Also blogging at Climate411
Last Wednesday, scientists published a study about global warming and hurricanes in the U.S. The authors report that warming could be associated with a "downward trend in U.S. landfalling hurricanes".
In its coverage of the study, the Miami Herald concluded that "global warming actually is diminishing the number of hurricanes that strike Florida and the rest of the United States -- and the phenomenon is likely to continue".
That downward trend should be reassuring, right? Actually - surprise! -- the science is more complicated than that. This study doesn’t mean that we can ignore global warming’s effects on hurricanes. Follow me over the fold for a look at what the paper actually says.
Some of the earliest work on hurricanes and global warming found a relationship between sea surface temperature (SST) and hurricane intensity – as SST has risen over the past 30 years, so has hurricane intensity. That result is not challenged by the latest paper.
The current study is not about intensity, but numbers of hurricanes, and a very small subset of hurricanes at that: just the storms that make landfall in the U.S. Of course, landfalling storms are the ones people are most worried about, so what do the data say?
Here's how the authors analyzed the effects of global warming on these storms.
First, they used regression analysis to show that a rise in global SST is associated with increased wind shear in the Atlantic. High wind shear, in which wind speed or direction changes with height, can inhibit storm development by "shearing" off the top of a nascent storm.
The authors also found competing effects of different ocean basins. Atlantic wind shear tends to increase with warming in the Pacific and Indian Oceans, but decrease with warming in the Atlantic Ocean. Keep these spatial differences in mind, because they figure prominently in the scientists' conclusions.
Next, they showed a negative relationship between Atlantic wind shear and the number of landfalling U.S. hurricanes: high wind shear is generally associated with fewer storms hitting U.S. shores.
Given that global warming has increased SST, and Atlantic wind shear tends to increase with global SST, the authors then asked whether the number of storms hitting the U.S. has actually decreased over time. They found that since 1854, the number of landfalling storms has decreased slightly, although the trend is not statistically significant. In other words, the math says the line is essentially flat.
Based on their findings, the authors concluded that future trends in landfalling storms will depend in part on the spatial distribution of warming:
...if the effects of warmings in the tropical Pacific and Indian Oceans cannot overcome that of Atlantic warming, global warming may favor landfall incidence for the United States.
Not surprisingly, the study is quite a bit more nuanced than the news stories made it out to be. But for the sake of argument, let’s assume that the oceans warm in a way that increases wind shear, which inhibits storm development, and the number of storms hitting the U.S. goes down. What then?
In discussing another paper about wind shear, climatologists at the RealClimate blog had this to say, and it’s equally valid here:
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.