Kerry Emanuel, professor of atmospheric science at the Massachusetts Institute of Technology, has been a weather groupie for a long time. As a child, he remembers making temperature and pressure measurements.

As an undergraduate at MIT, Emanuel had majored in earth science, only because there was no undergraduate major in meteorology; most of his courses, he adds, were in physics. He moved directly into MIT’s doctoral program in meteorology and managing to complete both degrees between 1973 and 1978.

Although he managed to pull up a memory from his preteen years in Florida that ties to his hurricane research — a day off from school due to a severe storm that was at least the periphery of a hurricane — it was not until the mid-1980s that he started to think seriously about these magnificent storms. He had been asked to teach a course on tropical meteorology and needed to scramble to get up to date on hurricane science. “I realized that what I had been taught, I didn’t understand,” he says, “and when I did, I realized it had to be wrong.”

Emanuel will present “Divine Wind: Hurricanes and Climate Change,” on Friday, April 25, at 7:30 p.m. at the Community Room in the Princeton Township Municipal Complex, 400 Witherspoon Street. The event is free and sponsored by the MIT Club of Princeton. For more information, contact Shahla Wunderlich at shahlaw@alum.mit.edu.

Emanuel spent eight years in Pennsylvania, then went to boarding school at the Deerfield Academy in Massachusetts. His father was a mechanical engineer for Philco, an appliance manufacturer, in its washing machine division and eventually moved over to the marketing side. Although Emanuel’s mother stayed at home with her children, before she married she served as a flight instructor during World War II. She was also an artist.

Emanuel started out in a branch of meteorology called mesoscale, which looks at the dynamics of medium-scale weather phenomena. For example, complexes of thunderstorms and bands of rain and snow. In his doctoral thesis he offered a possible explanation for squall lines, which are lines of thunderstorms that form in the spring and are common in the United States.

Meteorology also comprises the study of the very large-scale systems — the low and high pressure systems that show up on weather maps — and smaller-scale events like individual thunderstorms.

Hurricanes have a set operating procedure at least under ideal conditions. “Hurricanes are heat engines,” explains Emanuel. “They convert heat energy into wind energy.” They use the heat from evaporated ocean water in the tropics to create wind. Water evaporates under the eye wall of the hurricane, pumping in heat that in turn strengthens the winds; then as the winds get stronger, evaporation increases, yielding more heat and even stronger winds.

Several variable factors can affect a hurricane’s course and strength. Because hurricanes mix up the upper 500 feet or so of the ocean, they may bring to the surface deeper, colder water, which will cool and weaken the hurricane.

Hurricane winds may also be embedded in larger scale weather environments that affect them in different ways. Wind shear, where the winds vary by altitude, can also slow down a hurricane. “It tends to lead to dry air from the environment making its way into the core of the storm,” says Emanuel, “which is like throwing cold water into a fire.”

Hurricanes generally move at the average speed of the wind that embeds them, traveling with trade winds that blow east to west in the North Atlantic. Once they get farther west, they may also get caught in the middle-latitude winds that move west to east starting at about 25 degrees (a little south of Miami) to very high latitudes. These winds tend to blow offshore, which is why when hurricanes curve to the north, they usually start to curve away from land — at least by the time they reach New Jersey. Hurricanes can also get caught in a middle-latitude, low-pressure storm system, which can steer them back toward shore.

The saving grace of hurricanes is that once they come to shore, they immediately begin to die, usually progressing only about 100 miles. But even this rule has a caveat. As a result of variations in temperature at different latitudes, says Emanuel, “every once in a while a hurricane will become rejuvenated while it is inland. It starts to tap energy from a different source — the same source that powers the usual run-of-the-mill low pressure systems.”

The usual questions.Emanuel and other scientists have been asking: How do hurricanes respond to climate change, whether natural or manmade? Global warming, he explains, is caused by the addition of certain gases to the atmosphere, notably carbon dioxide, that absorb and then re-emit infrared radiation trying to escape from the earth’s surface, thereby making the earth’s surface warmer.

Emanuel says that four years ago he stumbled on an empirical connection between the power of a hurricane and the surface temperature of the ocean, particularly in the Atlantic, where the data is good. But in a study he just released, the New York Times reports that a new computer model looking at this relationship showed different effects on different regions, with some showing more activity and others less intensity and fewer storms.

