If it seemed liked the rapid intensification of Hurricane Michael into a Category 4 storm caught forecasters by surprise, that’s because it did.
On Sunday evening, the Tallahassee Democrat warned residents, “Destructive Michael on track to bring Cat. 2 hurricane fury to North Florida.” The same day, the forecasters at WeatherUnderground also predicted Michael would be “a Cat 2 at landfall.”
What’s driving that intensification? Michael is traveling over unusually warm waters, thanks to climate change.
“Once again we see a storm undergoing extreme rapid intensification over unusually warm ocean waters,” climatologist Michael Mann told ThinkProgress. “We saw this pattern last year with Harvey and earlier this year with Florence and now, with my namesake, Michael.”
Due to this swift development, Mann added, “once again the people in the storms path may not have had enough lead time to prepare for what is now likely to landfall as a Category 4 monster.”
Michael’s rapid development into a major hurricane points to the fact that global warming is not only making hurricanes more dangerous through stronger winds, higher storm surge, and much more precipitation; it also has begun to make them harder to forecast.
The National Hurricane Center described Michael as a “potentially catastrophic” storm on Wednesday morning.
Here is NOAA’s map of sea surface temperatures (SSTs) Tuesday in the Western Atlantic (compared to the average SSTs from 1961-1990 in degrees Celsius).
Sea surface temperatures compared to 1961-1990 average. CREDIT: NOAA.
The northern Gulf of Mexico is a hotspot.
So are mid-Atlantic coastal waters, as they have been for many weeks. Indeed, that’s why, back in September, the Weather Channel ran a story on the unexpectedly rapid intensification of Florence into a Category 4 hurricane, whose headline read in part, “We didn’t expect it to get so strong so soon.”
Last year, Harvey spun up from a tropical depression to a Category 4 superstorm in two days, while Maria intensified explosively in one day from a Category 1 storm to Category 5 superstorm.
What’s going on?
Hurricanes draw their ferocious power from warm ocean waters. One of the ways hurricanes are weakened is the upwelling of colder, deeper water due to the hurricane’s own violent churning action.
But if the deeper water is also warm, it doesn’t weaken the hurricane and often continues to intensify it. As human-caused global warming continues decade after decade, not only do sea-surface temperatures rise, but the warming penetrates deeper into the ocean.
“Storms are intensifying at a much more rapid pace than they used to 25 years back,” explained the author of a 2012 study on hurricane intensification trends. “They are getting stronger more quickly and also [to a] higher category. The intensity as well as the rate of intensity is increasing.”
A 2015 study on the impact of sea-surface temperatures on the intensity of hurricanes in the North Atlantic found “intensification increases by 16 percent for every 1°C increase in mean SST.” And a 2016 study warned that “the vast majority (79 percent) of major storms” are rapid intensification storms,” and “the most intense storms” are those that undergo rapid intensification.
One of the results of this warming-driven rapid intensification is that we are getting a higher proportion of super-hurricanes — Category 4 or Category 5 (see figure below).
CLIMATE MODELS PREDICTED HUMAN-CAUSED GLOBAL WARMING WILL BOOST THE PROPORTIONS IN THE HIGHEST HURRICANES (4-5) COMPARED TO THE LOWEST (1-2). THE ACCI IS A MEASURE OF HUMAN-CAUSED WARMING. CREDIT: NCAR 2013.
As climatologist and hurricane expert Greg Holland explained last year, “globally, the proportion of Cat 4 and 5 hurricanes has increased from ~20 percent of all hurricanes to around 40 percent due to climate change over the past 60 years.”
Bottom line: We are going to keep seeing more and more difficult to forecast super hurricanes in the coming years and decades — until we get serious about slashing carbon pollution and stopping global warming.