Image: NASA

We’re still living with the impacts of the 2017 hurricane season from Puerto Ricans living in hotels to the president saying grotesque stuff. It was the most costly season in history as Harvey, Irma, and Maria each caused tens to hundreds of billions in damage in the U.S.

There’s been a lot of ink spilled about how climate change impacted these individual storms, but a new study looks at how climate change impacted the season as a whole. The answer: An unusually hot Atlantic Ocean was the main driver in the uptick in the number of powerful storms.

Advertisement

There were actually six major hurricanes—defined as Category 3 or greater—that formed in 2017, three of which thankfully steered clear of land. Untangling whether it was just an outlier season from hell or another indicator of our changing climate is of critical importance.

“Major hurricanes are one of the most damaging and deadly natural disasters...understanding the character and causes of variations and changes (including potential influences of anthropogenic climate changes) is of profound scientific, economic and human interest,” Hiroyuki Murakami, a hurricane researcher at Geophysical Fluid Dynamics Laboratory, told Earther in an email.

Murakami led the new research published on Thursday in Science. There are a number of things that influence hurricane season, including a natural climate pattern known as the El Niño Southern Oscillation. We were in a weak La Niña in 2017, a state which can slacken winds in the atmosphere over the Atlantic, making conditions more conducive for hurricanes to spin up. But waters in the tropical Atlantic were also freakishly warm in 2017 in a place known as the main development region, which is where hurricanes tend to form and strengthen. During the peak of hurricane season in September, main development region temperatures were their third warmest on record—up to 5.4 degrees Fahrenheit (3 degrees Celsius) above normal.

Advertisement

To disentangle which of these factors played a dominant role, Murakami and his colleagues ran a high resolution hurricane model using 2017's weirdly warm Atlantic temperatures. They then ran it again with average sea surface temperatures for the main development region. In both cases, they also simulated the weak La Niña. Their results show that without the nudge of a warmer-than-normal Atlantic, there would have been fewer major hurricanes.

That suggests that climate change—which is warming the oceans nearly everywhere—played at least a bit of a role in the tragic 2017 hurricane season, though there’s certainly more research needed using other models and probing different variables.

“We have confidence that anthropogenic forcing is an important factor influencing hurricane activity, although separation of the effect of natural variability from that of anthropogenic forcing is difficult at this moment,” Murakami said.

Advertisement

The researchers then teased out what would happen in the future if greenhouse gas emissions continued to rise, cranking up the heat. By the end of the century, there could be an average of four major hurricanes annually if we start to cut carbon pollution and five if carbon pollution continues unabated. But layering an abnormally warm year like 2017 on top of the background rise in ocean temperatures could ratchet that number up to eight in either scenario.

Carl Schreck, a hurricane expert at North Carolina State University, told Earther the results “make sense.”

“We still have a lot of questions, but the scientific consensus is generally that more intense storms will become more common,” he said.

Advertisement

Murakami said his group is now doing similar work looking at the Pacific where the extremely active 2018 typhoon season has left a wake of destruction from Japan to the Philippines to Hong Kong.