Revisiting the global warming-hurricane link

A new study underscores the difficulty of estimating global warming's effect on weather.

Since the devastating 2005 Atlantic hurricane season, scientists have been briskly testing the notion that global warming's fingerprints have already appeared on tropical-cyclone activity worldwide.

Now, a new analysis from the scientist who helped trigger that flurry of studies suggests that the answer to questions about global warming's impact on current tropical cyclone trends may instead be: No, not yet.

The results come from a team led by Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology in Cambridge, Mass. He focuses on tropical cyclones, and his latest work seems to undercut his own conclusions in a 2005 study on climate change and tropical cyclones. That study, published shortly before hurricane Katrina struck the Gulf Coast, tied a global increase in the power expended by tropical cyclones to rising ocean temperatures in storm-prone areas. Those rising temperatures, in turn, have been linked to global warming.

There's no question, he says, that the old and new work "contradict each other to a degree."

His team drew its results from a new modeling approach it developed.

The research highlights the challenges scientists face as they estimate the effects rising global average temperatures could have on large-scale weather events important to society. It underscores, too, the risks some advocates and critics of climate-change policies face by using a single weather phenomenon as the poster child of the day for their position.

"There are only downsides" to traveling that path, regardless of one's political starting point, says Roger Pielke Jr., a science-policy specialist at the University of Colorado at Boulder. "If there's one thing we know about science, it changes, it evolves, it's counterintuitive, and we learn things we didn't expect before."

If climate policies make sense whether hurricanes are getting stronger or not, "then we should say that from the outset," he says.

The new study appears in the latest issue of the Bulletin of the American Meteorological Society. It is the latest of several projecting that if global greenhouse-gas emissions follow a business-as-usual path, the world would likely experience fewer tropical cyclones. But those that occur would either be more powerful or last longer. The study also shows that the size of changes varies, depending on the regions where the cyclones form.

The team used global-scale models to dictate conditions surrounding a given region where tropical cyclones form and travel. Then, they let smaller-scale regional models take over to fill in the details of storm behavior. And unlike other models, they designed this one to generate its own tropical cyclones from these conditions, rather than explicitly telling it where and when these storms should form.

After running the model against recently revised data from the US National Centers for Environmental Prediction, they confirmed that their new approach did a reasonable job of "hindcasting" storms between 1980 and 2006. Then they ran the model forward in time to see what the picture would look like over a 20-year period 200 years from now. The discrepancy between this and earlier work cropped up in the detailed results.

The results from 1980 to 2006 appeared to bolster Dr. Emanuel's earlier work; the amount of change in tropical cyclone activity grew by nearly 50 percent in the hindcasts, consistent with his earlier results. But the projections gave only a 20 percent increase in cyclone activity over the next 200 years.

This smaller long-term increase could mean several things, he says. The model could have problems. The increase in tropical-cyclone activity in the past 25 years may have nothing to do with global warming. Or it could be that once emissions – and hence temperatures – stabilized when the model reached the year 2100, increase in tropical-cyclone activity tapered off, lowering the overall figure for the full 200 years.

He and his team are now working to solve this puzzle.