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A huge number of hurricanes have blown in from the Atlantic Ocean in the past couple of decades. The 2005 season was the busiest in recorded history. As NPR's Jon Hamilton reports, there is now evidence suggesting you'd have to go back 1,000 years to find another period with so much hurricane activity.

JON HAMILTON: It's not easy to study hurricanes that occurred before reliable weather records came along, but there is a way. Thanks to an emerging field that some call paleotempestology.

Dr. MICHAEL MANN (Director, Earth Systems Science Center, Pennsylvania State University): Paleotempestology.

(Soundbite of laughter)

Dr. MANN: Okay? It's one of my favorite words actually.

HAMILTON: Michael Mann, of Pennsylvania State University, says the discipline relies on scientists who hunt for physical evidence of ancient storms. Mann says these researchers often look for a lagoon that is separated from the open ocean - except during a hurricane.

Dr. MANN: Typically they send a coring device into the bed of the lagoon. And, again, what one looks for are layers of sediment in that core that tell you that there was an event that was strong enough to take the stuff from the open ocean and bring it all the way across the barrier into that lagoon environment.

HAMILTON: Studying these layers is a bit like using tree rings to see what the weather was like hundreds of years ago.

Paleotempestologists also search for evidence of conditions that would have favored hurricanes. These include warm ocean temperatures in parts of the Atlantic and the presence of La Nina: the climate phenomenon that makes it easy for storms to grow. Coral growth patterns reveal when the water was warm. Ice cores help identify La Nina years. When Mann and his team reviewed that sort of evidence, they found that the conditions were ideal for hurricanes in the Middle Ages.

Dr. MANN: There appears to have been sort of a perfect storm, no pun intended, of conditions about a thousand years ago that relate to these various influence I just talked about.

HAMILTON: Even perfect conditions don't necessarily produce storms though. So Mann's team looked at medieval sediments taken from lagoons between Massachusetts and Puerto Rico. And the sediments confirmed that a number of big storms actually had struck the coast during that period.

Dr. MANN: We probably would've had seasons like that amazing 2005 season where we had 28 named storms and we had some very powerful storms, of course, like Katrina.

HAMILTON: But Mann says the current period of intense hurricane activity differs from the medieval one in an important way. He says today's activity is associated primarily with warmer ocean temperatures, rather than the influence of La Nina.

Dr. MANN: And we do believe that a substantial part of the reason for that anomalous recent warmth is, in fact, the human influence on climate: increased greenhouse gas concentrations due to fossil fuel burning.

HAMILTON: There is still debate about the effect of warmer water on hurricanes. Skeptics say it could've been a coincidence that the medieval storms came during a period of warm water and La Nina conditions.

But James Elsner, of Florida State University, says the new research on ancient hurricanes is providing a kind of information that modern satellites and aircraft surveillance just can't.

Dr. JAMES ELSNER (Florida State University): It does provide us some additional clues about how things might change in the long term.

HAMILTON: And at the moment, he says, those clues support the idea that global warming is one reason we're seeing so many hurricanes. The new research appears in the journal, Nature.

Jon Hamilton, NPR News.

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