David Easterling, a NOAA climate scientist, didn’t think much about the rainfall forecast until his wife called him on the evening of Tuesday, Sept. 24, asking whether he’d seen the weather outlook. 

Hours later, rain began falling. Over the next three days, it drenched the region, flooding homes, roads and entire communities. Easterling himself recorded 15 inches in his north Henderson County backyard, while some areas saw more than 30 inches in just 72 hours.

“I’ll be honest with you, I didn’t really pay that much attention to the forecast until she called,” said Easterling, who is chief of the climate assessment section of NOAA’s National Centers for Environmental Information and a co-author of the North Carolina Climate Science Report released in March 2020. 

“I thought: holy crap. Even then it just didn’t really click that the flooding would be this severe.”

Easterling and climate scientists around the globe are now sorting through the data to determine by what factor global warming intensified the amount of rainfall during Hurricane Helene. The results may influence how the region adapts to future extreme rainfall events.

“I’m concerned that these kinds of events are going to become much more common,” Easterling said. “Maybe not quite as severe as Helene, but the expectation in the future is that when storms occur, they will be more intense in terms of rainfall due to a warmer atmosphere.”

An October climate attribution study conducted by the World Weather Attribution initiative, or WWA, concluded that Helene’s rainfall totals were 10% heavier due to climate change.  A climate attribution study reconstructs extreme weather events by analyzing data and comparing the results with computer-generated weather simulations to determine whether climate change influenced the event.

Hurricanes and tropical storms, however, cannot be attributed to global warming since climate models can’t accurately simulate hurricanes or untangle the complex phenomena that form tropical systems. Instead, climate attribution studies flush out the side effects of a tropical system, such as extreme rainfall. 

The WWA initiative includes researchers from a range of institutions in Europe and the United States. Their assessment studied rainfall in two large subregions on Helene’s path: where the storm made landfall in Florida and an inland region that includes seven states.

According to the report, climate change enhanced conditions contributing to intense rainfall. 

“This is in line with other scientific findings that Atlantic tropical cyclones are becoming wetter under climate change and undergoing more rapid intensification,” the report said. The WWA did not respond to a request for an interview.

Among the contributing factors to Helene’s strength was exceptionally high sea surface temperatures in the Gulf of Mexico which provided a source of heat and moisture.

As the atmosphere warms, “it can hold more moisture so when storms occur, they will be more powerful,” Easterling explained

However, he criticized the studies’ analysis, telling CPP that the WWA used an overly broad area to average rainfall.

“Personally, I would have opted for a much smaller box,” Easterling said, rather than averaging rainfall over seven states. However, he agreed that climate change contributed to higher rainfall totals. 

“It’s very scientifically sound to say the rainfall totals were greater due to climate change,” he said. “Whether it’s 10% or 50% is debatable — that’s hard to determine precisely.”

Experts have also criticized attribution studies since they bypass the traditional scientific peer review process, which can span months. 

World Weather Attribution co-founder and climatologist Friederike Otto explained in her book Angry Weather that their goal is to release studies quickly to highlight the role of climate change in extreme weather events.

“The faster we are and the earlier we can make our answers public, the more impact we can have,” Otto wrote. For instance, understanding how climate change intensifies rainfall can help communities not only recover from weather events but also prepare for future, potentially more severe storms.

Easterling, whose research at NOAA focuses on helping society adapt to climate change and build resilient communities, recognizes the value of attribution studies. However, he expressed concern that their overuse might dilute their impact. “After a while, people may stop paying attention and just say, ‘yep,'” he said.

A larger concern, he said, is making sure that policy makers have “robust statistics” to make accurate decisions about future storms and climate risk.

More precise rainfall predictions can assist in timely evacuations. While flooding is obviously linked to rainfall amounts, there are also human factors that contribute, such as land use, inadequate infrastructure, or deforestation. More precise forecasts may also help to design infrastructure — such as dams, culverts and bridges — that are more likely to withstand extreme weather events. 

As the Earth’s surface temperatures increase, more liquid water evaporates from the land and ocean, adding more moisture to the air. Since warmer air temperatures can hold more water vapor, the increased moisture content in the air can produce more intense downpours.

The Southern Appalachians are also particularly vulnerable to flash floods since rainfall increases when moist air bumps into the Blue Ridge mountains. Known as orographic precipitation, when wet air hits steep slopes it’s forced to rise and condense at a faster rate, squeezing out more moisture in a relatively shorter period of time.

“An increase in rainfall of 10, 15, 20% can be a big deal in terms of designing a dam or a road. If the trend of more extreme precipitation continues, the real issue is whether our infrastructure is big enough to be able to drain off extreme amounts of rain,” Easterling said. “Helene was a real eye-opener for me.”

Increases in warming have accelerated since the mid-20th century and track closely with a rise in the output of greenhouse gasses — such as carbon dioxide and methane — into the atmosphere. Earth was about 2.45 degrees Fahrenheit warmer in 2023 than the preindustrial average, according to NOAA. The 10 most recent years are the warmest on record. 

Business as usual, said Easterling, means anywhere from 5 to 12 degrees (F) of global temperature rise by 2100. Technological adaptations and other mitigation strategies, he said, would put less pressure on temperature, “but that’s if we start taking climate change seriously.”

This article first appeared on Carolina Public Press and is republished here under a Creative Commons license.