Hurricanes: Back to normal?
The increase in the number of major Atlantic hurricanes since 1995 could be a recovery to normal activity, rather than a rise to unusually high levels. According to the study, which uses proxy records of vertical wind shear and sea surface temperature to reconstruct the number of major Atlantic hurricanes over the past 270 years, the storm frequency was anomalously low during the 1970s and 1980s.
The frequency of major hurricanes over the Atlantic Ocean has risen significantly since 1995, but it remains unclear whether this change is due to global warming or natural variability. One way to address this question is to consider changes in hurricane activity in the past, but reliable observations only cover the past few decades.
Johan Nyberg and colleagues used proxy records from corals and a marine sediment core that reflect changes in the two main parameters that influence hurricane activity — vertical wind shear and sea surface temperature — to reconstruct the frequency of major hurricanes over the Atlantic since 1730. The records show that the lull in hurricane activity from the late 1960s to early 1990s is an exception, and that the current active period from 1995 onwards reflects a return to ‘normal’ activity, rather than a direct response to increasing sea surface temperature. The records indicate that variations in vertical wind shear may have been responsible for the changes in hurricane frequency, although the underlying causes of changes in this parameter remain uncertain. The authors suggest that it is therefore crucial to understand future changes in the magnitude of vertical wind shear, because they may have a significant influence on hurricane activity.

CONTACT
Johan Nyberg (Geological Survey of Sweden, Uppsala, Sweden)
Tel: +46 18 17 91 94; E-mail: johan.nyberg@sgu.se

James B. Elsner (Florida State University, Tallahassee, FL, USA) N&V author
Tel: +1 850 877 4039; E-mail: jelsner@fsu.edu