The hurricane season of 2017 will likely be remembered for a very long time. Ten hurricanes, three of which peaked at category five status, reminded everyone that a hot ocean can wreak havoc. Harvey caused catastrophic flooding in Houston and Maria devastated most of Puerto Rico. Irma was the strongest hurricane ever recorded outside of the Gulf of Mexico and Caribbean Sea.
While no evidence exists that there will be more hurricanes or more hurricanes hitting our coasts, there is theoretical and statistical evidence that the strongest hurricanes are getting stronger as the oceans heat up due to global warming. The Atlantic ocean was very warm this year contributing to the duration and extent of storm intensification of the strongest hurricanes. Air pressures over the North Atlantic were favorable for steering the hurricanes toward the United States.
What might happen to hurricanes in the future as the climate warms continues to be an active research topic. Over the past decade, however, scientists have begun to gather clues as to the kinds of changes that are already occurring. The theory of maximum potential intensity, which relates intensity to ocean heat, refers to a theoretical upper limit of hurricane strength given the proper environment. The upward trend is obvious in the strong relationship between wind speed inside a hurricane and the underlying ocean temperature.
This sensitivity of hurricane intensity to ocean temperature suggests that stronger hurricanes are likely as the ocean warms. This is indeed happening. Hurricanes do not always find themselves in ideal environments for strengthening, but when the strongest ones like Marie do their intensity is strongly coupled to how hot the ocean below is as dictated by theory.
Unfortunately the theory is silent about the number of hurricanes. And there is no significant upward or downward trend in hurricane frequency since accurate record keeping began. So metrics that include frequency, like Accumulated Cyclone Energy (ACE) used widely by the National Oceanic and Atmospheric Administration (NOAA) are problematic for examining climate change effects.
Some scientists have suggested that the upward trend in hurricane energy is natural variability caused by the Atlantic Multidecadal Oscillation (AMO) rather than by global warming. But a test of these two competing hypothesis suggests that global warming causes the AMO.
Others have noted hurricane property losses have not gone up recently and the numbers of hurricanes hitting the United States have declined over the past century and a half. Historically, however, when a hurricane hits the coast the best predictor of the magnitude of property loss is ocean temperature with warmer oceans leading to greater loss amounts.
The 2017 hurricane season is certainly one for the record books. The records will be examined for additional clues about what might happen in the future. Stay tuned.
Dr. James Elsner is the Earl and Sophia Shaw Professor and Chair of Geography at Florida State University where he teaches applied spatial statistics and climatology. Dr. Elsner also is President and CEO of Climatek; a company that develops software for hurricane & tornado risk models.
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