Science Briefs

How Will the Frequency of Hurricanes be Affected by Climate Change?

Hurricanes are major storms spawned over tropical oceans whose surface temperatures are warmer than about 26°C (79°F). The extreme winds, often exceeding 100 mph, and very high precipitation rates associated with these storms pose a threat to life and property along vulnerable coastal regions.

The expected buildup of atmospheric "greenhouse" gases, such as carbon dioxide and methane, will likely increase ocean temperatures as well as the area of ocean surface water warmer than 26°C. It is therefore possible that hurricanes will become more frequent and/or more intense in future decades. However, there are other environmental factors that influence hurricane development, and these must also be taken into consideration in any assessment of future trends. Fortunately, climate simulation models can be used to evaluate how all of the relevant environmental conditions will change as the warming from greenhouse gas buildup is realized.

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Number of tropical cyclones in 20 years per 5° square adjacent to North America. The upper figure is based on a GISS model simulation of current climate, the lower figure for a future climate with double the 1959 concentrations of greenhouse gases.

Gray (1979) found that a mathematical combination of selected seasonally averaged climate indicators, combined into one index, realistically reproduces the historical pattern of hurricane frequency over each ocean basin. Druyan and Lonergan (1997) have adapted Gray's index so that it can be computed from simulations by the NASA/Goddard Institute for Space Studies climate model. Based on simulation experiments, they found that an atmosphere with double the present concentrations of carbon dioxide could lead to an increase of about 50% in the number of tropical storms (winds exceeding 40 mph) and hurricanes that form over the Gulf of Mexico, and an increase of more than 100% over the tropical North Pacific Ocean. Other experiments with the climate model, however, suggest that the natural variability of ocean surface temperatures is a much more important cause of recent year-to-year fluctuations in number of storms than are climate effects from injections of ash and aerosol particles during the volcanic eruptions of El Chichon in 1982 and Mt. Pinatubo in 1991.

Some of the largest climate fluctuations that affect ocean temperatures are related to El Niño cycles of warming and cooling of the tropical Pacific Ocean. During those years when the eastern tropical Pacific is warmest at the peak of the El Niño cycle, hurricane activity in the North Atlantic tends to be at a minimum, apparently because of the effect of warm ocean water on world-wide wind patterns. We plan to use the GISS climate model to examine how this impact of El Niño-caused ocean warming will enhance or diminish hurricane formation in the future, once significant greenhouse warming has occurred.

Reference

Druyan, L., and P. Lonergan. 1997. The impact of climate change on tropical cyclones. Risk Prediction Initiative Workshop, May 5-6, 1997, Hamilton, Bermuda.

Gray, W. 1979. Hurricanes: Their formation, structure and likely role in the tropical circulation. In Meteorology over the Tropical Oceans (D.B. Shaw, Ed.), pp. 155-218. Royal Meteorological Society. London.

Contact

Please address all inquiries about this research to Dr. Leonard Druyan.