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Climate Change Study Finds Big Storms on a 1,000-Year Rise - 13 Aug
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|The North Atlantic Ocean has spawned
more hurricanes and tropical storms over the last decade than it has
since a similarly stormy period 1,000 years ago, according to a new
The research, published yesterday in the journal Nature, tries to
trace the pattern of storms along North America's Atlantic and Gulf
coasts back to A.D. 500, well before humans were recording weather
The study's lead author, climate scientist Michael Mann of
Pennsylvania State University, said finding a reliable way to
reconstruct centuries of past hurricane activity could help
scientists tease out whether future climate change will alter storm
"One of the driving motivations for this research is to place in a
longer-term context ... Atlantic tropical cyclone behavior, and the
extent to which it may be anomalous," he said.
That meant trying to divine information about the past beyond
historical records collected by ships and observers on shore, and
later by storm-tracking aircraft and satellites.
The scientists relied on two different methods to reconstruct the
past -- a foray into the emerging field of "paleotempestology," or
the study of ancient storms.
First, they examined layers of sediment collected from coastal ponds
and salt marshes that tend to flood when hurricanes make landfall
nearby. Each flood deposits a layer of coarse barrier beach sand on
the muddy pond floor, creating a record scientists can examine by
collecting sediment cores. For the new study, Mann and his
co-authors at the University of Massachusetts and the Woods Hole
Oceanographic Institution collected cores at eight sites along the
U.S. Atlantic and Gulf coasts, ranging from southern Massachusetts
to Vieques Island, Puerto Rico.
The researchers also used a computer model to simulate 1,500 years
of Atlantic storms, feeding in information collected between 1851
and 2006 about factors known to influence hurricane activity,
including sea surface temperatures in the tropical Atlantic Ocean,
the occurrence of El Niño weather patterns, and fluctuations in the
The two methods produced similar overall results, Mann said, such as
showing a major peak in storm activity about 1,000 years ago.
But 'paleotempestology' doesn't end the debate
Taken together, he said, they suggest that warmer temperatures
produce more storm activity -- meaning that coming climate change
could increase the frequency of hurricane activity.
"The paleoclimate evidence seems to reinforce the notion that, all
other things being equal, when you have warm sea surface
temperatures in the tropical Atlantic, you see more activity," he
But Mann said there's also a possibility that climate change could
alter the frequency of El Niño, which blunts hurricane activity, and
counteract the effects of future ocean warming.
"Current state-of-the-art climate model projections are more or less
split between whether there will be more El Niño conditions or more
La Niña-like conditions," he said.
"The jury is still out. But this study is an independent data point
from the paleo record that gives more weight to the proposition that
warming the tropical Atlantic will continue to give us a higher
level of hurricane and tropical cyclone activity."
Kerry Emanuel, a climatologist at the Massachusetts Institute of
Technology who has published studies linking climate change to
stronger hurricanes, called the new research "an impressive piece of
work, melding two completely independent approaches to estimating
past hurricane activity."
Emanuel said the Nature study "shows that hurricane activity is
indeed quite sensitive to climate, and although we are still not
completely sure about global warming effects, the paper raises again
the flag that potentially they could be large."
But James Elsner, a climate scientist at Florida State University,
said the "rather large levels of uncertainty" in the new study's
results gave him pause.
"I don't see it as settling the debate on climate change and
hurricane activity," said Elsner, who helped develop the statistical
methods employed in the new study. "I think it does provide evidence
that warmth is important."
At the same time, he explained, "the lack of a real tight physical
theory between ocean warmth and frequency indicates this is not the
smoking gun that would allow us to confidently project what might
happen as oceans warm in the future."
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