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Hurricanes: are they strengthening globally or locally?
Climate trends can be difficult to determine, because methods of observing the Earth have changed radically from early to modern times, and the quality and reliability of measurements have changed as well. To establish a trend, we need to know what the early measurements were, as far back in time as possible. The trend in hurricane activity over the oceans is a great example of the difficulty. (All hurricanes form and strengthen over the oceans, often over remote expanses of water.)
Before airplanes, tropical storms far from land were discovered by chance, and many must have been unobserved. Knowledge about the existence and strength of hurricanes was spotty and inconsistent until quite recently. Until reconnaissance aircraft from the Weather Services began to target these storms around 1950, such storms were avoided, if they were detected at all. After “hurricane hunter” aircraft flew into the storms to fix their locations, and measure wind speeds and intensities, observations became more consistent and reliable than before. But it is a problem to compare the observations after 1950 with the less consistent observations before that time.
Then, when weather satellites began to observe the Earth and its clouds after 1970, hurricanes were observed in a consistent manner even in their early stages and in the most remote oceans. That change introduced another discontinuity in measurement of their strength.
Dr. James Kossin of the University of Wisconsin asks whether the observed trend in hurricane intensity is credible in all ocean basins of the world. In addition to the measurement problems through time, he points out that techniques for estimating the intensity have differed in the six ocean basins where hurricanes are monitored. He chooses to rely on satellite data since 1983 as a consistent data source, and ignore the trends before that time. Even after 1983, methods of calculating intensity differed around the world, so his team introduced a consistent technique for re-analyzing thousands of satellite images.
Kossin constructed a new measure of mean “storm energy,” focusing on those storms of hurricane intensity (64 mi/hr or greater). His results agreed well in the Atlantic and the Eastern Pacific Oceans in comparison with the traditional intensity estimates. Hurricanes have become more intense in the Atlantic, and less intense in the eastern Pacific, over the last 25 years. But the agreement was “terrible” for the other four basins in the Pacific and Indian Oceans, where 85% of hurricanes were found. Kossin found no trend in these four basins. Previous analysis by the Joint Typhoon Warning Center showed increasing intensity in the other four basins, especially in the South Indian Ocean. Kossin’s trend results differed the most from the Joint Typhoon trend in the two basins of the Indian Ocean, south and north of the equator.
Believing that the satellite-based technique is consistent around the globe, but the older technique is not, he asserts that the upward trend in the four Pacific and Indian Ocean basins, from the conventional record, is “spurious.” That 4-basin trend also strongly affects the trend for the globe, which must also be spurious, he concludes.
As the data (especially before 1970) cannot be trusted, Kossin looks forward to a better understanding of the physics of hurricanes, rather than the historical record of intensities, to illuminate how hurricanes will respond in a warming world. |