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NATURE
Published online: 6 June 2007; | doi:10.1038/news070604-7

Storm seasons back to normal?
Hurricane activity today looks much the same as the long-term average.

Quirin Schiermeier

The increased hurricane activity over the
tropical North Atlantic during the past 12 years
may have less to do with climate change than is
often assumed. The series of stormy seasons
experienced since 1995 could just be a return to
normal conditions, following a long unusually
calm period throughout the 1970s and 1980s, a new
study suggests.

As to how climate change will affect hurricanes
in the future, this will depend in part on what
happens with the wind, the researchers point out
– and this is largely unpredictable.

The series of powerful hurricanes that hit the
Caribbean and the US Gulf coast over the past few
years, including 2005’s devastating Katrina and
Rita, has fuelled speculation as to whether
global warming might be the culprit. Researchers
think that rising sea temperatures have a direct
impact on the strength of storms, increasing
their destructiveness1. But do hurricanes also
become more frequent in a warming world?

Reconstructing long-term hurricane trends is
notoriously difficult because no reliable
instrumental observations exist prior to around
1950. Johan Nyberg, a marine geologist at the
Geological Survey of Sweden in Uppsala, and his
team have now calculated a unique record of
strong Atlantic hurricanes over the past 270
years.

Stormy weather

Hurricane activity, they say, depends both on sea
temperature and wind ‘shear’ – when winds at
different altitudes blow in different directions
shear is high, and this tends to inhibit storm
formation.

To get a handle on wind, they looked at the
density of old corals. This depends upon
precipitation at the time of growth, which in
turn depends largely on trade-wind speed. The
trade winds in this part of the world blow in the
opposite direction to winds at very high
altitudes, so the stronger the trade winds, the
stronger the shear.

From their calculated wind-shear data, along with
sea-temperature records, they estimated hurricane
activity over history. Periods of significantly
enhanced hurricane frequency – more than four
major storms on average per year – didn’t
exclusively occur during exceptionally warm
climatic conditions, they report in Nature2. Six
such high-activity periods appear in the
270-year-long record, all at times when sea and
air temperatures were notably lower than today.

It’s hard to know what the future holds. The
ocean is getting warmer, but no one knows what
will happen with the winds. “Wind shear is high
at the moment,” says Nyberg. That could possibly
be a result of climate change, he notes, because
the atmosphere warms more quickly than the ocean,
and that can affect wind. But this may not be the
main driver of wind shear. “It’s hard to predict
the future value. If it were to decrease,
hurricane activity could remain high,” he says.

What the study can’t help determine, however, is
how factors such as climate affect the path that
storms typically take. It could be that this
study missed some storms that happened over the
past 270 years because it was looking in the
wrong place. More data from corals and marine
sediments are needed to reconstruct storm tracks,
says Nyberg.

References

    1. Emanuel K. Nature, 436 . 686 – 688 (2005).
| Article | PubMed | ISI | ChemPort |
    2. Nyberg J., et al. Nature, 447 . 698 – 701 (2007). | Article |

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