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Climate change fueled historic 2020 hurricane season’s rainfall, study shows

Researchers found hourly hurricane rainfall totals in 2020 were up 11 percent, compared with those in the preindustrial era

This GOES-16 GeoColor satellite image taken Sept. 18, 2020, at 12:20 p.m. Eastern time shows Hurricane Teddy, center, in the Atlantic; Tropical Depression 22, left, in the Gulf of Mexico; the remnants of Paulette, top right; and Tropical Storm Wilfred, lower right. (AP)
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Over the course of six months, hurricane activity in 2020 churned as never observed before in the Atlantic Ocean. The season marked the most named storms on record and the most storms to make landfall in the continental United States, and it chalked up more than $40 billion in damage. The season also spawned two Category 4 storms to hit the same region in Central America within weeks of each other.

A new study quantifies how human-caused climate change fueled the historic season.

In 2020, climate change increased hourly rainfall rates from tropical storms by as much as 10 percent, while hourly rainfall rates from hurricanes were as much as 11 percent higher than preindustrial conditions, according to a study released today in Nature Communications. The most extreme three-day rainfall totals were 5 percent higher for tropical storms and 8 percent higher for hurricanes.

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Extreme rainfall is one of the most significant threats from tropical storms, often triggering flooding and fatal events. In 2020, excessive rainfall from Category 4 Hurricane Eta triggered a massive landslide that buried a village and killed 100 people across multiple countries in Central America. Rainfall from Hurricane Laura flooded coastal areas along the Gulf, also taking numerous lives.

“One of the clearest signals of climate change is appearing within our extreme weather events, particularly with cyclones,” said Kevin Reed, lead author of the study. He said he expects to see such increased rainfall rates and accumulation in a study of 2021 and in the upcoming season “because the climate change signal is only increasing in time.”

Human-induced climate change has increased the global average surface temperature by more than 1.8 degrees Fahrenheit (1 degree Celsius) since preindustrial times. As the atmosphere warms, research has shown how the elevated temperatures increase threats by hurricanes.

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For one, oceans absorb more than 90 percent of Earth’s excess heat, primarily attributed to greenhouse gases. Warmer ocean waters provide more energy for hurricanes to form and intensify more quickly.

Reed and his colleagues estimated sea surface temperatures in 2020 rose by 0.72 degrees to 1.6 degrees Fahrenheit (0.4 to 0.9 degrees Celsius) across the Atlantic. With temperatures approaching record levels, September 2020 became the most active storm month on record for the Atlantic basin.

A warmer atmosphere can also hold more water vapor — about 7 percent more water per 1.8 degree (1 degree C) of warming. As a result, more tropical storms — especially intense, slower-moving storms — are dropping remarkable amounts of rain.

In 2017, Hurricane Harvey dropped up to 60 inches of rain in Houston, which was three times more likely to occur because of climate change. Researchers attributed excessive rainfall by hurricanes Florence in 2018 and Dorian in 2019 to climate change, as well.

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While the effect of climate change on large individual hurricanes has been researched, Reed and his colleagues chose to focus on the change in rainfall over an entire season (June 1 to Nov. 30).

“What this work shows us is that even a storm, maybe it’s a tropical storm that only dumped a couple of inches of rainfall in a given region … has still been impacted by climate change,” said Reed, an atmospheric scientist at Stony Brook University. “As far as we know, this is kind of the first study to objectively apply an attribution framework regardless of intensity.”

The team used hindcasting modeling, or the opposite of forecasting, to estimate how much extreme rainfall across the entire Atlantic hurricane season could be attributed to human-induced climate change. The researchers looked at the rainfall accumulation every three hours and every three days if there was a tropical storm and hurricane.

While a climate effect was detected in small storms, the highest increases were seen in stronger storms.

“This climate change signal actually increases. It becomes larger in more intense storms,” Reed said. “If we focus just on hurricanes instead of all storms throughout the season, we see those percentages go up a little bit.”

The study’s results are not surprising to others in the field. Climate scientist José Javier Hernández Ayala said his own research has shown that Hurricane Maria, which unloaded 41 inches of rain in Puerto Rico in 2017, was nearly five times more likely to happen because of conditions that were altered by human-induced climate change.

“Climate change is enhancing ideal conditions for higher tropical cyclone activity,” said Hernández Ayala, an assistant professor of climate science at Sonoma State University, in an email. “A warmer tropical Atlantic can then produce record-breaking hurricane activity that can lead to extreme rainfall accumulations like we have not seen before in the basin.”

Hurricane researcher Cindy Bruyère said such attribution studies — or the ability to test how much climate change contributed to a certain extreme event — have become more important as our environment changes.

“We are starting to observe the effects of climate change, something that has been difficult [until] recently,” said Bruyère, director of the Capacity Center for Climate and Weather Extremes at the National Center for Atmospheric Research.

She said the study “highlights the fact that … climate change is not a future problem — it is already changing our weather systems and impacting humans.”

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Researchers have already predicted above-average hurricane activity for 2022 in the Atlantic basin, partly because of La Niña conditions, which were also present in 2020 and 2021. A forecast by Colorado State University called for 19 named Atlantic storms, including nine hurricanes.

In addition to increased rainfall rates, Bruyère said we could see other climate-driven changes in hurricane activity this season. Studies have shown the forward speeds of hurricanes are slowing down, allowing them to linger longer over areas and drop more rain. Other research has shown hurricanes are also becoming stronger, more likely to be a Category 3 storm or higher.

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“All countries in the corridors of these storms should prepare for the impacts of intense winds, extreme rainfall, flash flooding and storm surge associated with the passages of these tropical cyclones,” Hernández Ayala said.