Hurricane Idalia charged ashore into Florida’s Big Bend coast on Wednesday morning, Aug. 30, as a powerful high-end Category 3 storm with 125 mph winds, bringing an extremely dangerous storm surge, destructive winds, and torrential rains to an area unused to major hurricanes. The story of Idalia’s devastation has yet to be written, but there is little doubt that human-caused climate change will be identified as a key contributor.
Hurricanes are heat engines that take heat energy out of the ocean and convert it to the kinetic energy of their winds. A hotter ocean will allow hurricanes to grow more powerful, assuming that the other factors that power hurricanes, including low wind shear and a moist atmosphere, are present.
As far back as 1987, MIT hurricane scientist Kerry Emanuel theorized that the wind speeds in hurricanes can be expected to increase about 5% for every one degree Celsius (1.8°F) increase in tropical ocean temperature, assuming that the average wind speed near the surface of the tropical oceans does not change. Computer modeling has found a slightly smaller magnitude (4%) for the increase.
A 4-5% increase in hurricane winds may not seem like a big deal, but damage from a hurricane increases exponentially with an increase in winds. For example, according to NOAA, a Category 2 hurricane with 100 mph winds will do 10 times the damage of a Category 1 hurricane with 75 mph winds. This includes damage not only from winds, but also from storm surge, inland flooding, and tornadoes. Bottom line: A 4-5% increase in winds yields about a 40-50% increase in the destructive potential of a hurricane (Figure 1).
According to NOAA’s Coral Reef Watch, sea surface temperatures along Idalia’s path through the western Caribbean and eastern Gulf of Mexico were about 1-2 degrees Celsius (1.8-3.6°F) above the long-term average. Using the theoretical results above, this increase in sea surface temperatures equated to a 50-100% increase in Idalia’s destructive power.
Not all of this year’s unusual warmth in the Gulf of Mexico can be confidently attributed to human-caused global warming. If we restrict ourselves to looking at just the long-term trend in August sea surface temperatures in the Gulf, these have warmed about 0.84 degree Celsius (1.51°F) per century, according to NOAA (Figure 2), which is close to about a 1-degree Celsius increase since preindustrial times. That also aligns with the long-term trend for the global oceans, which have warmed about 1 degree Celsius since preindustrial times in the late 1800s.
It thus appears reasonable to theorize that increased sea surface temperatures of about 1 degree Celsius since preindustrial time in the Gulf of Mexico from human-caused global warming led to a 40-50% increase in Idalia’s destructive power by increasing the hurricane’s winds by at least 4-5%. This could well be an underestimate of the influence of human-caused climate change on the record-warm sea surface temperatures in the Gulf, since the unusual atmospheric circulation patterns responsible for the hot and windless conditions experienced there this summer, which contributed to the record sea surface temperatures, could have had a climate change connection.
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