Scientists say Atlantic hurricanes may be getting weaker but critics disagree

http://www.theguardian.com/world/2015/sep/07/atlantic-hurricanes-getting-weaker-scientists-disagree

Version 0 of 1.

The busy Atlantic hurricane seasons of recent decades may be coming to an end, according to a new and controversial study.

Writing in the journal Nature Geoscience on Monday, two prominent hurricane researchers say the Atlantic looks like it is entering into a new quieter cycle of storm activity, as it did in the 1970s and 1980s.

Scientists at Colorado State University, including the professor who pioneered hurricane seasonal prognostication, say they are seeing a localised cooling and salinity level drop in the North Atlantic, near Greenland. Such conditions, they theorise, change local weather and ocean patterns and form an on-again, off-again cycle in hurricane activity that can be traced back to the late 1800s.

Warmer and saltier water produces periods of more and stronger storms followed by cooler less salty water triggering periods of fewer and weaker hurricanes, the scientists say. The periods last about 25 years, sometimes more, sometimes less.

The busy cycle that just ended was one of the shorter ones, perhaps because it was so strong that it ran out of energy, according to Phil Klotzbach, the lead author of the new study.

Klotzbach said that since about 2012, there has been more localised cooling in the key area and less salt, suggesting a new, quieter period. But he said it was too soon to be certain that one has begun.

“We’re just asking the question,” he said.

Related: Heatwaves but El Niño reins in the hurricanes

Klotzbach said he thinks the answer is yes. He said the busy cycle started around 1995 and probably ended in 2012. In 2005 alone, hurricanes Katrina, Rita and Wilma killed more than 1,500 people and caused billions of dollars of damage.

The quiet cycle before that went from about 1970 to 1994; before that it was busy from 1926 until 1969, he said.

Klotzbach does not take into account where a storm hits; rather how strong storms are and how long they last, regardless of whether they make landfall. So even though no major hurricane hit the US in 2010, overall activity was more than 60% higher than normal.

Klotzbach also said cities can still be devastated in quiet seasons. Hurricane Andrew hit south Florida in an otherwise quiet 1992 season, as a top-of-the-scale storm.

Other scientists either reject the study outright or call it premature.

“I think they’re pretty much wrong about this,” said MIT meteorology professor Kerry Emanuel, who also specialises in hurricane research. “That paper is not backed by a lot of evidence.”

Emanuel does not believe in the cycle cited by the researchers or the connection to ocean temperature and salinity. He thinks the quiet period of hurricanes of the 1970s and 1980s was connected to sulphur pollution and the busy period that followed was a result of the cleaning of the air.

Jim Kossin of the National Oceanic and Atmospheric Administration (NOAA) said cooler water temperatures earlier this year might be due to Atlantic dust, and August temperatures in the area have risen.

Another NOAA scientist, Gabriel Vecchi, said while there seemed to be signs of a change in the circulation of the Atlantic, it was far too early to say that the shift had happened.

“So what happens in the next few years is going to be very exciting to watch as it may help settle or at least refine some intense scientific debates,” Vecchi said.