New research suggests hurricane seasons in the United States are not only becoming more active, but the strength of the storms has also increased.

"We are at levels now that are about as high as anything we have seen in the past 1,000 years," said Michael Mann, director of the Earth System Science Center at Pennsylvania State University and the lead author of the paper. Mann and colleagues Jeffrey Donnelly of the Woods Hole Oceanographic Institution, Jonathan Woodruff of the University of Massachusetts and Zhihua Zhang of Pennsylvania State University examined sediment samples from across the North Atlantic coast and statistical models of historic hurricane activities.

They said they measured the severity of hurricane seasons during the past 1,500 years and found the sediment samples matched relatively well with computer models, both of which show a period of high activity around 1,000 A.D., followed by a lull in activity The study also adds validity to the theory that the La Nina effect and higher ocean surface temperatures result in higher hurricane activity. The researchers said if climate change continues to warm ocean waters, it could lead to even more active hurricane seasons.

This hurricane season has, so far, been lighter than usual, Mann said, because of the El Nino effect that's believed to have the opposite effect of La Nina patterns.

The study is reported in the journal Nature.

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