My interest in this, at least my more intense interest right now (which will probably dissipate in about 10 minutes), was spawned by an article on the Environmental News Network [LINK], apparently originating from Reuters and written by Michael Chirstie. Here's a quote:
"...studies by Kerry Emanuel, a professor of meteorology at the Massachusetts Institute of Technology, indicate the 2 degree Celsius (3.6 degrees Fahrenheit) increase in sea surface temperatures predicted by the IPCC would raise the upper limit on a storm's intensity by 10 percent. Landsea said those changes were largely imperceptible given the overall ferocity of hurricanes. "
The Landsea referred to is Chris Landsea, a well-known atmospheric scientist. Kerry Emanuel is an expert on hurricane intensity and atmospheric convection.
Reading this story made me wonder about the possibility of hurricanes being affected by a warming climate. This is not something I haven't thought of in the past, but I never thought of it as particularly provocative or controversial.
There are a couple of issues here. Well, more than a couple, but only a couple that I want to touch on here. First, we need to address how this story is being covered by this article. After that, I want to just think about the science briefly, mostly to raise some points that are not related to what is being discussed in the article but are obviously relevant to the spirit of the article.
The first point is pretty straightforward. The article is focusing on the potential damage, in terms of dollars, of tropical storms, specifically hurricanes. A hurricane is just a tropical cyclone with a high degree of organization, allowing stronger winds and the famous "eye." The article is really trying to discuss a correlation between hurricane intensity and global warming. It mentions that there is believed to be a long term cycle associated with tropical cyclone intensity, and also notes that "climatologists" believe there is also a trend in "hurricane activity." It highlights this debate, but also tries to bring politics into it by mentioning that some climate scientists feel they are being forced to downplay climate change evidence. In short, the article is confused on several levels.
First let's actually ignore politics other than scientific ones. That is, it is safe to assume that whatever debate is at hand has nothing to do with the federal government or funding or anything like that. These are good scientists who are doing good work, and may or may not have differing opinions based on the evidence.
Next, let's decide what "hurricane activity" is. In my opinion, "hurricane activity" is really referring to the number of hurricanes, or more generally the number of tropical cyclones generated. This is a research area known as cyclogenesis, which maybe could be specialized by tropical cyclogenesis. However, the article itself goes on to discuss the strength, or intensity, of hurricanes. Kerry Emanuel, the same one as above, has a simple thermodynamic model that predicts the potential strength of tropical cyclones. It doesn't deal with cyclogenesis though. The model assumes a cyclone exists, and then just guesses how strong it could give based on the conditions in which it exists. In the article, everyone seems to conclude that in a warmer climate, there would be a slight increase in hurricane strength. This is not something that can be put in stone yet, but let's just take it as given: in a warmer climate conditions will allow hurricanes to be slightly (around 10%) stronger, in terms of maximum sustained wind speed. Fine. However, earlier in the article says, "Last season's $45 billion devastation, when 15 tropical storms spawned nine hurricanes in the Atlantic and Caribbean, prompted climatologists to warn of a link to warming temperatures." This doesn't say anything about hurricane intensity, but rather hurricane frequency.
Now let's think about each effect separately. Does global warming suggest changes in (A) hurricane intensity and/or (B) hurricane frequency.
For (A), we already agreed. Yes. There is probably going to be a slight increase in hurricane intensity, but it probably won't be incredibly dramatic. Warmer sea-surface temperature, greater wind shear, and higher low-level relative humidity will all contribute to stronger tropical cyclones.
What about (B) though? This is the hard part, since the subject of cyclogenesis is tricky. What "spins up" hurricanes? You need to have a big blob of organized convection in the tropics, which then propagates poleward and begins to spin and become more organized. That initial blob and the subsequent propagation and organization are hard to predict. They depend on a delicate balance of wind and moisture influences all happening over nice warm water. A lot of people who think real hard about this have been saying, for many years now, that a key ingredient for many tropical cyclones is a passing "easterly wave." That is, an organized motion in the atmosphere coming from the east. The idea is that the easterly wave can help the initial organization of the storm, and then moves away and allows the cyclone to move away and strengthen. (At least, that's the way I've interpreted it so far.) In the Atlantic, and maybe even in the eastern Pacific, these waves originate over northern Africa. Understanding of these waves, and the precise way they help organize convection over the ocean. is still just beginning. There's a lot of work to be done. What can we say then about cyclogenesis under a global warming scenario? Well, we can expect that these easterly waves have something to do with heating over the continent, which is likely to increase in a warmer climate. Whether this increases the number of waves or the amplitude is not yet known. How such changes would change the number of tropical storms formed is also unknown. The first guess would be that there would be more easterly waves of significant enough amplitude that more cyclones can form and propagate poleward.
If that guess is true, which can't yet be confirmed, at least by me, we can guess the future. More hurricanes in the future, with a slightly higher average intensity. This is not what Chris Landsea thinks will happen, which you can read about on NOAA's Atlantic Oceanographic and Meteorological Laboratory website [LINK]. He cites a few studies, and comes to approximately the same conclusion about peak intensity, but refrains from saying anything about average intensity. However, he doesn't seem to see any reason to believe more or less hurricanes will form. The main reason is that the whole atmosphere will change, so the thresholds for cyclogenesis will change. This, I think, neglects the importance of easterly waves in cyclogenesis. As you can see, this is a complicated problem, and there's no reason to think there should be consensus at this point.
As a further caveat, it is also conceivable that more and/or more intense easterly waves originating over northern Africa could also reduce hurricane formation. By carrying very warm, dry air over the ocean, where young hurricanes might be forming, these layers, called "Saharan air layers," could inhibit convection and stop the hurricane in its tracks. So, yeah, basically you read all the way down to here to find out that we (or I at least) have no answers for you. Sorry about that.