The question we should be asking is whether or not anthropogenic climate change has exacerbated Hurricane Harvey. In other words, have human actions made it worse and more damaging than it otherwise would have been? This we can confidently answer in the affirmative, for two reasons.
Firstly, the sea level around the world is rising. Thanks to a combination of unprecedented amounts of terrestrial ice falling into the sea, and to the heat-driven expansion of the oceans, there’s no doubt about it: humanity is triggering an aquatic reclamation of the land.
This affects the hurricane’s storm surge. Hurricanes are extremely low-pressure beasts, and when they appear over water, the sea level rises. Wind also pushes seawater towards the shore. If the sea level is already far higher than it should be, this storm surge will be far worse. Harvey’s was historic by any measure, and one expert described it as a “tsunami arriving in the middle of a hurricane.”
This 3.7 meters (12 feet) high storm surge was certainly devastating, and it likely robbed a few Texan residents of their lives, but it still wasn’t the worst part of the hurricane. That was the precipitation, which broke all kinds of American rainfall records.
Harvey spent most of its time stalled over the city of Houston, a major metropolitan center built on a flat landscape. At the time of writing, it’s unleashed 86 trillion liters (about 19 trillion gallons) of water on that city, about three times more than was dropped during the entirety of Hurricane Katrina.
Thanks to a strong understanding of some very basic physics, scientists can say with certainty that climate change made this worse than it should have been too. As explained by renowned climatologist Michael Mann on a Facebook post, warmer air holds more moisture than colder air – and eventually, all that moisture will fall out as rain.