Hurricane Harvey, now Tropical Storm Harvey, has brought devastation to Texas and parts of Louisiana. Although it doesn’t compare to the monsoon-driven floods that are ravaging South Asia at the moment, it’s safe to say that it’ll be one of, if not the most costly natural disaster in US history. The Federal Emergency Management Agency (FEMA) have said that they’ll be around for “years” to come.
Inevitably, climate change has been brought up – and right now, depending on where you look, the man-made phenomenon has either definitely caused Hurricane Harvey or it has nothing to do with it. As it so happens, there are a few things you can definitely say about climate change in this sense, and a few things you certainly cannot say.
So let’s start with what we definitely cannot say: namely, that climate change caused this natural disaster to occur, because it didn’t. No one can say with any confidence that this exact cause-and-effect relationship can be found here, and no climatologist worth their salt would ever do so.
Hurricanes have been happening ever since Earth has had significant amounts of water. Although the process is still not completely understood, they form when specific types of wind currents coalesce around spots of warm oceanic water located close to the equator.
Hurricanes like Harvey gather strength over warm water, and they lose strength when they drift over cool, dry land. There’s more to it than that, but those are the basics. If humanity hadn’t been changing the climate, hurricanes would still be taking place today.
We also cannot say whether or not hurricanes would be more frequent as a result of climate change. Instinctively, it feels like that this should be the case. After all, all that carbon dioxide we’re pumping into the atmosphere is directly linked to warmer oceanic surface waters, the primary “fuel” source of hurricanes – so you might expect that warmer waters would lead to more hurricanes.
There’s no solid evidence in the scientific community that this is what we’re seeing. Plenty of multidisciplinary studies looking into this link have come up empty handed. More specifically, there’s not enough evidence yet to come down on either side.
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.
“There is a roughly 3 percent increase in average atmospheric moisture content for each 0.5°C (about 1°F) of warming,” Mann notes, citing a rigid formula known as the Clausius-Clapeyron equation.
“Sea surface temperatures in the area where Harvey intensified were 0.5-1°C warmer than current-day average temperatures, which translates to 1-1.5°C warmer than the 'average' temperatures a few decades ago. That means 3-5 percent more moisture in the atmosphere.”
This is what triggered the record-breaking rainfall in Hurricane Harvey. It may have still been record-breaking without man-made climate change playing a role, but it’s 100 percent certain that our addiction to fossil fuels made it worse.
There are a few other more ambiguous climate change-related points to make here, but for the sake of clarity, we’ll leave it at what we definitely do and do not know for now.
In sum, though, climate change made Harvey worse. How much worse is hard to quantify at this point – the science of attribution (i.e. what percentage of Harvey was a direct result of human activity?) is still a very nascent field.