September 19, 2006

Global Warming May Raise Hurricane Activity

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After the incredibly devastating 2005 hurricane season, the public, the government and scientists had to ask: did we bring this onto ourselves?

Until recently, global warming may have been earning some credence with the general public and some response by the federal government, but it wasn’t until Hurricane Katrina and the destruction of New Orleans that the dangers of global warming got through to the public conscience and many realized how large a threat a changing climate could be. Prof. Kerry Emmanuel, earth, atmospheric and planetary sciences, Massachusetts Institute of Technology, spoke directly on this issue with the intent of answering the question, “Is Global Warming Increasing Hurricane Activity?”

The number of hurricanes occurring each season worldwide has been about 90 per year, and there is no indication of that number increasing globally. However, approximately 11 percent of total hurricanes are in the North Atlantic Ocean and this small fraction of the whole is increasing. This upswing in hurricane frequency has been occurring since 1995 and corresponds to an increase in sea temperatures of the North Atlantic. These sea temperatures are well correlated with a Northern Hemisphere mean surface temperature, which includes land. This fact eliminates local Atlantic weather cycles as a cause of this increase in sea temperature and hurricane frequency. Emmanuel also said that there is no evidence for “natural cycles” and explained how statistics can be manipulated to make it look as if there is. All of this indicates a non-cyclic, non-local change in sea temperature: global warming.

The number of hurricanes appearing in a particular season is one measure of an increase in activity, but Hurricane Andrew, one of the most destructive hurricanes of all time, appeared during a slow season with only six other hurricanes (most seasons average nine).

The temperature of the ocean not only indicates high frequency storm seasons but can actually increase the strength and duration of hurricanes. Worldwide since 1975, wind speeds measured inside hurricanes have increased 15 percent and hurricanes have lasted 60 percent longer than in the past. This adds up to a 70 percent increase in the energy released by hurricanes in the last 30 years.

Sea temperature has been mentioned already as a cause for this increase in hurricane power, but what does the ocean have to do with the weather? The ocean can be viewed as a giant heat sink that absorbs energy efficiently from the sun and re-radiates it inefficiently back out. If the ocean had no other medium to release its heat, rising temperatures would produce conditions inhospitable to most life. Luckily, oceans lose a majority of their heat through evaporation and cool to habitable temperatures. The higher the sea temperature, the greater the evaporation. Sea evaporation is a key factor for the construction and powering of hurricanes. Hurricanes can even be thought of as a special kind of engine using thermal energy and transferring it into mechanical energy. This mechanical energy powers the intense winds that accompany hurricanes.

Hurricanes are very efficient engines, and nature uses them as a primary tool for heat transfer. Global warming reduces the already poor efficiency of other heat transfer methods, such as radiation. The oceans will still desire to release this energy, which means an increase in nature’s reliance on hurricanes as a heat-releasing option. Theory predicts and modeling supports that every one degree Celsius increase in average sea temperatures corresponds to a five percent increase in average wind speeds. Global warming will also change hurricane dynamics because it melts ice caps at the poles and increases global sea levels. Rising sea levels will increase the surface area of shallow tropical seas and possibly bring hurricanes to geographical locations not yet reached.

However, Emmanuel explained that while changing hurricane characteristics may be of concern for the future and require some of our attention today, a much larger problem is cultural and political in nature. Hurricane Katrina, for all its damage and destruction, was by no means the first hurricane as big and strong as it was. It just happened to directly hit one of the biggest cities in the region. The recent rush to the sunshine belt as well as federal and state subsidies that reduce the risk of developing coastline property have increased the costs each hurricane incurs on the average American in both taxes and property insurance.

Hurricanes are increasing in strength, duration and frequency in the North Atlantic. Recent rising sea temperatures from global warming are powering this shift and will continue to do so as long as greenhouse gasses remain uncontrolled and out of equilibrium. The recent rise in cost incurred by hurricanes, though, is not due to increases in hurricane power, but rather to our nations cultural migration and investment in hurricane-prone shorelines.