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A recent study raised the possibility that global warming could decrease the number of hurricanes hitting the U.S.
On Thursday, Nature published a paper showing that local sea surface temperature has a strong influence on the frequency and intensity of Atlantic hurricanes -- in fact ocean warming accounted for about 40 percent of the increase in hurricane activity that has occurred since 1996.
Confused? That's understandable, given the complex nature of this issue (not to mention how it's covered by the press). What does the new study show, and how does it relate to earlier research? What does it say about global warming?
The short answer is that this latest study provides a more sophisticated understanding of some processes scientists already knew about.
First, let's go over the basics from the latest paper. It essentially asks two questions:
- What are the relative contributions of sea surface temperature and wind to Atlantic hurricane frequency and activity?
- What effect has recent warming had on Atlantic hurricanes?
Influence of sea surface temperature and wind
Meteorologists know that warm ocean water fuels hurricanes, and that certain wind patterns (called wind shear) can inhibit storm development. So, the authors' choice to focus on sea surface temperature (SST) and wind has a solid physical basis. In fact, their statistical model, with just these two factors, did extremely well in "predicting" hurricane activity over the past 40 years (using a procedure called hindcasting). When they teased apart the individual effects of SST and wind, they found that both were important (in fact, wind explained a bit more variation than SST).
Sensitivity of Atlantic hurricanes to SST
Hurricane activity since 1996 has been much higher than normal (here, "normal" is the 1950-2000 average). How much of this has been due to rising SST? If you take the statistical model and hold wind constant, you can isolate the effect of warming by itself. The results are striking: warming has been associated with approximately 40% of the increase in Atlantic hurricane frequency and activity since 1996. (In fact, the influence of warming is evident as far back as 1965.)
What’s new?
The strong relationship between SST and hurricane intensity has been shown before. What this paper does is put that earlier observation in a more complex framework, giving a better understanding of the relative importance of different factors.
Putting it in context
What might surprise you is that this study doesn’t address any link to global warming. They simply looked at the relative influence of two different factors which were already known to affect storms. However, other studies have linked rising sea surface temperatures to human-caused global warming (especially this one (PDF), this one, and this one.)
And what about that study that said warming could decrease the number of hurricanes hitting the U.S.? Isn’t that a contradiction? Actually, no, and here’s why.
Take the central conclusions of the two papers and put them together, and here’s what we know:
- both winds and SST affect storms (and now we have a better understanding of their individual influences);
- wind patterns in the Atlantic appear to be influenced by the spatial distribution of warming (but we don't know what that spatial distribution will be in the future); and
- Atlantic hurricanes are already responding to rising SST, which we know from earlier research is related to human-caused global warming
Future spatial distribution of warming (and other factors) will obviously be important given the strong influence of winds on storm development. But to quote RealClimate (again):
While increases in wind shear could offset the impact of tropical temperatures in some — maybe even the majority — of storm seasons, one might worry about what happens during those seasons where there is anomalously low shear (e.g., a very strong La Niña event). The warm ocean will still be sitting there, waiting to produce tropical cyclones and Hurricanes–and the prospects for destructive Hurricane activity during those seasons could be especially grim
Further Reading
If you’re interested in learning more about hurricanes and global warming, Environmental Defense has a website devoted to the topic. The site includes summaries of the latest research and maps of storm surge risks for various states.
The tremendous RealClimate blog has numerous posts about hurricanes.
Two good books about hurricanes are Chris Mooney's Storm World and Kerry Emanuel's Divine Wind.
You can find more posts on climate change science, policy, and news on Climate411.