Hurricane theory comes under fire
For scientist, theory hurricanes will slow doesn't hold water
One researcher breaks from his peers with his idea that storm activity isn't cyclical
Published 5:30 am, Saturday, April 8, 2006
As an increasing cadence of storms with names such as Opal, Mitch, Allison, Ivan and Katrina have battered the Americas during the past decade, hurricane scientists have patiently explained that this was to be expected.
The north Atlantic Ocean, they say, goes through decades-long warming and cooling cycles when it spawns more and fewer hurricanes. The current "active" period for hurricanes began in 1995.
Scientists have dubbed this natural cycle the "Atlantic Multidecadal Oscillation," or AMO, and since ocean temperatures have been on the upswing for the past decade, so too has hurricane activity. The seas should settle down again in a decade or two, scientists say.
But what if they're wrong?
One of the world's most prominent hurricane researchers, the Massachusetts Institute of Technology's Kerry Emanuel, has proposed a stunning theory that suggests the AMO has a minor effect on hurricanes — and may not even exist at all.
"I'm worried about what the data is telling us," Emanuel said. "It's a pessimistic message, but barring a major (El Niño event), and barring a big volcanic eruption, there just isn't an objective basis for expecting a downturn in Atlantic hurricane activity."
During large El Niño events, when the Pacific Ocean warms, hurricane activity in the Atlantic generally diminishes. Volcanoes spew ash that reduces sunlight, lowering temperatures and likely also diminishing hurricane activity.
The recent upswing in storms is largely because of human-induced climate change, Emanuel contends. The unrelated, noncyclical events that reduce hurricane activity, including volcanic eruptions, have masked a progressive rise in sea temperatures because of global warming, he said.
Emanuel and a handful of other scientists previously have courted controversy with articles suggesting that climate change already is making hurricanes more forceful, with measurably stronger winds.
Global warming debateThis earlier work, especially after the 2004 and 2005 hurricane seasons, has ignited a scientific debate about the effect of a warming world on hurricane activity. If there's a general consensus, however, it's that climate change is likely to have some yet unknown effect on tropical storms, and that such an effect would be too small to measure yet.
A majority of scientists think if climate change is affecting hurricanes around the globe, its effect on the North Atlantic basin is minor compared with the influence of the AMO. Indeed, near the end of the record 2005 season, the National Oceanic and Atmospheric Administration released this statement:
"NOAA research shows that the tropical multi-decadal signal is causing the increased Atlantic hurricane activity since 1995, and is not related to greenhouse warming."
Emanuel first presented his new theory late last month at a hurricanes and climate change conference at Columbia University.
Among those in attendance was Phil Klotzbach, a hurricane researcher at Colorado State University who now does much of the research for Bill Gray's seasonal hurricane predictions, the oldest and best-known annual forecast. Klotzbach said he remains unconvinced.
"Dr. Gray and I believe that the AMO plays a large role in North Atlantic sea surface temperatures," he said.
Klotzbach said the data indicates a large increase in tropical Atlantic sea surface temperatures in 1994 and 1995. This sudden spike, he said, does not fit well with a greenhouse warming theory that would likely exhibit a smoother trend.
Other climate scientists who viewed Emanuel's talk admitted, however, that uncertainty exists within the AMO theory. Most agree it's difficult to determine exactly what is driving late summer temperatures in the tropical Atlantic — be it natural climate variability, greenhouse gases or some other factor, such as volcanic eruptions.
Arriving at solid answers is difficult because of limited data. Although reliable sea surface temperature records exist back to the mid-1850s, hurricane records are less reliable. It is difficult for scientists to compare modern accounts of storm frequency and, more importantly, wind speeds with historical records because of vastly more measurements today from satellites, airplanes and sophisticated equipment in the seas.
A modern hurricane's windspeed is measured dozens if not hundreds of times before landfall. The great Galveston storm of 1900 has just one such measurement.
The AMO theory holds that an active period ended in 1900, followed by relative calm until 1930, high activity until about 1970, and calm again until the recent period of higher-than-normal storm activity began in 1995. The oscillation closely tracks the ups and downs in Atlantic sea temperatures.
About a decade ago, a hurricane researcher named Chris Landsea re-examined wind and pressure data for hurricanes from the last active period, between the 1930s and 1960s. He found that the intensity of storms from the era may have been slightly overestimated, and suggested a "bias correction."
Looking for an explanationEmanuel has seized upon Landsea's correction and inserted it into his measure of a storm's intensity called the "power dissipation index," which estimates the overall destructive potential of a storm.
Emanuel's calculations show a rise in activity to about 1950, followed by a decline to about 1980, then a rapid increase. This does not conform to any AMO, he says. Rather, it follows the entire northern hemispheric temperature record, which shows a similar pattern.
The cooling between 1950 and 1980, he and some other climate scientists say, is a consequence of sulfate aerosols, which blacken the atmosphere and block some sun rays from reaching the Earth's surface.
Landsea, now a meteorologist at the National Hurricane Center, and Emanuel have sparred about whether the bias correction should be employed in current theories without verification.
Although he says the AMO doesn't explain all of the cyclical nature of hurricanes in the Atlantic, Landsea still thinks the Americas will continue to experience active and inactive periods of storminess.
"I thought his analysis was interesting, but I think it's far from definitive," Landsea said of Emanuel's presentation.
Landsea is leading a collaborative work among hurricane scientists to reanalyze the storm record and try to reach consensus about the data.
For example, the study led to raising 1992's Hurricane Andrew to Category 5 status a decade after it struck land. With better data, Landsea said, perhaps scientists can achieve a final answer on the role of the AMO and hurricanes.
One problem with the AMO idea is that despite the apparent relationship between oscillating sea temperatures, atmospheric changes and hurricane activity, scientists have yet to find an explanation in nature for why such a cycle would exist. There's some thought that it may be because of large-scale circulations of water deep in the oceans, but no one's sure.
For this reason, Florida State University hurricane specialist James Elsner said he finds Emanuel's theory plausible.
But for now, most leading hurricane scientists will continue to adhere to the idea that at some point, storm activity will quiet down again.