Hurricane season in the Atlantic is arriving ahead of schedule as the oceans warm, a new study finds. Big storms in the North Atlantic are forming earlier in the year than they used to, and forecasters say this means coastal communities need to be on the alert sooner, too.

Tropical storms that reach a certain strength are named by the World Meteorological Organization. And the first named storms to develop each year have come about five days earlier each decade since 1979, according to the study published today in the journal Nature Communications. Named storms that make landfall in the US, meanwhile, have shown up about two days earlier every decade since 1900.

That means communities that frequently find themselves in the path of those storms might need to prepare for hurricane season earlier than they have in the past. And officials might want to rethink the timeframe they’ve established for the Atlantic hurricane season, which has officially started on June 1st every year since 1965.

“As a hurricane forecaster, in the late 20-teens, I noticed that there’s a very unusual preponderance of storms developing before the start of hurricane season,” says Ryan Truchelut, lead author of the new study. Truchelut co-authored the paper and co-founded the forecasting company WeatherTiger with his wife, physicist Erica Staehling.

The authors of the new study based their research on storm development on observational data from 1979 to 2020. They were limited to studying those four decades because that’s about how long satellites have been around to help forecasters see more storms develop. They felt that including earlier years might have led to a biased assessment of seasonal trends over time.

In the past, it’s been difficult for researchers to work with such limited data to establish a connection between warming temperatures and a longer hurricane season. But the past decade has been pretty remarkable for forecasters to watch. The seventh consecutive year in a row that a named storm formed before June 1st was 2021.

Higher sea surface temperatures are likely the driving force behind that unusually early tropical storm activity, Truchelut and his co-authors found. Those higher temperatures have been strongly tied to climate change, and hurricanes gain strength in warmer waters.

Temperatures in May typically aren’t warm enough to churn up major hurricanes. But the public still ought to take precautions for earlier storms even if they’re weaker, Truchelut warns. Heavy rains from those storms can pose a significant threat, even if the storm’s wind speeds don’t surpass the necessary threshold (111 miles per hour) to be considered a major hurricane.

The National Weather Service (NWS) is already considering whether to move the first official day of the Atlantic hurricane season up to May 15th instead of June 1st. And even though the date hasn’t officially moved, some hurricane forecasting efforts have already shifted. Last year, the National Hurricane Center decided to start issuing Atlantic Tropical Weather Outlooks on May 15th. Those are routine forecasts that typically don’t come out until June.

Besides, “hurricane season does not have a rigorous scientific definition,” Truchelut tells The Verge. “It’s purely a social construct.” In 1935, the US Weather Bureau, the predecessor to the NWS, designated hurricane season for the Atlantic basin as June 15th through November 15th. In 1965, the dates shifted to June 1st to November 30th to encompass 97 percent of storm activity in the region.