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Did Climate Change Cause Hurricane Harvey?

Written by Alexandra Naegele, a 2017-2018 Sustainability Leadership Fellow and PhD Candidate in the Department of Atmospheric Science.

This past summer, Hurricane Harvey rolled into west Texas and stalled, causing disastrous flooding. The National Hurricane Center just released their final report on Hurricane Harvey and the final numbers are in: Maximum recorded rainfall of over 60 inches. Widespread recorded rainfall of 36 to 48 inches (compared to the 50 inches of rainfall that Houston generally receives over the course of an entire year). Storm surge up to 10 feet. Over 300,000 structures flooded. An estimated 40,000 people evacuated. At least 68 direct deaths. Months later, Houston is far from being fully recovered.

If there is one thing that’s certain, it’s that Hurricane Harvey devastated the region. With such an impactful storm, it predictably received an onslaught of national media attention. What was less certain, however—at least from the public perspective—was if and how Hurricane Harvey was a result of climate change.

After Harvey hit, headlines—admittedly from sources with a wide range of credibility—ran the gamut, from “Hurricanes Harvey and Irma Can’t Be Blamed on Global Warming” to “Scientists Link Hurricane Harvey’s Record Rainfall to Climate Change.” Among those from the most reliable sources, the best headlines avoided sensationalism; the worst were rife with ambiguity. Thus, the question that was on everyone’s minds remained: Did climate change cause Hurricane Harvey?

As tempting as it may be to hastily draw conclusions and answer that question with a resounding “Of course!,” the answer is unfortunately far from straightforward. But perhaps the best place to start when thinking about the attribution of extreme weather events to climate change is with a helpful analogy, courtesy of Marshall Shepherd, a Meteorologist and Professor of Geography at the University of Georgia. As he explains it, “Think of bad storms like home runs in baseball’s recent steroid era. Sure, the big hitters have always hit home runs, and I can’t say that any one homer resulted from steroid use. However, I can make the argument that steroids made a lot more balls go over the fence than normal.”

Applying this line of thought to hurricanes, we know that hurricanes existed before humans first began emitting massive quantities of greenhouse gases (GHGs) into the atmosphere, and we know that hurricanes exist at present. But it is also true that such large-scale GHG emissions can lead to conditions favorable for hurricane formation and persistence. So although we cannot conclusively say that anthropogenic climate change is the clear cause of one particular hurricane, such as Hurricane Harvey, we can confidently say that there is a likely connection between the two. But perhaps a more important question is, what is the impact of climate change on extreme weather events in general?

Although it is misguided to attribute an event to climate change outright, it is entirely different to ask about the ways in which it might have been affected by climate change. It is questions like these that drive the emerging field of event attribution. A distinguishing aspect of this subfield is that they are making a point to ask the right questions, primarily: Is climate change affecting the likelihood of an event? And, is climate change affecting the intensity of an event? Additionally, these questions are accompanied by a deep understanding of the assumptions and uncertainties that are inherent in the data and the methodologies.

Perhaps the biggest takeaway from this growing body of research is that not all events are equal—at least in terms of event attribution. The factors that determine our ability to attribute an event to climate change are the capabilities of climate models, the data record, and an understanding of the mechanisms that would cause these events to change (National Academy of Sciences, 2016). As shown in the table and figure below, these three conditions are all well met for events related to extreme temperatures; the same cannot be said for severe convective storms (e.g. thunderstorms).

With these points taken into consideration, the Bulletin of the American Meteorological Society recently published their latest special issue on event attribution, Explaining Extreme Events of 2016 (Herring et al., 2018). Remarkably, this is the first year in which several events have been found to lie outside the bounds of natural variability. Perhaps somewhat unsurprisingly, each of these events were extreme heat events. To be clear, however, this is not the first time, by any means, in which the fingerprint of climate change has been detected on specific extreme weather events.

To some, it might be interpreted that by detaching Hurricane Harvey (or any extreme weather event) from climate change—even if only slightly, by saying that it is not a 100% cause and effect relationship—it serves either to trivialize the event, to trivialize climate change, or potentially, to do both. On the contrary, I would argue that by not dismissively answering these questions with a simple yes (or no), this opens us up to more nuanced consideration of the complexity of our dynamic climate system—and the ways in which we are intimately and actively connected to it. With that in mind, it’s in our own best interest to gain a more comprehensive understanding of the ways in which we are affecting the climate system, as well as the ways in which it will affect us in turn. Our improved understanding requires that we find the right answers, but to get the right answers we must first ask the right questions.

References:

Herring, S. C., N. Christidis, A. Hoell, J. P. Kossin, C. J. Schreck III, and P. A. Stott, Eds., 2018. Explaining Extreme Events of 2016 from a Climate Perspective. Bull. Amer. Meteor. Soc., 99 (1), S1–S157.

National Academies of Sciences, Engineering, and Medicine, 2016. Attribution of Extreme Weather Events in the Context of Climate Change . Washington, DC: The National Academies Press.

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