For storm-battered residents of the Caribbean, the Southeast and the Gulf Coast, new research on hurricanes is rarely good news, with recent studies showing trends toward stronger storms that intensify suddenly near the coast and maintain their strength longer after hitting land.

A study published on Wednesday in the journal Weather and Climate Dynamics reinforces the growing consensus that the hurricane threat to vulnerable coastal communities will keep increasing. The research shows global warming has “contributed to a decisive increase in Atlantic Ocean hurricane activity” in the last 40 years and doubled the chances for extreme seasons like 2020.

That was the most active hurricane season on record, when tropical storms started early, ended late and included 11 tropical systems hitting the United States, with seven major hurricanes and one subtropical system even making it all the way to Portugal. Every single mile of the U.S. Atlantic coastline was under a tropical storm watch or warning during the 2020 season.

The study reinforces the growing consensus that vulnerable coastal communities need to prepare more for years like 2020, said lead author Peter Pfleiderer, a research scientist with Climate Analytics, a nonprofit climate science and policy think tank. 

The rapid run of destructive late season storms that year prompted the World Meteorological Organization to rethink its system of naming storms to avoid confusion. And because tropical storms have formed before the official start of the hurricane season several years in a row, the WMO also decided last year to start issuing hurricane bulletins starting May 15 instead of June 1. The National Ocean and Atmospheric Administration estimates that each degree Celsius warming lengthens the hurricane season by 40 days.

Related: How is climate change connected to hurricanes?

There is a Clear Global Warming Fingerprint

To find the global warming fingerprint against the smudges of year to year natural variability in the climate system, the scientists used two relatively simple metrics: sea surface temperatures and Accumulated Cyclone Energy (ACE), an index that adds together the wind energy from all named tropical systems in a season.

The research does not definitively answer all questions about how global warming affects tropical storms, but it’s one way of measuring how the warming of the ocean has affected total seasonal hurricane activity in the last 40 years, Pfleiderer said.

“We had our approach, we tested it, that’s what came out,” he said. “This study looks at the changes stemming from increasing sea surface temperatures, but it has to be expected that atmospheric circulation changes from global warming also affects tropical cyclone activity, so our results could be conservative,” he added.

The ACE index doesn’t help determine the risk to a specific area from any single storm, but it can help show large-scale trends over time. Pfleiderer said that ocean warming accelerated in the early 1980s, making the study period (1982-2020) useful for assessing how that change affected tropical storms during that time. 

He said sea surface temperatures weren’t increasing all that much before the 1980s, so seeing the effect of that warming on hurricanes happen at the same time was somewhat surprising, given the previous uncertainty. 

The Atlantic Ocean has warmed more, relative to other global oceans, during the study period, but he said he still “wouldn’t have expected the effect (on cumulative seasonal hurricane intensity) to be that strong.” 

The study was aimed at “separating the influence of natural variability in atmospheric circulation from the part that might be directly linked to climate change, which is the ocean surface warming.” 

The increase in total seasonal cyclone energy has been well-known for quite a while, but “for a long time people were saying a large part of this is natural variability. We tried to quantify how much is natural variability and how much is due to the warming of the ocean surface,” he said. 

It’s quite complicated and I wouldn’t say we did it perfectly,” he continued. “They are obviously linked. If the ocean surface is warmer this affects atmospheric circulation. We thought about this for a long time, and it’s not something that can be completely separated.”

Nevertheless, the research “underscores the importance of rising sea surface temperatures that lead to more extreme outcomes in terms of cyclone intensity for the same seasonal atmospheric patterns,” the scientists wrote in the study. 

Hurricane Impacts are Devastating Caribbean Island Nations

Pfleiderer said that, if he were helping communities plan for the coming decades, he would emphasize the need to be prepared for more bad hurricane years.

“The main factor that determines whether we get an active season is still atmospheric circulation and there is still a lot of uncertainty about that,” he said. “So looking at the last five or ten years does not really tell us what is the new norm. But I would still say that, assuming there is no change in atmospheric circulation that leads to less frequent tropical cyclone formation, we should be prepared to get these more extreme seasons.” 

Even if the new study uses simplified metrics that don’t account for year-to-year variability, the paper has some merit, with qualifications, said climate scientist Keven Trenberth, with the University of Auckland, New Zealand and with the National Center for Atmospheric Research.

The study captures some important aspects of the climate hurricane link related to overall winds and the distribution of hemispheric weather patterns that influence tropical storm formation, Trenberth said. He also noted that, since “the main global warming has occurred since the late 1970s and early 1980s,” the idea that there is a much bigger risk for extreme seasons is supported by the research. “There is a global warming component, I can go along with that statement,” he said.

In 2019, research led by scientists with Climate Analytics showed that, in the past 20 years some Caribbean Small Island Developing States have been hit by a major hurricane eight times, with GDP losses adding up to an average of about 3.7 percent per year. Dominica, the most affected  Caribbean country, averaged yearly losses of 21.2 percent of GDP. Forecasts for this year’s season by the same researchers predict it will be extremely active again. 

Up to now, the 2017 season remains the costliest on record with devastating storms in the Caribbean. In Barbuda, more than 90 percent of structures were destroyed, resulting in the island being completely uninhabited for the first time in 300 years.

Pfleiderer said his new study shouldn’t be too surprising because it’s in line with recent assessments from the Intergovernmental Panel on Climate Change, which also found that there will be an increase in the most intense and dangerous storms.

“I think there is now kind of a consensus about this,” with regard to individual storms, and the new study shows a similar trend at the seasonal scale, he said.

“In a way it’s not super-surprising but the headline looks surprising.”