A Q & A with Kerry Emanuel, author of Divine Wind: The History and Science of Hurricanes.
1.) Q: Is global warming causing more hurricanes?
A: No. The global, annual frequency of tropical cyclones (the generic, meteorological term for the storm that is called a tropical storm or hurricane in the Atlantic region) is about 90, plus or minus 10. There is no indication whatsoever of a long-term trend in this number.
2.) Q: But I’ve noticed that there seem to have been lots more hurricanes, beginning around 1995.
A: You probably live in North America, Central America, or Europe and are talking about hurricanes in the North Atlantic. (It’s important to remember that only 11% of all hurricanes occur in the Atlantic, the rest are in the Pacific and Indian Oceans.) There has been a large upswing in the frequency of Atlantic hurricanes, beginning in 1995. This is owing to natural cycles in North Atlantic climate that we have observed for many decades and, to the best of our ability to discern, has nothing obvious to do with global warming. This upswing was predicted at least 10 years in advance by meteorologists familiar with the 150 year record of Atlantic hurricanes.
3.) Q: Is the intensity of hurricanes increasing with time?
A: There is some evidence that it is. Records of hurricane activity worldwide show an upswing of both the maximum wind speed in and the duration of hurricanes. The energy released by the average hurricane (again considering all hurricanes worldwide) seems to have increased by around 70% in the past 30 years or so, corresponding to about a 15% increase in the maximum wind speed and a 60% increase in storm lifetime.
4.) Q: But aren’t there lots of errors in the hurricane record?
A: Yes, there are. Reliable records of wind speeds in hurricanes over the open ocean go back only to around 1950, when aircraft reconnaissance of hurricanes began over the North Atlantic and western North Pacific; before that, the only good measurements of wind speed were made when hurricanes made landfall or passed over islands or ships with measuring equipment. Unfortunately, methods of measuring or estimating wind speed from aircraft have evolved over time, and these changes were not always well documented. Since about 1980, there are wind estimates for all hurricanes globally, based on satellite images, but these are not as good as aircraft measurements.
5.) Q: Then how can you determine trends with such data?
A: Fortunately, the means of estimating the central surface pressure in hurricanes have remained fairly constant with time. In practice, central pressure is well correlated with maximum wind speed, and therefore can be used to detect changes in the way winds were estimated from pressures. Also, in a large enough sample of events, the wind speeds are well correlated with a quantity call the “potential intensity”, which is a function of the temperature of both the ocean and atmosphere. We have fairly good records of the information needed to calculate potential intensity, and so can compare estimated wind speeds with estimated potential intensity for large enough samples. This is another check on the quality of the wind estimates. Even in the Southern Hemisphere, where there have never been aircraft observations of hurricanes, the satellite-based estimates compare well with estimates of potential intensity.
6.) Q: You say that reliable records of hurricane wind speeds go back only to about 1950, so how can you say that there were not even more intense storms before 1950? How can you assert that the upswing in the last 50 years is a consequence of global warming?
A: We cannot say for sure. What we can say is that everywhere we have looked, the change in hurricane energy consumption follows very closely the change in tropical sea surface temperature. When the sea surface temperature falls, the energy consumption falls, and conversely, when it rises, so too does the energy consumption. Both theory and models of hurricane intensity predict that this should be so as well. In contrast to the hurricane record, the record of tropical ocean temperature is less prone to error and goes back 150 years or so. Moreover, geochemical methods have been developed to infer sea surface temperature from corals and from the shells left behind by micro-organisms that live near the surface; these can be used to estimate sea surface temperature for the past several thousand years. These records strongly suggest that the 0.5 degree centigrade (1 degree Fahrenheit) warming of the tropical oceans we have seen in the past 50 years is unprecedented for perhaps as long as a few thousand years. Scientists who work on these records therefore believe that the recent increase is anthropogenic.
7.) Q: Does this mean that we are seeing more hurricane-caused damage in the U.S. and elsewhere?
A: There is a huge upward trend in hurricane damage in the U.S., but all or almost all of this is due to increasing coastal population and building in hurricane-prone areas. When this increase in population and wealth is accounted for, there is no discernible trend left in the hurricane damage data. Nor would we expect to see any, in spite of the increase in global hurricane power. The reason is a simple matter of statistics: There are far too few hurricane landfalls to be able to discern any trend. Consider that, up until Katrina, Hurricane Andrew was the costliest hurricane in U.S. history. But it occurred in an inactive year; there were only 7 hurricanes and tropical storms. Data on U.S. landfalling storms is only about 2 tenths of one percent of data we have on global hurricanes over their whole lifetimes. Thus while we can already detect trends in data for global hurricane activity considering the whole life of each storm, we estimate that it would take at least another 50 years to detect any long-term trend in U.S. landfalling hurricane statistics, so powerful is the role of chance in these numbers.
8.) Q: I gather from this last discussion that it would be absurd to attribute the Katrina disaster to global warming?
A: Yes, it would be absurd.
9.) Q: OK, maybe we won’t see global warming effects in landfalling hurricanes for another 50 years or so, but shouldn’t we still be worried about it?
A: The answer to this question is largely a matter of one’s geographical and time horizons. For U.S.-centric concerns over the next 30-50 years, by far the most important hurricane problem we face is demographic and political. Consider that Katrina, as horrible as it was, was by no means unprecedented, meteorologically speaking. More intense storms have struck the U.S. coastline long ago. The big problem is the headlong rush to tropical coastlines, coupled with federal and state policies that subsidize the risk incurred by coastal development. Private property insurance is heavily regulated by each state, and political pressure keeps rates low in high-risk regions like tropical coastlines, thus encouraging people to build flimsy structures there. (Those living in low-risk regions pay for this in artificially high premiums.) Federal flood insurance pays for storm surge damage, and like private insurance, its rates do not reflect the true risk. We are subsidizing risky behavior and should not be surprised at the result.
On the other hand, if one’s view is not confined to the U.S. but is global, and/or one’s time horizon is more than 50 years, global warming may indeed begin to have a discernible influence on hurricane damage, especially when coupled with projected increases in sea level.
To read more by Professor Emanuel, including more detailed answers to these questions, you can read Emanuel’s essay on the subject.
You can also take a peek at all of the fabulous satellite images and ‘hurricane hunter’ photos from his book at www.divinewindbook.com