In 36 hours, Ian blew up to Category 4. Climate change may make that more common

By Nicolas Rivero, Miami Herald

MIAMI — Before Hurricane Ian slammed into Florida’s southwest coast with 155 mph winds, it went through two separate bursts of so-called “rapid intensification” when a cyclone’s top wind speeds rise by 35 mph in a single day.

This process took Ian from tropical storm to Category 4 monster in 36 hours. It’s a dangerous phenomenon that climate change may make more common in future hurricane seasons.

“It’s too early to say exactly how climate change affected this one storm,” said Kieran Bhatia, a climate researcher at Princeton University who studies hurricanes. “But, on average, we’ve seen multiple studies that show the conditions in the North Atlantic basin are providing more opportunities for storms to intensify.”

<p>Jake Moses, 19, left, and Heather Jones, 18, of Fort Myers, Florida, explore a section of destroyed businesses at Fort Myers Beach on Thursday.</p>

Douglas R. Clifford/Tamps Bay Times

Jake Moses, 19, left, and Heather Jones, 18, of Fort Myers, Florida, explore a section of destroyed businesses at Fort Myers Beach on Thursday.

There are three main factors that lead to rapid intensification: warm waters, stable atmospheric conditions, and high humidity in the middle layers of the atmosphere. All three will be exacerbated by climate change.

“One can anticipate these kinds of events will happen more and more often in the future,” said Karthik Balaguru, a climate scientist at the Pacific Northwest National Laboratory who studies ocean warming and tropical cyclones.

Hurricanes gather their strength from the heat stored in ocean waters. The warmer the surface temperature of the sea, the more fuel storms have to feed on. The temperature of the water deeper down matters, too. Hurricane winds tend to churn up the sea, causing the water at the surface to mix with deeper layers of water. If the deep water is cold, it can slam the breaks on rapid intensification. But if the deep water is also warm, it will add more fuel to the storm.

Oceans have absorbed about 90% of the heat from man-made climate change. A third of that heat has gone into surface waters, which have warmed, on average, 0.14 degrees Fahrenheit per decade since 1901, according to the National Oceanic and Atmospheric Administration (NOAA).

The rest of the heat is stored in deeper waters, which have become a hidden reservoir of fuel for cyclones.

“Looking at the surface, you only see a small change in temperature,” said Balaguru. “But it takes a lot of energy to be stored in the ocean to change the surface temperature.”

Hurricanes also need relatively stable atmospheric conditions to thrive. If a storm hits a bumpy patch of atmosphere where the speed and direction of prevailing winds varies a lot at different altitudes — a phenomenon called “vertical wind shear” — it can become disorganized and weaken.

Vertical wind shear often acts as a stumbling block that prevents storms from intensifying. But researchers at Columbia University and NOAA predict that climate change will weaken vertical wind shear along the U.S. east coast, removing an obstacle for growing hurricanes.

The final ingredient is moisture in the middle layers of the atmosphere. Pockets of dry air can pump the brakes on hurricane intensification while moist air helps hurricanes grow. The United Nations’ Intergovernmental Panel on Climate estimates that total atmospheric water vapor is increasing between 1% and 2% per decade.

Conditions were ripe for Ian

On its way to Florida, Ian found plenty of warm water and moist air. After moving past an area of vertical wind shear, the storm experienced its first growth spurt over the balmy waters of the Caribbean, where surface temperatures neared 90 degrees, according to NOAA.

In less than 36 hours between Sunday night and Tuesday morning, National Hurricane Center bulletins show that Ian strengthened from a tropical storm with 60 mph winds into a Category 3 hurricane with 125 mph winds.

After ravaging western Cuba, Ian rapidly intensified again over a particularly warm pocket of the Gulf of Mexico. The storm’s track hewed closely to the Loop Current, which drags warm water from the Caribbean up between Cuba and the Yucatan peninsula before swooping around the coast of Florida. In the span of eight hours between 11 p.m. Tuesday and 7 a.m. Wednesday, Ian’s top wind speeds increased from 120 to 155 miles per hour, putting it two miles per hour shy of a Category 5 designation.

How much did climate change fuel Hurricane Ian?

While climate change has made the conditions that help hurricanes rapidly intensify more common, it may take years to determine how big a role man-made global warming played in boosting Hurricane Ian’s strength.

Researchers in the field of “attribution science” will try to answer that question by comparing real-world observations of Ian’s developments to weather models that simulate how the storm might have developed if there were no climate change. Attribution studies released in the years after Hurricane Harvey struck Texas and Louisiana in 2017 estimated that climate change had increased the storm’s damaging rainfall by at least 15 percent.

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