The 2020 North Atlantic hurricane season was historic, producing some of the heaviest rain and winds on record, a groundbreaking 30 named storms, and billions of dollars in damage. A computer modeling study of the entire hurricane season, published April 12 in Nature Communications, found that human-induced climate change played a major role in fueling this intense storm season, with hourly hurricane rainfall totals up to 10% higher than those of the pre-industrial era.

The research team, which included scientists from Lawrence Berkeley National Laboratory (Berkeley Lab), applied hindcast attribution methodology to the entire 2020 North Atlantic hurricane season. Just as a weather forecast predicts future events, a hindcast details what has already happened.

“The intent was to simulate the storm that was, and then the storm that might have been had there not been global warming,” said Michael Wehner, a senior scientist in Berkeley Lab’s Applied Mathematics and Computational Research Division and one of the study co-authors. “We’ve used this method to look at many individual storms, but this was the first study that looked at a whole season of storms.”

The research team — which included lead author Kevin Reed of Stony Brook University and co-author Colin Zarzycki of Pennsylvania State University — used data from the National Center for Atmospheric Research’s Community Earth System Model Large Ensemble Community Project, which encompasses transient simulations from the year 1850 onward, to estimate the environmental changes caused by humans. They found that human activities that increase greenhouse gases in the atmosphere have resulted in a more than 1°C rise in the global average surface temperature in 2020 compared to 1850. This increase led to increases in sea surface temperature in the North Atlantic basin of 0.4-0.9°C during the 2020 hurricane season. Hurricanes are fueled in part by moisture linked to warm ocean temperatures. Over the last century, higher amounts of greenhouse gases due to human emissions have raised both land and ocean temperatures.

“Before Hurricane Harvey [a Category 4 hurricane that made landfall in Texas and Louisiana in August 2017], we hadn’t really done hindcast modeling on individual hurricanes,” said Wehner. “That’s when our understanding of how climate change was affecting weather systems really started to deepen.”

When Wehner and colleagues set out to study Hurricane Harvey at the National Energy Research Scientific Computing Center (NERSC), they anticipated that the scaling of extreme precipitation would follow previously established patterns. “It had already been established from 19^th century steam engine measurements that the amount of moisture that can be held in the air when its fully saturated increases at about 7% per degree centigrade of warming, so I assumed that climate change would have a 7-10% effect,” Wehner said.

However, after Hurricane Harvey, three related studies came out around the same time, one of which was authored by Wehner and Mark Risser, a research scientist in Berkeley Lab’s Earth and Environmental Sciences Area. That study found that the average of the effect of climate change in those three studies was not 7% but 19%.

“Those results spurred more of our research with Christina Patricola [who holds an affiliate faculty position in Berkeley Lab’s Climate and Ecosystem Sciences Division and is an assistant professor at Iowa State University] and Kevin Reed, and we continued to find the same results, where the rate of scaling of precipitation was larger than the rate of scaling of the moisture,” Wehner said. “With this most recent study of the 2020 hurricane season, we went from looking at individual storms to looking at a whole season and found the same result again: that the scaling rate of precipitation was greater than the scaling rate of moisture. It’s clear that storms are more efficient at raining out the available moisture that’s in the air.”

Within five years, he added, “our understanding of how climate change affects precipitation has changed dramatically because of the high performance computing resources at NERSC and elsewhere. And the story is, unfortunately, worse than we initially thought.”

This research used supercomputing resources at the National Center for Atmospheric Research’s Computational and Information Systems Laboratory.

For more information about this research, see this Stony Brook University news release.