As the thunderstorms multiply, they can get picked up by low-pressure systems drifting through equatorial waters, forming a tropical depression.
The spin of the Earth causes the winds within the storm to whirl around the center of the low pressure. This spinning can form an eye of a storm.
The strongest part of a hurricane is the eye wall, on the edge of the calm center. "The size of the eye wall can vary, and the intensity of the storm can vary depending on how much heat is available" and other factors such as high altitude winds, Barnes said.
According to Emanuel, if global temperatures continue to rise, it is reasonable to assume that hurricane activity will increase, as there is more heat to drive the storms.
Previous studies have tried to measure whether typhoons and hurricanes were becoming more frequent. Emanuel's research, however, focused on the total energy generated by the storms over their duration.
"They can have the same frequency, but if they get stronger or last longer this metric will show an increase," he said.
When Emanuel looked at the hurricane record in the North Atlantic, where the storms of most interest to U.S. residents form, he found that intensity fluctuated from decade to decade.
This fluctuation roughly corresponded with factors such as the El Niño weather phenomenon, which has been shown to influence hurricane formation.
However, North Atlantic hurricanes account for only 12 percent of the total number of hurricanes and typhoons that form globally each year, Emanuel said.
"If you look at a more global measure of this metric, you don't see these strong interdecadal swings. They cancel each other out between one ocean and the other," he said. "You see instead a large upward trend."
According to Emanuel, on a global scale, the strength of storms corresponds with ocean temperatures: It goes up when temperatures go up, down when temperatures goes down.
Most scientists say the rise in sea surface temperature in the last 30 to 50 years is a signal of global warming.
"That's their conclusion, not mine," Emanuel said. "[But] it would follow reasonably well from this metric that the upswing [in intensity] is a result of global warming."
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