Typhoon flogs Philippines
As grieving people in the central Philippines bury their loved ones and begin to restore shattered lives, we are again wondering: Was the awesome typhoon Haiyan a response to global warming?
(Typhoons, tropical cyclones and hurricanes are the same thing, so we use the terms interchangeably.)
We don’t know. Nobody knows, but when you add more energy to a system, you can get more energy out of it. And so for a decade or two, we’ve heard that warming is going to make more hurricanes, stronger hurricanes, or both.
Hugh Willoughby, a research professor at Florida International University, has seen the inside of hundreds of hurricanes from an airplane. He hews to the received wisdom: Typhoon Haiyan “is the sort of thing that we expect to see more of as the globe warms.”
Kerry Emanuel, a professor of atmospheric science at MIT, has been a leader in assessing the danger of hurricanes. He expects that tropical cyclones will continue getting stronger, and says, “Certainly there has been a near-doubling of power dissipation in the Atlantic.”
Measurements of numbers and intensity are much better in the Atlantic than the Pacific.
Although the overall literature is mixed, Emanuel told us, “Projections from theory, observation, and other means, all seem to show that the frequency of high-intensity tropical cyclones goes up as the climate warms.”
But the issue of warming and tropical cyclones is open for debate. “The picture is still a little murky, models disagree on even the sign about frequency in the Atlantic,” says Thomas Knutson, a researcher at NOAA’s Geophysical Fluid Dynamics Laboratory. “Our model shows a decrease of tropical storms and hurricanes; only the very strongest increase in frequency in the Atlantic, but there are other models that show a decrease in the strongest storms. It’s similar for models in the Northwest Pacific: the majority show a decrease in frequency.”
However, in a recent study in the Proceedings of the National Academy of Sciences (PNAS) Emanuel projected a substantial increase in activity in the Northwest Pacific – an area that includes the Philippines
At its simplest, the expectation for increased tropical cyclones is based on the idea that because greenhouse gases reflect energy back to the planet, more energy is available to power stronger storms.
A hurricane is a cyclonic machine that transfers energy from the warm ocean to the cold atmosphere above. But that shape, with organized updrafts, can be blown apart by winds moving in different directions at various levels of the atmosphere, and some climate models predict that stronger wind shear will actually blow tropical cyclones apart before they mature.
The biggest single problem in understanding the present and forecasting the future of tropical cyclones resides in a shortage of data, especially in the Northwest Pacific, Haiyan’s neighborhood. “We have records of various quality going back in time for various periods, depending on the location,” is how Knutson sums up the situation.
That forces scientists to massage data to account for differences in the number and accuracy of measurements. “The best study I know” that accounted for changes in the quality of satellite data, “corroborates that the frequency of the highest intensity events is going up,” Emanuel says. That study1, concluded, “Our results are qualitatively consistent with the hypothesis that as the seas warm, the ocean has more energy to convert to tropical cyclone wind.”
It’s not exactly a firm conclusion, but Willoughby agrees that overall, projections do favor an increase in monster tropical cyclones. “Simulations with global climate models point to the very strongest tropical cyclones becoming stronger.”
The problem with statistics
None of this proves that deadly typhoon Haiyan was a product of global warming. That’s frustrating, and frustrated journalists tend to stretch for a sports metaphor. Imagine Beardy Barry, a first baseman who is batting .333. Getting one hit in three times at bat makes him happy on payday, and Beardy will get more hits than Furry Freddy, who is batting .250, if they both play enough games for statistics to apply.
But a good average does not insure Beardy against striking out three times in a row. And a lousy average does not prevent Furry from swatting the occasional home run.
Statistics apply to groups, not to single people or events.
And unlike statistic-obsessed sports like baseball, the data on tropical cyclones is wretched over the Pacific. That paucity, combined with “a lack of consensus” on the physical links with climate, lead a large group of scientists to conclude that “attribution of any observed trends in tropical cyclone activity in these basins to anthropogenic forcing remains controversial.”2
All of these limitations apply in spades to Typhoon Haiyan. “The scientifically correct answer is that you can’t say anything based on a single event, you just can’t, however horrendous it is,” says Emanuel. “You have to look at the statistics, and you have to be careful when you say that we are not seeing global warming or that we are seeing it.”
The answer you get depends on the question you ask. And if you are concerned about the danger of tropical cyclones, you need to think beyond wind speed. Consider rainfall: In 1998, Hurricane Mitch soaked Honduras with rains of 30 centimeters or more, causing floods and landslides. Overall, 11,000 died and an equal number disappeared.
Hurricanes are measured on a scale from 1 to 5, and Sandy was only a category 2 when it smashed into New Jersey and New York in October, 2012. Sandy killed an estimated 286 people in seven countries, and has so far cost $68 billion in the United States, where shoreline communities are still struggling.
