Warmer seas creating stronger hurricanes

Natural cycles?
Some scientists have explained the rising strength of hurricanes as being part of natural weather cycles in the world's oceans.

In the North Atlantic, this cycle is called the Atlantic multi-decadal mode. Every 20 to 40 years, Atlantic Ocean and atmospheric conditions conspire to produce just the right conditions to cause increased storm and hurricane activity.

The Atlantic Ocean is currently going through an active period of hurricane activity that began in 1995 and which has continued to the present. The previous active cycle lasted from the late 1920's to 1970, and peaked around 1950.

These cycles definitely do influence hurricane intensity, but they can't be the whole story, Curry said.

While scientists expect stronger hurricanes based on natural cycles alone, the researchers suspect other contributing factors, since current hurricanes are even stronger than natural cycles predict.

"We're not even at the peak of current cycle, we're only halfway up and already we're seeing activity in the North Atlantic that's 50 percent worse than what we saw during the last peak in 1950," Curry said.

Some scientists still think it's too premature to make any definitive links between sea surface temperatures and hurricane intensity.

"We simply don't have enough data yet," said Thomas Huntington in of the U.S. Geological Survey. "Category 5 hurricanes don't come around very often, so you need the benefit of a much longer time series to look back and say 'Yup, there has been an increase.'"

Huntington is the author of a recent review of more than 100 peer-reviewed studies showing that although many aspects of the global water cycle — including precipitation, evaporation and sea surface temperatures — have increased or risen, the trend cannot be consistently correlated with increases in the frequency or intensity of storms or floods over the past century. Huntington's study was announced this week and is published in the current issue of the Journal of Hydrology.

Brace yourselves
Whatever the underlying cause, most scientists agree that people will need to brace themselves for stronger hurricanes and typhoons in the coming years and decades.

However, most regions around the world will not experience more storms. The only exception to this is the North Atlantic, where hurricanes have become both more numerous and longer-lasting in recent years, especially since 1995. The reasons for this regional disparity are still unclear.

The team's findings are controversial because they draw a connection between stronger hurricanes and rising sea surface temperatures — a phenomenon that has itself already been linked to human-induced global warming.

The study by Curry and her colleagues therefore raises the frightening possibility that humans have inadvertently boosted the destructive power of one of Nature's most devastating and feared storms.

"If humans are increasing sea surface temperatures and if you buy this link between increases rising sea surface temperatures and increases in hurricane intensity, that's the conclusion you come to," Curry said.

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