Storm Gods: Emanuel sees portents in increased intensity of hurricanes

October 6, 2006

AUSTIN, Texas—Kerry Emanuel, one of the world’s foremost experts on hurricanes and climate, told a packed house at The University of Texas at Austin that global warming is increasing the intensity of hurricanes in the Atlantic.

Drawing on a series of graphs and data stretching back to the 1860s, Emanuel, a professor of meteorology at the Massachusetts Institute of Technology, illustrated his contention that intensities of hurricanes have been stair-stepping their way upward in close correlation with rising sea surface temperatures in the Atlantic for at least the past 35 years.

The sea surface warming seen in recent decades, said Emanuel, “has been almost certainly due to man-made influences.” While debate over the causes of global warming may continue in the media, “there really isn’t a debate in my profession,” he said.

Though Emanuel made a strong case linking hurricanes and global warming, he stopped short of attempting to attribute individual hurricanes to recent climate change. “I think it’s a mistake to think that Katrina was a result of global warming,” he said. The majority of tropical storms never reach land, dissipating over the oceans. “A lot of this is just bad luck, in terms of [which storms make] landfall,” said Emanuel.

Hurricane Man

Emanuel has been at the eye of the media storm over hurricanes and global warming since his August 2005 paper in the journal Nature correlated global warming with the increasing destructiveness of tropical cyclones over the past 30 years.

His Oct. 5 lecture, organized by the university’s Environmental Science Institute and the Jackson School of Geosciences and co-sponsored by the SBC Foundation and ConocoPhilips, drew on materials from the paper and his new book, “Divine Wind: The History and Science of Hurricanes.”

Hurricane intensity is strongly linked to summertime sea surface temperature in the Atlantic. The first graph shows the intensity of Atlantic hurricanes (plotted as PDI, Power Dissipation Index) from 1970 to 2004 (green). The second graph shows the same intensity curve, overlain with summertime sea surface temperature (blue). See larger images.

Only 30 major hurricanes (category 3-, 4-, and 5-storms) have made landfall since 1870, said Emanuel, and yet the majority of U.S. insurance losses from natural disasters come from these storms. “Over half of all hurricane damage in the U.S. has come from just the top five most powerful hurricanes,” he said, “all of which were category four or five,” the largest category measured.

Monetary damage from hurricanes has increased phenomenally in the past three decades, jumping 900 percent in constant 2004 dollars from the ten-year period of 1975-1985 to the ten-year period of 1995-2005.

Why such dramatic increases? In part, explained Emanuel, because of equally dramatic increases in population along the coasts most affected by hurricanes.

Perfect Storms

The reason hurricanes are becoming more intense as Earth warms, explained Emanuel, is fairly straightforward: Hurricanes are heat engines powered by warmth from the ocean. They take hot, moist air from the sea surface and lift it up several kilometers into the atmosphere, where the air releases heat and then sinks back to the base of the storm. As heat engines, hurricanes convert heat energy into wind energy.

With or without anthropogenic global warming, oceans generate seasonally varying levels of heat due to the natural greenhouse effect that keeps Earth habitable. “Hurricanes are almost perfect heat engines caused by the greenhouse effect,” explained Emanuel. “So it makes sense that increasing the greenhouse effect will increase hurricane activity.”

Some climate scientists further speculate, said Emanuel, that hurricanes might regulate heat through a feedback mechanism, and that changes in hurricane activity in turn change climate.

Ironically, Emanuel noted that efforts to reduce air pollution have actually slightly increased the rate of global warming. Aerosol particles, produced by both industrial emissions and volcanic eruptions, reflect solar radiation, effectively cooling the planet. Successful efforts to reduce industrial aerosol emissions have increased the planet’s exposure to greenhouse effects.

Emanuel acknowledged one lingering mystery in his field: in the Atlantic ocean, over the last 35 years, hurricanes have become more frequent, even while the number of tropical storms globally has remained constant. According to predictions, the storms should become more frequent everywhere.

The view from inside a hurricane taken by a crewmember in a "hurricane hunter" aircraft. The encircling eyewall is often described as looking like a coliseum. See larger image.

During a Q&A after the talk, an audience member asked if there was a cap on how powerful hurricanes could get. “In this climate we live in, yes, there is a cap,” Emanuel said. “The speed limit is probably around 200 miles per hour. But if you change the climate, the speed limit goes up.”

The word “hurricane” derives from the ancient Mayan storm god, Hunraken. Ancient Mayan images of Hunraken, Emanuel noted, showed an uncanny understanding of how hurricanes rotate, a discovery Western science did not make until the 19th century.

Mayans were also quicker to appreciate the threat from hurricanes. “The Mayans stopped building cities on the coast,” noted Emanuel. “They got tired of being clobbered by storms. We haven’t progressed as far as the Mayans.”

For more information about the Jackson School contact J.B. Bird at jbird@jsg.utexas.edu, 512-232-9623.