Jennifer Akst is a graduate student at the University of Indiana. She contacted me a few weeks ago about a class project in which she had to write a new story. She chose to focus on the recent Elsner et al. Nature paper that claims to have documented an increasing intensity of the very strongest storms. The paper, like so much in this area, has been subject to some considerable debate in the community. Jennifer’s class project news story was the best piece of reporting on this paper that I saw, and so I asked if I could reproduce it here. See it after the jump.

Seas Are Getting Warmer, Storms Are Getting Stronger: New Evidence that Global Warming May be Fueling More Costly Hurricanes

The strongest hurricanes may be getting even stronger, a recent study found. Paralleling increases in ocean temperatures, wind speeds of extreme tropical cyclones appear to be rising since the early 1980s.

Tropical cyclones plague the American coasts each year and are the costliest natural catastrophes in the U.S. With Hurricane Gustav resulting in the largest evacuation in Louisiana’s history and Hurricane Ike costing an estimated $30 billion in damage, this storm season once again begs the question: are tropical storms growing in intensity?

“We showed that over the last 25 years or so that, globally, the strongest tropical cyclones are getting stronger,” said Dr. James Elsner of Florida State University, lead author of the current study, published in the September 4 edition of Nature. Further, these upward trends were still apparent when the data were reanalyzed using surface sea temperature (SST) instead of year.

Theoretical work has long predicted increases in hurricane intensity with the increases in ocean temperatures. According to the heat-engine theory of cyclone intensity, the warmer SST provide more energy to fuel the winds of the storm.

“We’d expect that as the seas continue to warm that we would continue to see an increase in the strength of the strongest storms,” Elsner explains.

At first glance, the data appeared to support the theory when a 2005 study showed that tropical cyclone threat increased from the mid-1970s to present and was highly correlated with SST.

Three weeks later, Hurricane Katrina devastated the Gulf coasts of Mississippi, Louisiana, Alabama, and Florida, and quickly became the costliest natural disaster in U.S. history. The theory’s predictions had come to fruition, and the evidence seemed impossible to ignore.

However, this early study was criticized due to inconsistencies in the global hurricane records that it analyzed. Two years later, one of the authors of the present study, Dr. James Kossin of University of Wisconsin-Madison, published another collaborative study that suggested the observed trends may inflated or may not exist at all.

“We took a consistent record of satellite imagery and we estimated hurricane intensity from that imagery,” explained Kossin. “What that did is remove some of the possible problems that the existing data sets had.” With this new data set, Kossin found no global changes in average hurricane intensity.

In the new study investigating changes in hurricane intensity, researchers examined differences between storms of varying strength, rather than look at the average intensity across all storms.

The heat-engine theory of cyclone dynamics is a way of thinking about hurricanes “in an idealized setting,” Elsner explains. Because the strongest storms are on average closest to their idealized potential, those are the storms in which the increases in intensity predicted by theory are most likely to be visible and not masked by environmental influences.

Thus, the increases in intensity of only the strongest storms found in this study “ are consistent with the simple heat-engine theory of tropical cyclone intensity,” said Elsner.

Some experts are still skeptical, however. When the data were split geographically, only the Atlantic and Indian oceans showed consistent significant increases in wind speed, and “in the Indian Ocean, the data set they’re using is problematic,” Dr. Chris Landsea of the National Hurricane Center warns.
“Prior to 1997,” Kossin explains, “there was no satellite that was in that area, so the satellites that were off to the side were looking somewhat obliquely at these storms in the Indian Ocean. That affects the actual measurements of that temperature and biases them.”

The researchers attempted to correct for this bias by uniformly adjusting the data collected before 1997, but Landsea worries that this may artificially inflate the numbers in the Indian Ocean. “At best what we have is anecdotal evidence that that’s the right correction to make,” Kossin admits.

Additionally, the Atlantic basin shows cyclical variations in tropical storm intensities on a multidecadal time scale. Because the current study only investigates the storms of the last 26 years, this paper cannot address how this current busy period in the Atlantic compares to previous busy periods, Landsea explains. “Are we seeing a trend, or is this more of a cycle?”

Part of the trend may be caused by global warming while part of the trend may be the result of some natural variability, Kossin agrees. “We can’t really separate those two things. Certainly not with this study.”

Finally, Elsner warns that while increasing intensity of extreme tropical cyclones mirror the increasing ocean temperatures, this does not prove that global warming is causing this increase.

Consequently, this study is by no means an answer to the question of whether global warming is affecting hurricane intensity. “There’s no smoking gun here,” Kossin admits.

“This is a debate that I have every expectation will continue for decades,” said Dr. Roger Pielke of the Center for Science and Technology Policy Research in Boulder, CO. For now, he warns against conflating the scientific debate with the politic debate: “Let’s let science take its course, and in the meanwhile, let’s take all of those steps that make a lot of good sense regardless of how the science eventually turns out.”