Hurricane Harvey swamped Houston with seven days of pounding rain last August. When scientists went back to look at historical weather patterns, they reported Harvey dumped 20 percent more rain than it typically would have. The culprit: climate change.

Today, we know that Harvey wasn’t an outlier. A new study, published Wednesday in Nature by the same lab, reports that climate change intensified the rains of Hurricane Katrina, Hurricane Irma and Hurricane Maria by between 4 and 9 percent. The researchers predict that future warming could increase rainfall totals for the most extreme hurricanes and tropical cyclones by up to 30 percent.

Humans can also affect these torrential rains through how we engineer cities. A second analysis, also published today in Nature, found the urbanization of the Houston area — namely its skyscrapers and the pervasive use of pavement and other concrete surfaces — made Harvey’s rain dump and subsequent flooding worse.

Here’s how both teams used supercomputers to analyze these superstorms and what their research means for future storms.

What the scientists did

To model how a changing climate impacts tropical weather, climate scientists Christina Patricola and Michael Wehner of Lawrence Berkeley National Laboratory first had to model real life hurricanes.

“It’s actually pretty challenging to be able to diagnose how climate change may be influencing tropical cyclones so far,” Patricola said. “There’s a lot of missing data or potentially missing data before the satellite era. Before satellites, a lot of observations were taken by ships, so you can imagine sometimes they would just miss a tropical cyclone.”

Recreating those storms with computers can help fill in gaps in those hurricane observations. To do that, the team used supercomputers to model 15 major tropical cyclones that formed in the Atlantic, Pacific or Indian Oceans in the last threes decades. Storms included Hurricanes Andrew, Katrina and Matthew as well as Typhoon Haiyan and Tropical Cyclone Gafilo.

The researchers simulated each storms’ formation, track and wind speeds and compared them to the real-life events.



The clouds of a simulated Hurricane Katrina tracked along the storm’s actual path, noted in red dots. Video courtesy: Berkeley Lab

Once confident in their ability to mirror reality, Patricola and Wehner tweaked the climate inputs so that the model behaved in a cooler, pre-industrial world — as if climate change didn’t exist. They then re-ran the simulations for the 15 tropical cyclones under the warmer ocean and atmosphere temperatures we experience because of climate change.

Running such simulations is a time-consuming process, given each test requires weeks of setup and a full day to run. But modeling the storms under cooler and warmer conditions allowed the team to better understand how climate change impacted the storms.