Link between hurricanes and global warming questioned

Link between hurricanes and global warming questioned

Study questions linkage between severe hurricanes and global warming

New research calls into question the linkage between major Atlantic hurricanes and global warming. That is one of the conclusions from a University of Virginia study to appear in the May 10, 2006 issue of the journal Geophysical Research Letters.

In recent years, a large number of severe Atlantic hurricanes have fueled a debate as to whether global warming is responsible. Because high sea-surface temperatures fuel tropical cyclones, this linkage seems logical. In fact, within the past year, several hurricane researchers have correlated basin-wide warming trends with increasing hurricane severity and have implicated a greenhouse-warming cause.

But unlike these prior studies, the U.Va. climatologists specifically examined water temperatures along the path of each storm, providing a more precise picture of the tropical environment involved in each hurricane’s development. They found that increasing water temperatures can account for only about half of the increase in strong hurricanes over the past 25 years; therefore the remaining storminess increase must be related to other factors.

“It is too simplistic to only implicate sea surface temperatures in the dramatic increase in the number of major hurricanes,” said lead author Patrick Michaels, U.Va. professor of environmental sciences and director of the Virginia Climatology Office.

For a storm to reach the status of a major hurricane, a very specific set of atmospheric conditions must be met within the region of the storm’s development, and only one of these factors is sufficiently high sea-surface temperatures. The authors found that the ultimate strength of a hurricane is not directly linked to the underlying water temperatures. Instead, they found that a temperature threshold, 89?F, must be crossed before a weak tropical cyclone has the potential to become a monster hurricane. Once the threshold is crossed, water temperature is no longer an important factor. “At that point, other factors take over, such as the vertical wind profile, and atmospheric temperature and moisture gradients,” Michaels said.

Global warming causing stronger hurricanes confirms study The link between warmer ocean temperatures and increasing intensity of hurricanes has been confirmed by scientists at the Georgia Institute of Technology. Last year, two studies published in the journals Nature and Science found a strong correlation between rising tropical sea surface temperatures and an increase in the strength of hurricanes. La Nina will not affect 2006 Atlantic hurricanes NASA oceanographers agree that the recent La Nina in the eastern Pacific Ocean is not expected to have an effect on the Atlantic hurricane season this year. That’s good news, because normally a La Nina tends to increase Atlantic hurricane activity and decrease Pacific Ocean hurricanes. Birthplace of hurricanes heating up say NOAA scientists The region of the tropical Atlantic where many hurricanes originate has warmed by several tenths of a degree Celsius over the 20th century, and new climate model simulations suggest that human activity, such as increasing greenhouse gas emissions into the atmosphere, may contribute significantly to this warming. This new finding is one of several conclusions reported in a study by scientists at the NOAA Geophysical Fluid Dynamics Laboratory in Princeton, N.J., published today in the Journal of Climate. 2005 Atlantic hurricane season worst on record The 2005 Atlantic hurricane season is the busiest on record and extends the active hurricane cycle that began in 1995 — a trend likely to continue for years to come. The season included 26 named storms, including 13 hurricanes in which seven were major (Category 3 or higher). Number of category 4 and 5 hurricanes has doubled over 35 years: The number of Category 4 and 5 hurricanes worldwide has nearly doubled over the past 35 years, even though the total number of hurricanes has dropped since the 1990s, according to a study by researchers at the Georgia Institute of Technology and the National Center for Atmospheric Research (NCAR). The shift occurred as global sea surface temperatures have increased over the same period. The research appears in the September 16 issue of Science.

While there has been extensive recent discussion about whether or not human-induced global warming is currently playing a role in the increased frequency and intensity of Atlantic hurricanes, Michaels downplays this impact, at least for the current climate.

“The projected impacts of global warming on Atlantic hurricanes are minor compared with the major changes that we have observed over the past couple of years,” Michaels said.

He points instead to naturally varying components of the tropical environment as being the primary reason for the recent enhanced activity.

“Some aspects of the tropical environment have evolved much differently than they were expected to under the assumption that only increasing greenhouse gases were involved. This leads me to believe that natural oscillations have also been responsible for what we have seen,” Michaels said.

But what if sea-surface temperatures continue to rise into the future, if the world continues to warm from an enhancing greenhouse effect?

“In the future we may expect to see more major hurricanes,” Michaels said, “but we don’t expect the ones that do form to be any stronger than the ones that we have seen in the past.”

Michaels’ co-authors are Robert E. Davis, associate professor of environmental sciences and Paul C. Knappenberger, former U.Va. graduate student in environmental sciences.

Reference: Michaels, P. J., P. C. Knappenberger, and R. E. Davis, 2006. Sea-surface temperatures and tropical cyclones in the Atlantic basin. Geophysical Research Letters, 33, doi:10.1029/2006GL025757.

This article is a modified news release from the University of Virginia .

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