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Hurricanes Getting Stronger

The number of Category 4 and 5 hurricanes worldwide has nearly doubled over the past 35 years, even though the total number of hurricanes has dropped since the 1990s. The shift occurred as global sea surface temperatures have increased over the same period.

This research has just been published in the prestigious journal Science. Atmospheric scientist Peter Webster says, "What we found was rather astonishing. In the 1970s, there was an average of about 10 Category 4 and 5 hurricanes per year globally. Since 1990, the number of Category 4 and 5 hurricanes has almost doubled, averaging 18 per year globally."

Category 4 hurricanes have sustained winds from 131 to 155 miles per hour; Category 5 systems, such as Hurricane Katrina at its peak over the Gulf of Mexico, have winds of 156 mph or more. Researcher Judith Curry says, "Category 4 and 5 storms are also making up a larger share of the total number of hurricanes. Category 4 and 5 hurricanes made up about 20% of all hurricanes in the 1970's, but over the last decade they account for about 35% of these storms."

The largest increases in the number of intense hurricanes occurred in the North Pacific, Southwest Pacific and the North and South Indian Oceans, with slightly smaller increases in the North Atlantic Ocean. All this is happening as sea-surface temperatures are rising across the globe-anywhere from around one-half to one degree Fahrenheit, depending on the region, for hurricane seasons since the 1970s.

"Our work is consistent with the concept that there is a relationship between increasing sea surface temperature and hurricane intensity," says Webster. "However, it's not a simple relationship. In fact, it's difficult to explain why the total number of hurricanes and their longevity has decreased during the last decade, when sea surface temperatures have risen the most."

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