There are essentially two global warming mechanisms at play here, according to Mann. First, sea levels in the Houston region have risen by more than half a foot over the past few decades due to global warming. That obviously makes it much more likely for an area to flood.
More from Vox:
Hurricane Harvey: where you can donate to help with disaster relief and recovery
Catastrophic flooding in Houston from Hurricane Harvey: what we know
The military has been deployed to Houston to search for survivors
The second — and more complicated — factor is the amount of moisture in the air. As Mann explained, the rising temperatures in the region add up to 1°C to 1.5°C higher temperatures than average a few decades ago. Based on the Clausius-Clapeyron equation, Mann calculated that amounts to roughly 3 to 5 percent more moisture in the atmosphere — which means more rain.
"That large amount of moisture meant the potential for much greater rainfalls and greater flooding," he noted.
He also goes into one last, but less certain, issue: It's possible that global warming contributed to Harvey "stalling" near the Texas coast, due to "'stationary' summer weather patterns" that Mann argued global warming may contribute to. (He cited a recent paper in Nature about it.) This stalling is one of the reasons Harvey has become so dangerous: It's expected to stay in the area for days, blasting the region with literally feet of rainfall.
"In conclusion, while we cannot say climate change 'caused' hurricane Harvey (that is an ill-posed question), we can say that it exacerbate several characteristics of the storm in a way that greatly increased the risk of damage and loss of life," Mann wrote. "Climate change worsened the impact of Hurricane Harvey."
Other climatologists agree with Mann. Kevin Trenberth, a senior scientist at the US National Center for Atmospheric Research, told the Atlantic, "The human contribution can be up to 30 percent or so up to the total rainfall coming out of the storm. … It may have been a strong storm, and it may have caused a lot of problems anyway — but [human-caused climate change] amplifies the damage considerably."
That helps explain how Harvey suddenly caused what experts have called a 500-year flood.