SEEING A LINK BETWEEN CLIMATE CHANGE AND TROPICAL CYCLONES
MANILA, September 15, 2005 (STAR) By Emmanuel G. Anglo, Ph.D. - (First of two parts) Was it global warming? When Hurricane "Katrina" devastated Louisiana and Mississippi last week, many observers were quick to cite the disaster as evidence that accumulating greenhouse gases were causing tropical cyclones to get stronger. Such proof would hardly be news to Dr. Kerry Emanuel of the Massachusetts Institute of Technology, who recently put forward the connection in an article published in the August 2005 edition of the prestigious science journal Nature. By his calculations, climate change has caused hurricanes in the Atlantic to become twice as destructive as they were 30 years ago. Over the Western Pacific where the Philippines lies, the intensification is about 75 percent. After our own experiences with two major disturbances in November 2004, Emanuel’s article didn’t need validation.
Not so fast, says a team of scientists headed by Roger A. Pielke Jr. of the University of Colorado, whose results are to appear in the Bulletin of the American Meteorological Society. Disputing Emanuel’s claims, this article essentially reiterates the conclusions of a 2001 review for the Intergovernmental Panel for Climate Change (IPCC) by a meteorology Dream Team that included Emanuel himself. Pielke and his group maintains that while there may be theoretical basis for suspecting that greenhouse warming can bring about stronger typhoons, the empirical evidence is just too weak, and that our understanding of the physics of tropical cyclones is too rudimentary to make any such conclusions. Dr. William Gray of the Colorado State University, arguably the biggest name in that Dream Team, also notes that it is impossible to distinguish between what is happening today and the natural variability that tropical cyclones have historically displayed. Present Atlantic hurricanes are strong because the basin is in a period of high activity that was last recorded around the 1920s well before the first smoke-belching jeepney took to the road.
What do these two clashing articles mean to the Philippines, which typically runs out of letters of the alphabet to name its typhoons with? What do we know about climate change and its connection to tropical cyclones?
The theory behind Emanuel’s prediction boils down to a few basic facts. First, tropical cyclones get their energy from heat released by large-scale condensation of water vapor, which finds an unlimited supply in the ocean. Second, the warmer the ocean, the more water vapor it releases. Indicative of the importance of sea surface temperatures is the observation that hurricanes almost never form during winter and strike only the lower half of the continental United States facing the tropics. The Pacific Ocean, which is warmer than the Atlantic, has a longer typhoon season and a larger area for tropical cyclone formation. ‘Destructiveness Index’ What Emanuel constructed was a "destructiveness index" based on the recorded maximum velocities of tropical cyclones in the Atlantic and western Pacific. When he plotted the values of this index for each basin over the past 30 years, he found a clear and striking increase over the period. More important, the graphs mimic the rise in sea surface temperatures in each basin. Results like these have changed the minds of the many undecided in the scientific community, including former skeptics such as Emanuel himself.
Yet many choose to remain cautious, particularly among those who find Emanuel’s thermodynamic approach too simple to model a system like the atmosphere. The objections of Pielke’s team is premised on the fact that other metrics of tropical cyclone behavior do not show dramatic changes over the decades that could be attributed to greenhouse warming. For instance, the rise in mean global sea surface temperatures do not appear to have caused a similar increase in frequency over the decades as the theory might predict. Hurricane "Katrina" crossed the city of New Orleans to become the most destructive of cyclones to hit the United States in a hundred years, but it was not particularly strong. Last year’s "Winnie," which struck Northern Luzon, was only a depression.
One reason is that while a warm ocean is critical to typhoon formation, this factor influences only the first few hundred meters of a towering mass of vapor that can be 20 kilometers tall. Another crucial requirement is the temperature profile of the atmosphere, which must be unstable enough for vapor-laden air masses to rise to the upper atmosphere where they can cool down, condense and release their energy. The wind profile along the entire column must also be fairly uniform, or else the circulation will never get organized. Unlike the heating of the sea surface, it is not clear how global warming will influence these factors.
Since there is no way to test the connection between global warming and tropical cyclones in the laboratory, climate scientists can only do so in virtual reality. Using the world’s fastest computers to simulate the entire atmosphere-ocean system, research teams from around the globe can test the consequences of scenarios resulting from the doubling of CO2 levels. But even starting from virtually identical assumptions, different global climate models yield frustratingly disparate outputs. Their estimates of typhoon intensification range from five to 20 percent; others find no significant change.
One key shortcoming of these models is resolution, which defines how finely the atmosphere can be represented in a model. Even models that run on the most powerful computers are unable to adequately resolve the features of the atmosphere smaller than a kilometer, let alone clouds, a cyclone’s most important feature. As a result, the forecasts remain imperfect. Since improving resolution demands more processing power, understandably any new advance in high-speed computing is quickly recruited for climate modeling.
For now the race to improve resolution postpones the question of whether we really know enough about the science of weather phenomena to make good forecasts with. After all, computer models only solve the arcane equations expressing the laws of geophysics that we know of. Many of the fundamental theories of fluid motion were discovered in the 19th century, while processes involved in clouds, friction and turbulence are still understood through statistical relationships and not as "true" scientific laws. Among many theorists there is therefore an impression that if weather prediction is to meet the heightened demands of modern society, what must take place is a scientific revolution of the magnitude that gave rise to the theory of relativity.
(To be concluded)
* * * (The author is an associate professor of the Department of Physics of the Ateneo de Manila University. E-mail him at firstname.lastname@example.org)
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