We read today in Le Monde, that although the number of hurricanes remains relatively constant, their intensity is increasing. It is reported that there are more in category 4 and 5, with the rise directly proportional to an increase in ocean temperature. Being curious about storm intensity in the wake of Hurricane Irene, we asked a climatologist for his view.
Dr. Peter Hildebrand* replied as follows:
Le Monde presents the hypothesis as fact, that Category 4 and 5 tropical storms are on the increase with global warming. While this is probably going to turn out to be true, and perhaps even with an increase in the number of tropical storms, the scientific jury is still out. The reason for the current lack of clarity lies in the length of the record of accurate global tropical storm observations. This is because the tropical storms occur at sea, and many of them never make landfall. Thus, it is only since the weather satellite era that we actually know when, where and how intense the tropical storms may be. Since this record only goes back to about the 1970s, we only have the past forty years to assess changes in tropical storm occurrence. Although forty years seems like a long time, the occurrence and intensity of topical storms is highly variable from year to year, so in ten more years, we will have better assessed the situation.
Nevertheless, the data from the satellite era pretty clearly show an increase in tropical storm intensity, with a roughly constant number of hurricanes per year; an increase in the proportion of category 4 and 5 storms; and a decrease in the category 1 and 2 storms. The scientists who dispute this finding usually argue that there needs to be a longer record of measurements before this is known.
As for a possible increase in the number of hurricanes, we will have to wait and see. Tropical storms can occur over warm oceans, where the water temperature is at about 82F (= 28 C) or greater. There also needs to be a disturbance to set off or trigger the tropical storm. These disturbances are called tropical waves, and are areas in which there are circulations in the counter-clockwise direction in the northern hemisphere, or clockwise in the southern hemisphere. These circulations create areas for enhanced development of the individual thunderstorms that make up the tropical storm. For the Atlantic ocean, many of these disturbances originate from the diurnal thunderstorm that occurs near the horn of Africa. Although that particular thunderstorm dissipates, the residual circulation is then blown across Africa, and then out across the equatorial Atlantic Ocean. As they move out over the Atlantic, a few of these disturbances can create areas of enhanced storm development that can form into tropical storms. Presently, we cannot forecast which of these disturbances will develop into a tropical storm. One other thing that is needed to have a tropical storm occur and become strong, is to not have strong winds at the storm top level. This is because the strong winds will cause the thunderstorm cells in the tropical storm to be tilted over, and this tilting will weaken the tropical storm circulation. Right now our measurements of the winds at storm top level, way out over the ocean, are not very good. Several NASA proposals will fix this with new wind-measuring satellites, plus use of the research versions of Global Hawk aircraft to fly our to measure these storm top winds. Thus we still have a lot to learn about tropical storms.
Putting all this together, we know that with global warming, the oceans will continue heating, and that this alone may possibly make tropical storms more intense, and more likely to occur. We also can expect that the disturbances that create the tropical storms will likely continue, and possibly about as before. What we do not know is whether the disturbances will become more intense; hence, possibly increasing the number of triggering events that can initiate tropical storms, or possibly grow weaker. And finally, we also do not know if the high level winds are likely to increase (fewer tropical storms), or to decrease (more storms). Hence, for now, the jury remains out.