Hoogewind, K.A., D.R. Chavas, B. A. Schenkel, and M. E. O’Neill. “Exploring Controls on Tropical Cyclone Count through the Geography of Environmental Favorability.” J. Climate (2020), 33 (5): 1725–1745. DOI: https://doi.org/10.1175/JCLI-D-18-0862.1
NO ONE KNOWS, but read on to hear about some of the ways scientists are thinking about this question!
How will climate change affect hurricanes?
Since hurricanes are such dangerous and costly natural disasters, we all want to know how they’ll be affected by the climate change. Hurricane researchers are in agreement on many of the effects that are directly related to climate change: hurricanes are likely to become more intense, more rainy, more likely to drift further towards the poles (which is bad news for those of us living in New England), and maybe most worryingly, more difficult to forecast as a result of climate change. One seemingly simple question, however, continues to puzzle hurricane researchers, and that is: how will climate change affect the number of hurricanes?
I think it’s interesting to see how creative scientists have been in their approach to the question of how climate change will affect the number of hurricanes per year. There is no confirmed answer yet, but in this article, I’ll talk about some of the ways scientists have approached the problem. Most studies seem to fall under one of these three categories:
(1) Take a computer model that simulates the global climate. Run this model at a high resolution (meaning with small small grid boxes) so that you can actually see hurricanes in the climate simulation. Then run the model under different global warming scenarios and see if the number of hurricanes it simulates will change with warming.
Why this is difficult: Climate models are not designed to study weather phenomena that are relatively small and transient, like hurricanes. Past studies have shown the results from this approach to be very sensitive to small changes, to a point where different experiments have displayed opposite trends in the number of hurricanes. It’s also harder than you might expect to even count the number of hurricanes in these simulations!
(2) Look at how the number of hurricanes has changed in the past.
Why this is difficult: we only have reliable data on hurricanes stretching back to the 1970s and 80s. This does not give us a large enough window of time to be sure that any trends we see are actually real and not just random noise. There has been a really interesting push to use sediment cores from corals and swamps to estimate hurricane activity from hundreds of years ago. So far, though, it seems like even those efforts are affected by the high degree of randomness in the number of hurricanes at any given site.
(3) Write a hurricane model that simulates how a single hurricane evolves based on its environment. Then, feed it data about different environments, run it a whole lot of times, and see if hurricanes are more or less likely to develop in different climate conditions.
Why this is difficult: a key property of these models is that they simulate each hurricane in isolation. This bakes in the assumption that hurricanes don’t interact with each other or change their environment in any meaningful way. As we will see later in this article, this may not actually be true in the context of tropical cyclogenesis, or the process by which hurricanes form.
So what can we do??
Hopefully all of this demonstrates why this is such a tough question!
A recent study by Kimberly Hoogewind et al. took a new, and completely different approach, to tackling this problem. The authors drew from an existing body of work that studies tropical cyclogenesis specifically. Hurricanes form in a very particular kind of environment, which is roughly characterized by warm temperatures, high humidity, low wind shear, and high vorticity. (Vorticity is a measure of the rotational component of a wind field – tornadoes, for instance, have really high vorticity.)
The authors of this study asked the question: what if the number of hurricanes is set by the geographical area that has the right conditions for tropical cyclogenesis to occur? If that’s true, then the question “How does climate change affect the number of hurricanes?” just becomes, “How does climate change affect the area in which you can find favorable weather conditions?” It would be great if this were true, because this is a much easier question to answer!
To test their hypothesis, the authors used data from the years 1979-2016. First they identified the geographical region in which it was favorable for hurricanes to develop. Then, using data on the typical distances between hurricanes in real life, they developed a packing algorithm that they used to determine the number of hurricanes that could fit inside that favorable region. Finally, they totaled up those numbers of potential hurricanes, and then compared that to the numbers of hurricanes per year we actually observed.
Is the number of hurricanes determined by the area of favorable conditions?
The authors found that from season to season, the size of the favorable region corresponded fairly well to the actual number of hurricanes. Their numbers were roughly within an order of magnitude, which means that they were too big by about a factor of 10. They attributed this over-estimation to some of the factors that they neglected in their analysis. Those were: (1) not including any information about the weather disturbances that “seed” hurricanes in real life, and (2) not including any information about the ways that hurricanes can potentially modify their environment and make another hurricane more or less likely to form.
I think this study is really interesting, both because its approach is completely different from the three main categories outlined earlier in this article, and because there’s insight to be gained even from the ways that the authors’ hypothesis fell short. In particular, these results point to the importance of resolving questions about both about the disturbances that lead to hurricanes, as well as the effects the hurricanes themselves have on their environments.
I think research tends to be like this. Many of the questions about the effects of climate change are hard to answer either because of observational or modeling challenges or both. Even if these studies don’t entirely answer the original question, they still provide insight and clues to keep us moving in the right direction.