It's that time of year again…hurricane season! The news media keep us informed of the latest storm, how strong it is, where it might strike--constantly reminding us of our vulnerability to nature's fury. With the 2005 season off to a record-breaking start and the busy 2004 season still a vivid memory, it was inevitable that scientists would renew the debate over the possible connection between the recent upswing in the frequency of these atmospheric maelstroms and global warming.

Hurricanes are a source of endless fascination for the public and researchers alike. The interest is not limited to the United States--our web site, WeatherStreet.com, receives visitors to our hurricane pages from almost every country in the world.

What causes these whirlwinds of destruction? While cyclonic storms outside of the tropics rely on air masses of different temperature for their energy, hurricanes are unique in that they extract their power from thunderstorms. Evaporation of water cools the surface of most of the Earth, a process which stores the latent heat of evaporation in the water vapor. This heat is later released when clouds form and thunderstorms release massive quantities of this heat, with the condensed water returning to the surface in the form of rainfall.

Whether global warming will cause an increase in hurricane strength or frequency has been a subject of considerable debate. Global warming is the expected temperature increase of the lower atmosphere in response to a slight increase in the trapping of infrared radiation by greenhouse gases emitted when fossil fuels are burned. For all of the sunlight absorbed by the Earth, an equal amount of infrared radiation must be emitted back to outer space in order for the temperature of the Earth to remain the same. We are now over 100 years into the industrial era, and well on our way to a doubling of the natural concentration of carbon dioxide in the atmosphere. This doubling could occur late in this century, and the resultant warming is expected to be somewhere in the range of two to ten degrees F.

Since hurricanes represent one mechanism for the Earth to rid itself of excess heat, it might seem obvious that global warming should cause an increase in hurricane activity. What complicates matters is that hurricanes only form when there is relatively little wind shear over the depth of the atmosphere that contains thunderstorms. Normally, there is a sufficiently large change in wind speed with height to carry the thunderstorm-warmed air away from the storm, and low pressure never forms at the surface. It is when a tropical air mass "stagnates" over a region of sufficiently warm sea-surface temperatures that the hurricane formation mechanism kicks in. It is not yet clear what kinds of changes in wind shear, if any, will accompany global warming.

One of the more recent studies of the effect of global warming on hurricanes suggests that there will, on average, be a slight increase in wind speeds within hurricanes accompanying global warming. The effect is small--about a 5 percent increase in maximum sustained winds over the next 80 years, if the model-predicted level of warming is correct. These results are based upon extended runs of a global climate model that actually "grows" hurricanes--albeit crude ones.

But for hurricane researchers, of greater concern are the natural, multi-decadal fluctuations in hurricane activity. Long-term oscillations in the general circulation of the global atmosphere lead to extended periods of high wind shear and lower sea surface temperatures, suppressing hurricane development and intensification. People are lulled into a sense of complacency when 20 or 30 years pass by with relatively few hurricanes striking the United States. Meanwhile, the population within hurricane-prone areas, which includes the entire Atlantic and Gull coast of the U.S., has increased dramatically in recent decades. Hurricane experts have warned year after year that it is only a matter of time before we experience a new and extended period of increased hurricane activity when low wind shear and high sea surface temperature conditions return.

And it looks like that time has arrived. After a lull in hurricane activity during the 1970s and 1980s, Hurricane Andrew in 1992 provided a wake-up call as it slammed into south Florida with peak sustained winds of 165 mph and widespread destruction. Hurricane Ivan was the worst of four major hurricanes that ravaged Florida last year. This year, a record four named tropical storms (sustained winds of at least 39 mph) occurred by early July. Hurricane Dennis was the earliest category 4 hurricane (sustained winds of 131-155 mph) to develop in the Caribbean Sea, as well as the strongest hurricane so early in the year. Superimposed on these multi-decadal cycles are the El Niño and La Niña climate oscillations which appear every few years. During El Niños, hurricane activity is suppressed, primarily due to increased amounts of wind shear. If this all sounds like the processes that control the level of hurricane activity are complicated, you are right.

What about the surge in tropical activity since last year? While global warming could conceivably be involved in some way in the current upswing, this is more speculation than anything. Scientists are just as prone to speculation as anyone else, and this has led to conflicts within the climate research community on a wide variety of global warming topics. One group will contend that computer modeling experiments "show" that global warming will cause a certain aspect of the climate system to change. Another group will protest that the computer models are still too primitive to make any kind of conclusion with any confidence.

It is a popular misconception that the rapid rise in hurricane-related damage in the last century has been due to global warming. In fact, there has been no long-term rise in hurricane activity. The increase in damage has simply been the result of the greater number of man-made structures along the nation's coastline.

What we do know with much more confidence is that we have indeed entered a new upswing in the natural cycle of hurricane activity. While it may seem like the Earth is suddenly swarming with hurricanes, it should be noted that the tropical Pacific, usually a region of more frequent tropical cyclone activity than the Atlantic, has been relatively quiet this year. Again, this illustrates the dominant influence of natural changes in the general circulation of the atmosphere on hurricane activity, causing cycles that usually last anywhere from 25 to 40 years.

From a practical point of view, if people are prepared for the worst hurricane that would occur naturally, then we are prepared for the small increase in activity that global warming might contribute. Despite the uncertainties regarding the effect of global warming on future hurricanes, it seems a good bet that we have now entered a new period of increased activity that will be with us for years to come…so stay tuned.