for National Geographic News
Following the devastation caused by Hurricane Katrina, climate scientists began a heated debate: Was last year's superstrong hurricane season a result of global warming?
Several experts say no, pointing to a natural long-term fluctuation in sea-surface temperatures in the Atlantic Ocean called the Atlantic multidecadal oscillation (AMO).
Few scientists doubt that human-induced global warming is occurring. But some maintain that a natural cycle played a larger role in creating last year's bumper crop of storms.
Now a new study argues that global warming is probably the larger of the two factors.
Kevin Trenberth and Dennis Shea of the National Center for Atmospheric Research, in Boulder, Colorado, tried to untangle the competing factors by looking beyond the Atlantic records. Their research appears in this month's issue of the journal Geophysical Research Letters.
They collected 130 years of temperature records for other tropical and mid-latitude waters, which they used to determine what portion of the changes in the Atlantic were due to global temperature shifts and which were because of regional factors, such the natural sea-temperature cycle.
Warming Is the Major Player
Sea-surface temperatures are widely believed to play a major role in hurricane formation and growth by fueling the storms with extra heat and moisture (interactive feature: how hurricanes form).
The 2005 hurricane season saw record sea-surface temperatures. The thermometer readings were 1.6°F (0.9°C) higher than the average between 1901 and 1970 across the hurricane-generating belt of the tropical Atlantic.
Last year also saw a record 28 named storms, 4 of whichEmily, Katrina, Rita, and Wilmareached Category Five hurricane strength. Such storms are the strongest on Earth, with winds exceeding 155 miles (249 kilometers) an hour.
Many experts believe that warm sea-surface temperatures from the AMO enhanced hurricane activity in the 1930s and '40s. When the cycle was at its minimum in the 1970s and '80s, hurricane activity was much lower.
This trend has led some climate scientists to speculate that the recent spate of hurricanes is simply a result of another AMO upswing.
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