How Much Is Climate Change to Blame for Tropical Storm Harvey?

Scientists are already starting to weigh in on how humans may have helped exacerbate the weekend’s catastrophic flooding in Texas. The consensus so far: we can’t say climate change caused Harvey, but it certainly made it worse than it could have been.

Writing for the Guardian, Michael E. Mann, a professor of atmospheric science at Pennsylvania State University, points out that sea levels have risen by over six inches around Houston during the last few decades as a result of human action. That means that the water surge forced inland by the storm was six inches taller than it would have been in the past.

But, as Hal Needham, from research firm Marine Weather and Climate in Galveston, Texas, says to the New York Times: “A two- or three-foot storm surge alone would not have been catastrophic.” Indeed, it’s the quantity of rain on top of the surge that appears to have made it such a disaster. The storm has already deposited at least 20 inches of rain, and some estimates suggest that the figure could climb to 50 inches in some areas.

That, too, is likely to have been exacerbated by humans. Mann points out that average temperatures in the area where Harvey developed were over 1 °C higher than they would have been several decades ago. The elevated temperatures will have caused between 3 and 5 percent more moisture to evaporate into the atmosphere than would have been the case in the past, meaning that there was more water to fall as rain when the storm hit.

Kevin Trenberth, from the U.S. National Center for Atmospheric Research, agrees. He tells the Atlantic that “the human contribution can be up to 30 percent or so of the total rainfall coming out of the storm.”

Mann also speculates that the way Harvey has hung over Houston, offering no respite, could also be our fault. He points to his own research, which suggests that stationary weather anomalies appear to be linked to human-caused global warming.

So, how much blame can we lay at our own feet for Harvey? By no means all: it’s a natural disaster, the result of a series of complex systems that happened to act together in a deeply undesirable way. But we definitely made things worse.