The human element and the scams that go with it. Emanuel, however, is in some sense less concerned with the science, which is ever-accumulating more knowledge about the relationship between hurricanes and climate change, than he is with the effects of hurricanes on human beings. He attributes the ever-increasing damage wrought by hurricanes running ashore to the nonstop building near the ocean. “The main effect is not climate change, it’s human change,” he maintains. “People are flocking to the coastline like lemmings; and our government, both at that state and federal levels, heavily encourages and subsidizes that migration.”

What has happened, explains Emanuel, is that the disproportionately wealthy families who have homes on the beach have used their political connections to subsidize their love of the ocean. “They have succeeded over last 80 years in coming up with a system of regulations that keeps property insurance low on the coast,” he says. “The state insurance commissioner can cap premiums, which is almost unheard of in any other market sector.”

Since insurance companies are not allowed to charge what they think is warranted by the risk, Emanuel continues, the regulators quietly allow them to overcharge people living in less risky places to make ends meet. “Like all successful scams,” he explains, “it works by robbing a large number of people of very small sums of money per person.” He estimates that New Jerseyans who do not live on the shore probably pay a 10 percent insurance premium to support those who do.

When the people that this scheme benefits are disproportionately well-connected politically — after all, Teddy Kennedy and John Kerry in Emanuel’s home state of Massachusetts both have beachfront property — the rest of us are left with little power to do what needs to be done to decrease hurricane destruction on the coast, which is to discourage people from living there.

Getting warmer. As far as global warming, it is for Emanuel a settled issue, even if theories as to the nature of the mutual interaction between global warming and hurricanes remain controversial. “The only people that deny it are a handful of retired physicists being paid by conservative think tanks,” he observes.

When asked what the world can do to stop global warming, Emanuel was quick to point out that the decisions we have to make as a world society have everything to do with balancing risks.

When asked what the world can do to stop global warming, Emanuel was quick to point out that the decisions we have to make as a world society have everything to do with balancing risks. Doing nothing about global warming carries very dangerous risks. “If the earth warms too much, we have the risk of a disintegrating Greenland icecap, which would raise the sea level by 22 feet and would submerge most of South Florida,” says Emanuel, “and we don’t know enough to rule that out.”

Of course we can try to adapt by curbing emissions, and Emanuel jokes that perhaps moveon.org needs to transmute to moveback.org. But more seriously, he points to two proposals for engineering our way out of global warming: finding ways to artificially remove carbon from the atmosphere, and adding material to the stratosphere that would slightly decrease the amount of sunlight reaching the earth’s surface. The latter path would counterbalance the effects of greenhouse gases that are locking in more heat.

In terms of global warming, then, we have to weigh the risks of doing something versus doing nothing. But psychology and ethics play a part in whether we choose to be passive or proactive, as Emanuel illustrates with an analogy. Say John has a tree on his property that he thinks might be in danger of falling on his next-door neighbor George’s house, but he’s not sure and doesn’t do anything about it. One night in a bad storm, sure enough the tree falls on George’s house. Emanuel asks: How will George treat John and what position will John be in legally? Probably the insurance companies will just hash out responsibility and George’s house will get fixed.

But what if John told George he was worried that the tree he might fall in a storm, so he was going to chop it down — even though there was a slight risk that it might fall on George’s house as a result of the chopping. Then suppose the tree does fall on George’s house. Although in the first scenario George would likely have blamed John for his inattention, but in the second the accusation would be incompetence.

“If something bad happens as an unintended side effect it is treated differently than if you intended to do something and it has an unpleasant side effect,” says Emanuel. This assessment might not be rational but it is nonetheless psychologically likely.

Noting that there are risks of inaction and of action, Emanuel says that the role of scientists is to present information. “What scientists can do is try to be more precise about what those risks are,” he says.

When scientists are too outspoken about the implications of their work, says Emanuel, they may be putting their careers in danger because having an emotional stake in your work tends to erode credibility by impugning scientific objectivity. Yet there is a tipping point. “Only when things become very clear and no one else is doing something about it,” says Emanuel, “scientists should become more forceful in communicating information.”

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