NewsJune 17, 2011

Increase in Hurricane Numbers Due to Better Detection, Not Climate Change, Study Says

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Alyson Kenward

By Alyson Kenward

Follow @alysonkenward

The recent forecast for an above average 2011 Atlantic hurricane season comes at an anxious time for many Americans, already storm-weary from a year in which several record tornadoes, droughts, wildfires, and floods have already caused billions in damage. The possibility that global warming may be leading to more and stronger hurricanes has been a hot research topic during the past several years, ever since the devastating Atlantic hurricane seasons of 2004 and 2005.

Yet while dozens of research groups are trying to better understand how climate change is influencing Atlantic tropical storms, it’s been difficult for them to pinpoint how hurricane frequency and intensity have been changing during the last century, as average ocean and air temperatures have risen. Now, a new study suggests that an increase in the number of short-lived Atlantic hurricanes, starting around the mid-1940s, is largely due to the onset of better detection methods that developed around the same time, rather than a warming climate.

However, this does not mean that climate change is off the hook when it comes to influencing Atlantic hurricanes.

A crew member of an Air Force Reserve WC-130 “Hurricane Hunter” aircraft inspects the plane prior to a surveillance mission out of St. Croix, Virgin Islands, on Sept. 16, 2010. Credit: Department of Defense/U.S. Air Force Staff Sgt. Manuel J. Martinez.

Several years ago, researchers discovered that the number of North Atlantic hurricanes had increased during the last century. More recently, scientists found that beginning in the 1940s and 1950s, there appeared to be more short-lived and weaker hurricanes popping up in records of Atlantic Ocean hurricane activity, and that these types of storms — which maintain hurricane strength for just two days or less — accounted for most of the overall increase in storms.

“Suspiciously enough, that increase in the 1940s and 1950s was occurring at the same time that there was an increase in the observational systems out there that could detect smaller hurricanes,” says environmental engineer Gabriele Villarini from Princeton University. “It seems like it was our improved capability of detecting these short-duration storms that has driven what looked to be an overall increase in tropical storms.”

In the new study, published this week in the Journal of Geophysical Research, Villarini, along with colleagues from Princeton and the National Oceanic and Atmospheric Administration’s (NOAA) Geophysical Fluid Dynamics Laboratory, found that improvements in hurricane observation technology in the middle of the last century probably inflated the statistics.

How is that in the 1940s we all of a sudden got much better at detecting hurricanes? During and after World War II, Villarini explains, there were a lot more planes flying over the Atlantic Ocean. With more airplanes in the sky, there was a greater likelihood that one would come across a small or short-lived hurricane. In the 1950s, planes began flying into the Atlantic, deliberately in search of hurricanes, and these “Hurricane Hunter” flights continue today. 

Moreover, once satellites became a common feature in our skies, starting in the 1950s, they offered a new view of hurricanes from above.

A view of Hurricane Earl from the International Space Station. Credit: NASA.

While Villarini says the new analysis shows that airplane and satellite detection increased the number of short-lived (less than two-day) hurricanes that were recorded, it does not rule out the possibility that there has also been some influence on hurricane numbers from the warming climate. “A climate signal could be masked by the change in observational technology,” he cautions.

Nor does the new study address the concern over whether climate changes — in particular, rising Atlantic sea surface temperatures — have boosted the intensity of the average hurricane.

MIT atmospheric scientist Kerry Emanuel warns that simply counting up the number of Atlantic hurricanes won’t capture changes in hurricane strength or movement.

Nor does looking at short-lived hurricanes get to the heart of what worries people the most: how much damage might future hurricanes inflict?

“Most of the short-lived storms are weak,” says Emanuel, “and when it comes to societal impacts, these weak storms don’t really matter at all.”

While Category 1 and 2 hurricanes constitute about 80 percent of Atlantic hurricanes, they account for only about 15 percent of the damage, he explains.

“Most of the damage is done by Category 3, 4 and 5 storms,” says Emanuel, “so what we’re really worried about is the frequency of these intense events.”

For example, there were 12 Atlantic hurricanes in 2010, but none made landfall in the U.S.. On the other hand, 2008 saw fewer hurricanes, but billions of dollars in damages, with Hurricanes Gustav, Ike, and Hannah all striking the U.S..

An Air Force Reserve pararescueman from the 920th Rescue Wing scans the ravaged Texas landscape for signs of life in the aftermath of Hurricane Ike. Credit: U.S. Air Force photo/Tech. Sgt. Paul Flipse.

Emanuel and his colleagues have investigated how average hurricane intensity has been increasing in recent years, and he says there is a clear climate connection.

“There is a truly remarkable correlation between the power dissipation, or the destructive potential of a storm, and the temperature of the Atlantic Ocean during hurricane season,” says Emanuel.

While there is still little consensus among researchers regarding whether future ocean warming will cause the number of Atlantic hurricanes to increase or decrease, several studies have found that on average, hurricanes around the planet could become up to 10 percent more powerful over the next century. 

This year, the National Oceanic and Atmospheric Administration’s Climate Prediction Center predicts there will be between 12 and 16 named storms in the Atlantic Ocean, with as many as six to 10 developing into hurricanes.