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A man sits on debris of his house in Leyte province, central Philippines
A man sits on debris of his house in Leyte province, central Philippines
Stéphane Foucart

Does the classification of tropical storms need changing? With sustained winds of 195 miles per hour, and gusts close to 235 mph, the super typhoon Haiyan, which left a trail of destruction in the Philippines and beyond, was the most powerful cyclone to make landfall in recent history. So much so in fact that some climatologists are suggesting the addition of a sixth category to the Saffir–Simpson hurricane wind scale — a scale that allows experts to classify tropical storms depending on the potential destruction they can cause.

The argument is simple: The Saffir-Simpson scale is based on the windspeed and the difference from one category to another is about 20 mph. The fifth category, at the top of the scale, is reached with sustained winds of 157 mph. Haiyan smashed this threshold by almost 40 mph. Isn't it legitimate in this case to add a sixth grade?

The question keeps coming back, with increased regularity. Last year, after Hurricane Sandy hit the northeastern coast of the U.S., former Vice President Al Gore already called for a sixth category of cyclones to be created.

Rising temperature, sea levels

"A lot of tropical meteorologists think that the Saffir-Simpson scale is obsolete and should be replaced," admits Kerry Emanuel, professor at the Massachussetts Institute of Technology (MIT) and one of the leading figures of the research on tropical cyclones. "But personally, I believe that any scale can be deceptive. The problem is that no number can represent the particular nature of the threat posed by a cyclone. In some cases, the destruction will be mostly caused by the wind, in others it will be the storm tide, and in yet other cases, the floods."

For the advocates of Category 6, this decision would be likened to that taken in January 2013 by Australia's Bureau of Meteorology to add a new color — purple — to its temperature maps in order to show the latests measures that went over 50° C (122° F). This new convention also served as a warning: With global warming, such highs are destined to become commonplace.

Adding a new level to the official scale would represent a similar caveat, with one difference though: The connection between global warming and the frequency or intensity of tropical storms is a tricky one to make. Will there be more strong tropical storms in the future? The link seems logical — tropical storms form only when the temperature on the surface of the ocean rises above 26° C (78.8° F), and with the current warming process, these temperatures are certain to rise. Besides, the fact that the waters in the North Pacific were particularly warm is the very reason why Haiyan was so uncommonly powerful.

That said, the Intergovernmental Panel on Climate Change showed in its latest report (published in September) that the likelihood of a connection between global warming and tropical storms doesn't exceed 50%. The report also judges that the uncertain precision in the measures and the lack of hindsight are too notable to blame the recent trend merely on climate change. MIT's Emanuel believes this assessment to be "too conservative," as do numerous experts who have little doubt that a sharp increase in the number of Category 4 or 5 cyclones will happen this century.

If the question of the relationship between global warming and cyclones is still unresolved, there is no question whatsoever that other effects of climate change will reinforce their destructive potential. A cyclone always comes with a "storm tide" that causes the sea level to rise due to low pressure at the center of the cyclonic system. These sudden tides — more than four meters in the case of Sandy — are however exacerbated by the current sea level rise, which is one of the main consequences of global warming.

In some areas of the Philippines, Haiyan caused the ocean to penetrate inland over several miles, causing considerable damage. According to the Global Sea Level Observing System, the sea level on the Philippine coast has risen 20 centimeters since 1960, further weakening the country's coasts in the face of such phenomena.

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