Hurricane season doesn’t officially start until June 1, but in recent years named storms have been popping up ahead of schedule more and more often.
A newly published scientific paper found that the reason could be something that storm watchers and climate scientists have long suggested — climate change.
As human-caused carbon emissions heat the planet, they’ve warmed the patch of ocean where most Atlantic hurricanes and tropical storms are born. This paper, published Tuesday in Nature Communications, found that decades of warming have nudged the start of hurricane season and the first U.S. landfall weeks earlier, into May.
Ryan Truchelut, the chief meteorologist of private weather service Weather Tiger and lead author of the study, said the research team analyzed all storm landfalls since 1900 and all storms in the Atlantic basin since 1979. They found a direct connection between a hotter ocean and storms forming sooner.
“The thing that controls the start of hurricane season, we found, is sea surface temperature. It’s basically when do the sea surface temperatures get warm enough to sustain convection,” he said. “That threshold is being hit earlier every year.”
Average sea surface temperatures in the chunk of the Atlantic Ocean where storms most often form have gotten about three-quarters of a degree Celsius warmer since the 1970s, he said. That’s a little under 2 degrees Fahrenheit.
“Basically, for every degree of warming in that box in May, there’s the expectation that hurricane season would move earlier by about a month,” he said. “It matched very well with the amount of actual change we’ve seen.”
While this paper suggests we could see more weak and wet storms ahead of June 1, the rest of the season appears to have stayed about the same, he said. So the peak of the season has remained around August and September, and the end of the season still wraps up around the end of November.
‘This isn’t rocket science’
Other researchers have argued that a hotter Atlantic would lead to an earlier hurricane season for decades, but this latest article is one of few peer-reviewed published papers to show a direct connection.
Micheal Mann, director of the Earth System Science Center at the University of Pennsylvania, said the analysis “provides additional confirmation for what many of us have suspected.”
“This isn’t rocket science. You make the oceans warmer, and the Atlantic has warmed quite a bit, you’re going to have a longer and longer season during which sea surface temperatures support tropical cyclogenesis and tropical cyclone formation,” he wrote in an email.
Climate models suggest that global warming, if left unchecked, could cause another 1.5 degrees Celsius (2.7 degrees Fahrenheit) of warming to the ocean, which currently absorbs about 90% of the heat caused by human carbon emissions.
The paper didn’t project into the future, but Truchelut said he would expect the start of hurricane season to keep nudging earlier as the ocean warms, “at least for the next couple decades.”
Critics of the theory that warmer oceans are causing a shift in frequency and number of storms point to the vastly improved technology over the last few decades. It’s far easier to catch wind of storms on satellites that a century ago wouldn’t have been spotted unless a ship ran right through them
This new technology has made it easier to find smaller, short-lived storms that only reach tropical storm (or higher) status for under 48 hours. Some blame these “shorties” for an uptick in global counts of more or earlier storms.
Truchelut said his team was very careful to account for that and even ran the analysis without counting the short-lived storms.
“We actually found a stronger trend toward an earlier season start excluding the shorties than we did with them in the study,” said Truchelut. “It did not change our conclusions in any way, but there was an accentuating of our findings excluding shorties.”
An official shift
This paper could add to the mounting push to shift the Atlantic hurricane season earlier, something the National Hurricane Center is already mulling. The center has already shifted its unofficial season start — when its storm-watching team starts offering daily updates — to mid-May.
Truchelut said he believes moving the official start of storm season “should be given serious consideration” in light of these new findings.
“We have a very strong signal from the data that the onset of tropical storm risk to the continental U.S. can be fairly said to be in late May. I personally am of the opinion that hurricane season should reflect that reality so people can have a better perspective of the risk,” he said.
While his data show that earlier storms may be more common moving forward, Truchelut said it’s likely that these storms will be smaller, weaker and more of a rain and flooding threat than the major category storms that occur later in the season.
“Moving the season would be a strong statement that rainfall is every bit the killer and every bit the destructive force that storm surge and high winds are.”