the surface temperatures in the Gulf of Mexico during the last week in August 2005 “were one to two degrees Celsius above normal, and the warm temperatures extended to a considerable depth through the upper ocean layer.” The report continues, “Also, Katrina crossed the ‘loop current’ (belt of even warmer water), during which time explosive intensification occurred. The temperature of the ocean surface is a critical element in the formation and strength of hurricanes.”
In a 2013 paper, “Projected Atlantic Hurricane Surge Threat from Rising Temperatures,” Grinsted and his colleagues determined that the most extreme storm surge events “are especially sensitive to temperature changes, and we estimate a doubling of Katrina magnitude events associated with the warming over the 20th century.” The study concludes, “we have probably crossed the threshold where Katrina magnitude hurricane surges are more likely caused by global warming than not.”
Another 2013 paper, “Recent Intense Hurricane Response to Global Climate Change,” in Climate Dynamics, looked at hurricane frequency and intensity in recent decades as they relate to human-emissions of greenhouse gases and aerosols. Researchers at the U.S. National Center for Atmospheric Research found no human signal in the total number of tropical cyclone or hurricane that occur each year; however, they find that “since 1975 there has been a substantial and observable regional and global increase in the proportion of Cat 4–5 hurricanes of 25–30% per °C of anthropogenic global warming.”
A third 2013 paper is “Trend Analysis with a New Global Record of Tropical Cyclone Intensity,” in the Journal of Climate. The study was led by Dr. James Kossin of NOAA’s National Climatic Data Center. Hurricane expert Emanuel calls this “the best existing analysis of South Pacific tropical cyclones” in his article on Haiyan and Pam “two exceptionally intense tropical cyclones,” that caused devastation in the western Pacific. The 2013 Kossin et al. paper concluded: “Dramatic changes in the frequency distribution of lifetime maximum intensity (LMI) have occurred in the North Atlantic, while smaller changes are evident in the South Pacific and South Indian Oceans, and the stronger hurricanes in all of these regions have become more intense.”
Thus, the best evidence and analysis finds that although we are not seeing more hurricanes, we are seeing more of the Category 4 or 5 super-hurricanes, the ones that historically have done the most damage and that have destroyed entire coastal cities. At the same time, we are seeing a significant rise in the most damaging storm surges, whereby even a Category 1 hurricane (such as Sandy) that hits in precisely the worst possible place can cause unprecedented damage to coastal communities and major cities.