“It’s not fair to say that categories 3, 4 or 5 are the only ones that matter,” says Knutson. “They are important, and are responsible for the most damage, but a hurricane can lead to a very serious storm surge, even if the storm is not that intense.”
The giant Sandy, the largest hurricane in Atlantic history at 1,800 kilometers in diameter, illustrates the importance of size. Sandy pushed a huge wall of water — a storm surge – into a corner defined by New York and New Jersey, and the surge caused more damage than wind.
Surging sea level
And so we meet another aspect of global warming. When warming melts glaciers and causes the oceans to expand, it raises sea level. And because the surge rides above sea level, an elevated sea level allows the same surge to reach further inland.
Although the historic rise in sea level is clear, the rise over the next century is debated, says Knutson.
And so, when considering the danger of a tropical cyclone, “We have to be concerned about size, intensity, how susceptible the location is; it’s complicated,” says Knutson.
The damage question
Before looking at how much damage can we expect over the next century or so, what about the past century? We asked Willoughby, who noted a calculation by Roger Pielke of the University of Colorado, who factored in inflation, population increase, and national wealth, and found that the level of U.S. hurricane damage between 1900 to 2000, “Was dead flat. To a first approximation, the damage so far scales to the assets in peril,” and not to the size, number or intensity of storms, Willoughby said.
As more people build more structures in the danger zone along the U.S. Atlantic Coast, Emanuel says, “We expect, in a projection for the next 100 years, a doubling in hurricane damage due to population change and migration to the coast.”
The price of federal flood insurance, Emanuel notes, is finally rising, but it has been “a huge subsidy that relatively poor people are paying to the well off” who are building houses along the coasts. Hurricane damage “has been driven by a very unwise policy, greatly underwriting people who are moving into risky regions,” he adds.
And so even if warming does not increase the number or ferocity of tropical cyclones, the sea they are riding on is rising, shoreline populations are growing , and the price tag – in dollars or human lives – can only rise.
We asked Emanuel if he was frustrated about the difficulty of assessing the past and future effects of warming on tropical cyclones. But rather than lament the state of incomplete knowledge, he concentrated on how to answer the question. “What’s most frustrating to me is that we are not doing a good job of measuring tropical cyclones.” During the Cold war, the U.S. Navy flew into Pacific storms, but it stopped in 1987 to save money. “We are just not monitoring them well,” Emanuel says, and as a result, meteorological authorities “come up with wildly different estimates of how strong a storm is” outside the well-documented Atlantic.
Satellites are much better at seeing hurricanes than measuring their intensity, Emanuel adds. “Even the most vocal pro-satellite people will tell you that satellite-based data are not good enough, because they don’t measure wind speed or pressure, the things that allow us to make an inference about the storm intensity. A satellite can tell the difference between category 1 and 5, but not between category 2 and 4.”
“You have to put a probe into the storm,” Emanuel says, suggesting that drones could make the observations at low cost and risk.
What is the real question?
If the issue is confirming the reality of warming, tropical cyclones are the wrong place to look, says Knutson. “If you are looking for evidence of warming, look at the thermometer record, or sea level rise, look at the loss of ice in the Arctic, or the mountain glaciers disappearing or the change in temperature records toward more records on the warm side. These signals are stronger than any evidence we have relating to tropical cyclones.”
Willoughby, who flew into Pacific hurricanes with the U.S. Navy, recalls a “Catch phrase in naval aviation: ‘The moment when paralyzing panic replaces fatal overconfidence.’” When the issue is whether and how warming is affecting the biggest storms, “What we really need to get to is the moment when rational analysis replaces fatal overconfidence or simple heedlessness.”
If we look for a signal of global warming in tropical cyclones, he indicates, “There may not be a lesson until it’s too late for the lesson to be useful.”
Warming will not be the only thing going on, says Willoughby, who lived as a toddler in Cebu City, a Philippine city blasted by Haiyan. Sea level will be up, coastlines will be flooded, food may be scarce as the population rises – and the outcome cannot be predicted. “100 years ago, the world was 10 months away from World War I. In the books, you can read about people saying war was impossible. But they understated human stupidity.”
– David J. Tenenbaum
Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
- The increasing intensity of the strongest tropical cyclones, James B. Elsner et al, Nature 455, 92-95, 4 September 2008 ↩
- Monitoring and Understanding Trends in Extreme Storms State of Knowledge, Kenneth E. Kunkel et al, Bulletin of the American Meteorological Society, April, 2013 ↩
- Q&A: Anthropogenic Effects on Tropical Cyclone Activity ↩
- Did Climate Change Cause Typhoon Haiyan? ↩
- How Typhoons are Named ↩
- What is Storm Surge? ↩
- Geoengineering in the Spotlight ↩