The Imperfect Storm

Glenn Reynolds at Instapundit has a roundup of links on the debate over whether hurricanes are getting "worse". James Glassman argues that the frequency of giant storms has actually been decreasing over time, based on NOAA data.

Giant hurricanes are rare, but they are not new. And they are not increasing. To the contrary. Just go to the website of the National Hurricane Center and check out a table that lists hurricanes by category and decade. The peak for major hurricanes (categories 3,4,5) came in the decades of the 1930s, 1940s and 1950s, when such storms averaged 9 per decade. In the 1960s, there were 6 such storms; in the 1970s, 4; in the 1980s, 5; in the 1990s, 5; and for 2001-04, there were 3. Category 4 and 5 storms were also more prevalent in the past than they are now. As for Category 5 storms, there have been only three since the 1850s: in the decades of the 1930s, 1960s and 1990s.

The counterargument cited is MIT researcher Kerry Emmanuel's letter in Nature.

Theory and modelling predict that hurricane intensity should increase with increasing global mean temperatures, but work on the detection of trends in hurricane activity has focused mostly on their frequency, and shows no trend. Here I define an index of the potential destructiveness of hurricanes based on the total dissipation of power, integrated over the lifetime of the cyclone, and show that this index has increased markedly since the mid-1970s. This trend is due to both longer storm lifetimes and greater storm intensities. I find that the record of net hurricane power dissipation is highly correlated with tropical sea surface temperature, reflecting well-documented climate signals, including multi-decadal oscillations in the North Atlantic and North Pacific, and global warming. My results suggest that future warming may lead to an upward trend in tropical cyclone destructive potential, and—taking into account an increasing coastal population—a substantial increase in hurricane-related losses in the twenty-first century.

Emmanuel has an earlier version of his global warming thesis on his website entitled Anthropogenic Effects on Tropical Cyclone Activity. In that earlier paper, Emmanuel had not yet reached any definite conclusions about the effect of global warming on the frequency, energy and distribution of hurricanes.

The theory of tropical cyclones, in its present state of development, yields some useful insights into the relationship between tropical cyclone activity and climate. There is a rigorous upper limit to the intensity that hurricanes can achieve, and this limit can be easily determined from known states of the atmosphere and ocean. Elementary considerations show that this limit increases with the amount of greenhouse gas in the atmosphere, but the magnitude of the increase that would result from the present injection of anthropogenic greenhouse gases into the atmosphere is unknown, owing to large uncertainties about feedbacks in the climate system. Moreover, very few storms approach their limiting intensity, and the processes responsible for keeping storm intensities below their limiting value are poorly understood and not likely to be well simulated by present GCM's. The frequency with which tropical cyclones occur is a product of the prevalence of known necessary conditions for their formation and the frequency and strength of disturbances that have the potential of initiating tropical cyclones. Neither basic theory nor numerical climate simulation is well enough advanced to predict how tropical cyclone frequency might change with changing climate, and both give conflicting results on the change of tropical cyclone frequency on doubling atmospheric. There is no physical basis, however, for claims that the total area prone to tropical cyclogenesis would increase. The new field of paleotempestology entails the use of a variety of techniques for deducing the long-term history of hurricane activity from the geological record. Pushing the record of landfalling tropical cyclones well back into prehistory, perhaps even to the last ice age, may be the key to understanding from an empirical standpoint the relationship between tropical cyclone activity and climate. We should do what we can to encourage this endeavor. 

To be perfectly fair, the letter cited in Nature does not assert that the frequency of hurricanes is increasing. Emmanuel's letter says: "work on the detection of trends in hurricane activity has focused mostly on their frequency, and shows no trend." Emmanuel does argue that storm lifetimes and intensities has increased over time and that his data suggests it is correlated with "tropical sea surface temperature" which is in turn a function of "multi-decadal oscillations in the North Atlantic and North Pacific, and global warming". A New York Times article citing Emmanuel says:

In an article this month in the journal Nature, Kerry A. Emanuel, a hurricane expert at the Massachusetts Institute of Technology, wrote that global warming might have already had some effect. The total power dissipated by tropical cyclones in the North Atlantic and North Pacific increased 70 to 80 percent in the last 30 years, he wrote. But even that seemingly large jump is not what has been pushing the hurricanes of the last two years, Dr. Emanuel said, adding, "What we see in the Atlantic is mostly the natural swing."

There are several issues which should be individually highlighted before considering their interaction. The first is frequency. If hurricanes are considered to be natural heat pumps and hurricane frequency is constant or declining, individual storms will logically pack more energy to pump the additional available heat. But this additional heat is apparently contributed by non-human causes: Emmanuel says, "What we see in the Atlantic is mostly the natural swing." The third factor is storm distribution and it enters the picture in two ways. The first is where storms arise and track. The second, already mentioned by Emmanuel is the "increasing coastal population" of the world. What really gets people's attentions isn't storms, but landfalling storms. The most powerful storms on record remain relatively unknown as long as they are confined to the open ocean. Wikipedia notes:

The most intense storm on record was Typhoon Tip in the northwestern Pacific Ocean in 1979, which had a minimum pressure of only 870 mb and maximum sustained windspeeds of 190 mph (305 km/h). Fortunately, it weakened before striking Japan. Tip does not, however, hold alone the record for fastest sustained winds in a cyclone; Typhoon Keith in the Pacific, and Hurricane Camille and Hurricane Allen in the North Atlantic currently share this record as well, although recorded windspeeds that fast are suspect, since most monitoring equipment is likely to be destroyed by such conditions.

How strong was Typhoon Tip? For comparison, Katrina had a central pressure of 902 mb in midocean against 870 for Tip. (The lower the windpressure the worse and remember the scale is logarithmic.) One weather site estimates that if a storm like Tip "hit south Florida directly, tropical storm force winds would be felt as far north as Charlotte, North Carolina and as far south as Merida, Mexico and Kingston, Jamaica". Katrina became the most destructive US storm in history not by virtue of its power, but by a combination of power and geographical distribution. (But not in lives lost. That dubious honor belongs to the Galveston Hurricane of 1900 which killed between 6,000 and 12,000 people)

It should be pretty clear from the foregoing that Glassman's point about the decreasing frequency of hurricanes and Emmanuel's argument for their increasing power are not necessarily in conflict. They may both be true. But what of Global Warming? If, as Emmanuel himself suggests, the increased ocean heat is dominated by "the natural swing" even the most strenuous efforts at reducing the consumption of fossil fuels would bring meager results. What's left is the distribution variable. Emmanuel notes there is "an increasing coastal population", and which implies ceteris paribus, that typhoon damage will monotonically increase even if there were no additions to storm frequency or power. One of the reasons that typhoons in the northern Indian Ocean, comprising India, Bangladesh, Sri Lanka, Thailand, Myanmar, and Pakistan kill the most people year after year is because of population concentrations in the low-lying coastal areas. One of the reasons Katrina was so devastating was the location of Gulf coast cities and oil rigs. This suggests that zoning and resistant architecture and contingency planning might do far more to limit disaster than immense investments in windmills or solar panels.

(Speculation alert.) Environmentalists might take note of how inhabitants in the little-known Batanes Islands in the Philippines cope with typhoons. The Batanes are so small they are repeatedly struck by typhoons undiminished by the landfall effect. The inhabitants of the "typhoon islands", the Ivatans, developed a unique form of architecture that simply enabled them to ride out the storm. In other words, they used technology to adapt to the forces of nature. But they are indigenous people, so that's OK.