Climate Change Linked to Doubling of Atlantic Hurricanes

BOULDER, Colorado, July 30, 2007 (ENS) - Twice as many Atlantic hurricanes formed each year from 1995 to 2005, on average, than formed during parallel years a century ago finds a new statistical analysis of hurricanes and tropical storms in the north Atlantic. The researchers conclude that warmer sea surface temperatures and altered wind patterns associated with global climate change are responsible for the increase.

Hurricane hunters flew into and around the eye of Hurricane Katrina. August 28, 2005. (Photo courtesy NOAA)
The study, by Greg Holland of the National Center for Atmospheric Research, NCAR, and Peter Webster of Georgia Institute of Technology, is published online today by the Royal Society of London.

"These numbers are a strong indication that climate change is a major factor in the increasing number of Atlantic hurricanes," says Holland.

For the 2007 Atlantic hurricane season, NOAA scientists predict 13 to 17 named storms, with seven to 10 becoming hurricanes, of which three to five could become major hurricanes of Category 3 strength or higher. An average Atlantic hurricane season brings 11 named storms, with six becoming hurricanes, including two major hurricanes.

The analysis by Holland and Webster identifies three periods since 1900, separated by sharp transitions, during which the average number of hurricanes and tropical storms increased and then remained elevated and relatively steady.

The first period, between 1900 and 1930, saw an average of six Atlantic tropical cyclones each year, of which four were hurricanes and two were tropical storms.

From 1930 to 1940, the annual average increased to 10, consisting of five hurricanes and five tropical storms.

In the final study period, from 1995 to 2005, the average reached 15, of which eight were hurricanes and seven were tropical storms.

This last period has not yet stabilized, which means that the average hurricane season may be even more active in the future.

Holland and Webster say it is not possible at this time to predict the level at which the frequency and intensity of storms will stabilize.

The increases over the last century correlate closely with sea surface temperatures, which have risen by about 1.3 degrees Fahrenheit in the last 100 years, say Holland and Webster.

Housingin Pensacola Beach, Florida damaged by Hurricane Dennis. July 16, 2005. (Photo by Jocelyn Augustino courtesy FEMA)
The changes in sea surface temperatures took place in the years before to the sharp increases in storm frequency, with an sea surface temperature rise of 0.7 degrees Fahrenheit leading up to 1930 and a similar rise leading up to 1995 and continuing even after.

The authors note that other studies indicate that most of the rise in Atlantic sea surface temperatures can be attributed to global warming.

The unusually active hurricane seasons of 2004 and 2005 have prompted considerable research into the question of whether more intense tropical cyclones are correlated with natural cycles, global warming, or some other cause.

According to the 2007 assessment report of the Intergovernmental Panel on Climate Change, IPCC, on a global scale, "There is no clear trend in the annual numbers of tropical cyclones."

But considering the North Atlantic apart from the global pattern give a different picture.

The IPCC wrote, "the frequency of tropical storms has increased dramatically in the North Atlantic. Reasons for this increase are currently subject to intense debate among climate scientists."

"At least two recent peer-reviewed scientific studies indicate a significant statistical link between the increased frequency and global warming," the IPCC said earlier this year, "but research to identify a mechanism explaining this link is ongoing."

"All signs that I've seen show that it's related to natural variability," said Eric Blake, a meteorologist at the National Hurricane Center, whose view is supported by some scientists. "There could be some impact of global warming, but its role is probably a secondary or tertiary role."

In May, the Woods Hole Oceanographic Institution published research showing that the frequency of intense hurricanes in the Atlantic Ocean appears to be closely connected to long-term trends in the El Niņo/Southern Oscillation and the West African monsoon. Geologists Jeff Donnelly and Jonathan Woodruff made that discovery while assembling the longest-ever record of hurricane strikes in the Atlantic basin.

The Holland and Webster study indicates that natural cycles are probably not the entire cause of the increase in hurricane numbers because the increase has happened across the last century rather than oscillating in tandem with a natural cycle.

Better weather observations in recent decades cannot account for all of the increase either, according to Holland and Webster.

Hurricane Mitch killed 11,000 people and left 8,000 missing in Central America. October 26, 1998. (Photo courtesy NOAA)
To observe storms in the Atlantic more systematically, meteorologists began relying on data from aircraft flights in 1944 and satellites about 1970.

The distinct transitions in hurricane activity documented by Holland and Webster occurred around both 1930 and 1995.

"We are of the strong and considered opinion that data errors alone cannot explain the sharp, high-amplitude transitions between the climatic regimes, each with an increase of around 50 percent in cyclone and hurricane numbers, and their close relationship with sea surface temperatures," the authors state.

Holland says, "Even a quiet year by today's standards would be considered normal or slightly active compared to an average year in the early part of the 20th century."

When it comes to intensity of hurricanes, the IPCC says there has been an increase in intensity and it is "more likely than not" that there is a human contribution to the observed trend of hurricane intensification since the 1970s."

Future hurricanes will be even more intense, the IPCC predicted earlier this year, saying, "It is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation" associated with ongoing increases of tropical sea surface temperatures.

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