Climate change contributed significantly to the severity of the ferocious 2020 Atlantic hurricane season, increasing both rainfall rates and rainfall totals, according to a study published Tuesday.

The most extreme three-hour rainfall rates were 10% higher than they would have been without climate change, and the most extreme three-day accumulations were 5% higher than they would have been, the researchers said. The effects were even stronger — 11% and 8% — when looking only at storms that reached hurricane strength.

These numbers add to the growing pile of documentation that climate change is here, said Kevin A. Reed, an associate professor in the School of Marine and Atmospheric Sciences at Stony Brook University and the lead author of the study. “It isn’t this end-of-the-century problem that we have to figure out if we can mitigate or adapt to,” he said. “It is impacting our weather and our extreme weather now.”

The 2020 Atlantic hurricane season was the most active on record with 30 named storms. Fourteen of those storms reached hurricane strength, meaning sustained winds of at least 74 mph, and several — including Hurricane Sally — caused severe flooding.

It is well established that climate change can increase the frequency and the severity of hurricanes. However, numerous factors — including both day-to-day weather patterns and larger climatic cycles like La Niña, which contributed to the intensity of the 2020 hurricane season — are involved in the development and severity of any given storm. Quantifying the role of climate change in an individual hurricane or series of hurricanes requires detailed statistical analysis known as attribution science.

During the official 2020 Atlantic hurricane season, which ran from June through November, the average sea surface temperature in the Atlantic Ocean was more than 27 degrees Celsius, or 80.6 degrees Fahrenheit. The new study, published in the journal Nature Communications, used modeling to estimate that the average temperature would have been 0.4 to 0.9 degrees Celsius lower without the effects of climate change.

The researchers then simulated a hypothetical season without those 0.4 to 0.9 degrees Celsius in human-caused warming, and compared the rainfall rates and totals to models of the actual season.

Previous attribution studies have quantified the effects of climate change on individual Atlantic storms: For instance, researchers estimated that up to 38% of the extreme rainfall that Hurricane Harvey dumped on southeastern Texas in August 2017 was attributable to climate change. Mr. Reed was among the researchers who confirmed that climate change also played a role in Hurricane Florence in 2018 and Hurricane Dorian in 2019.

The new study is unusual in that it examined the effects of climate change not on a single hurricane but on a full hurricane season, including not only the headline-grabbing storms but also the seemingly ordinary ones. Mr. Reed said the findings provided strong evidence that the anthropogenic impact was not an anomaly confined to huge events like Harvey.

“If you just do this objectively through a whole season, you get similar results,” he said.

Rosimar Rios-Berrios, a research meteorologist at the National Center for Atmospheric Research who was not involved with the new study, said that examining a full hurricane season rather than individual storms provided a higher degree of confidence that the findings accurately reflected the role of climate change.

“There’s a lot of power in studying individual events, but in the end, a single event is not enough because every hurricane is different,” she said.

A separate analysis released on Monday found that climate change also most likely increased the intensity of the rainfall from two vicious tropical storms that battered southeastern Africa earlier this year. But the researchers said that because of a shortage of high-quality weather data for that region, they could not measure the precise influence of global warming on those storms.

Mr. Reed noted that the same methodology his team employed could be used to quantify the impact of climate change on a storm in close to real time — or to illustrate how much worse storms will get if nations continue to burn fossil fuels.

The study published Tuesday compared the 2020 hurricane season as we experienced it with the hypothetical 2020 hurricane season in a world that has not been warmed by human activities. Since the 19th century, the burning of oil, gas and coal has increased average global temperatures by 1.1 degrees Celsius, or 2 degrees Fahrenheit. It is also possible to compare the season as experienced with the version that might occur after, say, 1.5 or 2 degrees Celsius of warming — the threshold beyond which scientists say highly destructive storms become significantly more likely.

“It’s important to not plan for the 2020 hurricane season in the future,” Mr. Reed said. “It’s to plan for what the 2020 hurricane season plus climate change would look like in the future.”

First Published: April 12, 2022, 8:35 p.m.