Tropical storms stepping up with climate change

New Scientist news service: Climate change has triggered three major shifts in the number of tropical storms that rise up in the North Atlantic, according to a new analysis of 20th century records. The first change came in 1905, starting a 25-year period with an average of 6.0 tropical storms or hurricanes per year. In 1931, the number jumped to 9.4 per year, and stayed at that level until 1994. The last big shift came in 1995, starting a period through to 2005 with an average of 14.8 storms per year.

With only nine storms, 2006 was comparatively quiet, but Greg Holland of the US National Center for Atmospheric Research in Boulder, Colorado, says it would have been an average year just a couple of decades ago.

The series of jumps is clear in the data, says Holland, with almost no long-term variation from 1931 to 1994, then a rapid increase. In his study, he reports that rises in sea-surface temperatures occur at the same time as storm frequencies shift. He concludes that global warming is behind most of both changes.

Some earlier studies had found that storm counts rose and fell periodically, with 30 to 40 quiet years followed by 30 to 40 busy years, but no long-term change. "But that was always a very, very weak case," says Kerry Emanuel of the Massachusetts Institute of Technology.

Holland does see a small periodic variation in North Atlantic sea-surface temperature, but says it is much smaller than the long-term increase caused by greenhouse warming. The proportion of storms that go on to become major hurricanes does vary cyclically, and has shown no marked trend over the past century. The bad news, says Holland, is that this cycle is now driving an increase in the proportion of major hurricanes at the same time as the numbers of storms is rising due to global warming.

With the storm frequency climbing above those in historical records dating back more than 150 years, he warns "we are moving into territory that we don't understand." Concern about the problem is spreading. Bills have been introduced in the US House and Senate to create a National Hurricane Research Initiative, as recommended in a recent National Science Board report (pdf).

What the peak part of the 2007 season, from mid-August to mid-October, will bring remains uncertain. The mild El Niño that dampened the formation of North Atlantic hurricanes in 2006 has faded away, but sea-surface temperatures remain below the peak that contributed to the onslaught of storms in 2004 and 2005.