Hot air: A September 2006 satellite image shows Hurricane Gordon churning west of the Azores in the Atlantic Ocean. Studies suggest that Hurricanes such as this may be increasing in both frequency and ferocity.
Credit: NOAA
WASHINGTON DC: Global warming's effect on wind patterns and sea temperatures have more than doubled the annual number of Atlantic hurricanes over the past century.
An analysis published this week in British journal The Philosophical Transactions of the Royal Society identifies three periods since 1900, during which the average number of hurricanes and tropical storms surged dramatically and then remained elevated and relatively steady.
The first period, between 1900 and 1930, saw an average of six Atlantic tropical cyclones, 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 most recent period, from 1995 to 2005, the average reached 15, of which eight were hurricanes and seven were tropical storms.
Increasing hurricane activity
The data was compiled by Greg Holland of the U.S. National Centre for Atmospheric Research and Peter Webster of Georgia Institute of Technology in Atlanta. This latter period, they caution, has not yet stabilised, which means the average hurricane season may get more active in the future.
"These numbers are a strong indication that climate change is a major factor in the increasing number of Atlantic hurricanes," said Holland.
The scientists see a strong correlation between the spike in storm activity and rising sea surface temperatures, which "feed" hurricanes. Over the last 100 years, these temperatures have risen by about 0.7 °C, the study asserts.
The temperatures rose approximately 0.4 °C, in a period leading up to 1930, which was marked by some of the deadliest storms to hit the Atlantic Coast of the United States.
The so-called Galveston hurricane of 1900 that went down in history as the worst ever to hit the United States, took an estimated 8,000 lives. The second-worst storm that ravaged southeastern Florida and killed about 2,500 local residents occurred in 1928, according to data from the U.S. National Hurricane Centre.
Another hurricane that hit the Miami area in 1926 caused 372 deaths, a 1919 storm in the Florida Keys was responsible for 287 fatalities, and an unnamed category four hurricane that befell New Orleans in 1915 took 275 lives.
A similar water temperature rise, according to the study, was recorded leading up to 1995.
The second-costliest U.S. Atlantic hurricane, Andrew, hit Florida and Louisiana in 1992, causing US$ 26.5 billion in damage. Katrina, the costliest and third-deadliest US Atlantic storm, which killed about 1,500 people and caused 81 billion dollars in damage, ravaged the states of Louisiana and Mississippi in 2005.
Not better observation
Critics of the global warming theory have often held that data showing increased hurricane activity account for better observation rather than changing global weather patterns.
But Holland and Webster refuted these assertions, arguing that the distinct transitions in hurricane activity noted by the study occurred around 1930 and 1995, while aerial and satellite observation of hurricanes stated respectively in 1944 and in 1970.
"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 write.
The 2006 hurricane season, the study notes, was far less active than the two preceding years, in part because of the emergence of an El Nino weather phenomenon in the Pacific Ocean.
However, "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," Holland said.
