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17/5/19

Hurricanes in a warmer planet: Impacts of climate change

Written by Jorge Luis García Franco
Postgraduate researcher at the University of Oxford
@JorgeLGarciaF

Hurricanes are amongst the most destructive weather phenomena in our planet. Famous for their strong winds and massive rainfall, hurricanes are formed in most tropical oceans, specially in the Atlantic, the Pacific and the Indian Oceans, causing important economic and human losses everywhere they go. In recent times, when a powerful hurricane forms, particularly if the hurricane approaches mainland, some politicians and news media invoke climate change as the cause for the hurricane or as the reason for its strong power. Over time, hurricanes have become a sort of poster-boys for climate change, being used to illustrate the dire consequences of a warmer world. But exactly, how are hurricanes being affected by climate change? Is it true they are getting stronger? Are they getting more frequent?

 For a long time, scientists have worked on answering these questions. Perhaps one of the most influential scientists studying hurricanes, Kerry Emanuel, from the Massachusetts Institute of Technology, published in 2005 (1) one of the first strong indications of how climate change could affect hurricanes. He estimated that the potential intensity of hurricanes would increase if ocean temperatures increased, i.e., that in suitable weather conditions hurricanes were capable of becoming more intense at higher ocean temperatures. This study rapidly gained attention and led people to the conclusion that hurricanes would become more frequent and more intense over time, a conclusion which has stuck with public perception for some time.

 Not long after, in 2008, Gabriel Vecchi and Tom Knutson (2) analyzed whether the frequency of hurricanes was changing, in other words, whether we are getting more hurricanes as a response to climate change. Their results showed that no perceivable trend was observed, so it appeared as if climate change was having no effect on the amount of hurricanes we observe every year. Knutson (3) and others revisited the issue in 2010 and still found no impact of climate change on the frequency of hurricanes. They explained that detecting the influence of climate change on hurricane frequency is challenging but as far as we can tell, there is no clear trend in global hurricane frequency. However, recent studies have shown a small positive increase in the frequency of hurricanes in the Atlantic Ocean but only further studies will confirm this.
While this result might seem discouraging for those that usually use hurricanes as a powerful tool in arguments with climate deniers, there is a simple explanation for the apparent lack of relationship between climate change and the number of hurricanes. Most extreme weather events are part of what is known as natural variability, a term that simply accounts for the fact that weather is the chaotic result of many processes and factors that cause variations on many temporal scales. Hurricanes are particularly sensitive to natural variability, so, for example, even in a planet with no climate change, hurricane frequency would vary greatly on the scale of years and decades. The sensitivity of hurricanes to natural variability makes it particularly difficult to separate human from natural effects in hurricane characteristics, particularly with the short amount of time that we have properly been able to observe hurricanes. In other words, while there is yet no evidence to suggest that hurricane frequency is changing, this does not mean that climate change will have no effect in future hurricane frequency, it simply is a matter of time for us to have a more conclusive answer.
But frequency is not the only thing that we could be worried about regarding hurricanes, so how about intensity? In the last 10 years, several investigations (4,5) have analyzed how hurricane intensity has changed in recent decades and whether man-made climate change is to blame. The consensus of these studies was that very intense hurricanes are becoming more frequent. So we have the same number of hurricanes every year, but the number of extremely dangerous and powerful hurricanes is increasing. Modeling studies analyzing future climate have also found that powerful hurricanes will become more frequent in the upcoming decades. While super-hurricanes worthy of apocalyptic Hollywood movies might be interpreted to appear in the future as a result of these findings, there is little evidence to suggest that the maximum intensity of hurricanes is changing significantly. So, while intense hurricanes will become more frequent, their maximum intensity is not expected to change, at least not in a massive amount.

 What is changing, though, is how fast hurricanes intensify. A very recent study in 2019 from the Geophysical Fluid Dynamics Laboratory from Princeton (6) , analyzed whether the time that takes a hurricane to become very intense has changed and whether this change could be attributed to human emissions. Their results have found that hurricanes are intensifying faster, and rapid intensification, a concept used to describe the process of a hurricane that in a very short time becomes very intense, has become more frequent. This is particularly worrying because rapid intensification is very hard to predict and, historically, has caused heavy damage.

 News media and politicians have frequently asked -and probably will continue to do so- whether a powerful hurricane was caused by climate change. But this is not the right question, extreme weather events, including hurricanes, existed long before man-made climate change so asking whether climate change caused any of them is not the best approach. All weather events are now in a warmer world, and are, therefore, being affected one way or another. A warmer world will certainly change hurricanes in ways that we do not yet understand, but as far as our knowledge extends to today, we know that powerful hurricanes will become more common in our planet and intensify faster as a direct result of our emissions, and that is merit enough for climate action.

References

Emanuel, K. (2005). Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436, 686–688
Vecchi, G. A. & Knutson, T. R (2008). On Estimates of Historical North Atlantic Tropical Cyclone Activity*. Journal of Climate 21, 3580–3600
Knutson, T. R. (2010). Tropical Cyclones and Climate Change: An Indian Ocean Perspective. Indian Ocean Tropical Cyclones and Climate Change 47–49. doi:10.1007/978-90-481-3109-9_7
Vecchi, G. A., Fueglistaler, S., Held, I. M., Knutson, T. R. & Zhao, M. (2013). Impacts of Atmospheric Temperature Trends on Tropical Cyclone Activity. Journal of Climate 26, 3877–3891
Knutson, T. R. et al. (2013). Dynamical Downscaling Projections of Twenty-First-Century Atlantic Hurricane Activity: CMIP3 and CMIP5 Model-Based Scenarios. Journal of Climate 26, 6591–6617
Bhatia, K. T. et al. (2019). Recent increases in tropical cyclone intensification rates. Nature Communications 10

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