There's extreme, and then there's really extreme.
In terms of sheer volume of rainfall over a limited area, Hurricane Harvey was record-breaking. By my estimate, never before has a single rainfall event produced such a deluge anywhere in the U.S. It is as if six Mississippi Rivers poured all their water onto a small section of south-central and southeast Texas for three straight days.
As we wait for all that water to go somewhere, it's reasonable to wonder how much of this was our own doing. Once rainfall hits the ground, a lot depends on issues such as urbanization and flood control infrastructure. Those aspects are obviously up to us. But what about the rain? Did we also affect Harvey through climate change?
In one sense, everything is affected by climate change. Our atmosphere is different; our weather is different. If there's a parallel universe out there without man-made climate change, they'll be experiencing not just a different climate but an entirely different sequence of day-to-day weather. A more useful question is whether we've altered the likelihood or destructiveness of events such as Harvey.
The connection between hurricanes and climate change is being actively investigated. So far, the evidence leans toward an overall decrease in the number of hurricanes. Whether that applies to storms in the Gulf of Mexico is something we don't know yet. Studies are more consistent in suggesting an overall increase in the intensity of the strongest storms.
One of the things that made Harvey difficult to deal with was the short period of time between formation and landfall as a Category 4 storm. Harvey didn't even exist as a tropical depression three days before landfall. I call such storms "short-fuse hurricanes." It's easy to convince people to evacuate when they can see what's coming. With a short-fuse hurricane, you have to react before there's anything to see.
I'm sure scientists will be taking a closer look at climate change's impact on Harvey's intensity and its rate of intensification. We're lucky that Harvey made landfall in a relatively unpopulated area and made landfall before it reached peak destructive intensity.
By far the biggest impact of Harvey has been the rainfall and flooding. Extreme rainfall in general is increasing, mainly because warmer air can hold more water vapor. My analysis of Texas rain gauge data shows an increase of 5–7% in the magnitude of typical extreme rainfall events over the past century, independent of any improvements in the gauge network.
Harvey wasn't typical, but the same factors that lead to the observed increase in extreme rainfall would have been at work in Harvey too. Harvey's air picked up moisture from sea surface temperatures that were running warm, in part because of the long-term climate trend. The atmosphere was supercharged with water vapor compared to what might have happened in a similar storm without warming seas.
Based on past rainfall trends and the known causes of those trends, I estimate a 5–7% enhancement of intense rainfall compared to a Harvey-like event without climate change. For a place that received 30 inches of rain, that's an extra 1.5 to two inches. The extra flooding might be measured in feet, not inches.
Does that seem like much? For all those families with a foot of water in their houses, it's everything.
John Nielsen-Gammon is the regents professor of atmospheric sciences at Texas A&M University.