At the time Harvey intensified into a Category 4 hurricane, it was over a section of the Gulf that was about 4 degrees above normal, says
Martin Hoerling, a research meteorologist with the National Oceanic and Atmospheric Administration in Boulder, Colo.
"The water in the Gulf of Mexico is the heat reservoir to support these hurricanes," says
Ben Kirtman, an atmospheric scientist at the University of Miami. The warm water and air above the Gulf means there's more energy to drive a storm such as Harvey.
Kirtman says that doesn't mean Harvey was directly caused by climate change. Rather, climate change is shaping conditions for storms like this one. So if Harvey was a 1-in-100-year storm, for example, "maybe it becomes a storm that could happen one in 50 years, or one in 20 years, or one in 10 years," Kirtman says.