The effects of climate change are still being explored, many of which
are unknown and most others not fully understood. Several simulations
of the earth's climate indicate that the planet may undergo significant
warming during the enxt 100 years. This drastic warming may affect the
globe in many ways, on of which is the way it affects hurricanes. In
this paper, I will discuss research done on the impact of global warming
upon hurricanes and how this research applies to one specific
southeastern Florida basin.
It is difficult to predict what types of small scale changes could occur due to a warmer atmosphere. This is because global simulation models are typically too coarse to resolve effects on small land features such as drainage basins. A paper written by William J. Gutowski, George F. McMahon, Shing S. Schluchter, and Paul H. Kirshen describes the process they use to get around these climate model problems. The writers circumvent this difficulty by using a physical model for severe hurricanes to derive changes in hurricane rainfall under a predicted global warming. The modified rainfall is then transferred to a linked hydrologic - hydrodynamic model to compute the impact of global warming on hurricane induced flooding in a susceptible watershed in southeastern Florida. Their test was done on an area of land in southeastern Florida of 237 square kilometers using temperature changes when atmospheric carbon dioxide concentration is doubled.
The effects that climate change could have on hurricanes needs to be discussed first. The thermodynamics of a mature hurricane can be described in terms of a Carnot cycle driven by the temperature difference between the sea and the upper atmosphere. With this information it is possible to determine the intensity of a hurricane in terms of the pressure difference between the center and the outside of a hurricane. There are three factors that determine the pressure difference: sea surface temperature, atmospheric relative humidity, and the temperature difference between the surface and the lower stratosphere. From global climate models, we know that a doubling of the atmospheric carbon dioxide concentration will likely increase sea surface temperature around the globe. This sea surface temperature increase could have an impact on the severity of hurricanes. Using the increased sea surface temperatures, it has been estimated that in the world's tropical oceans, hurricane strength could increase by thirty to sixty percent. IN this specific region of Florida, it is estimated that hurricane intensity coudl increase by about forty percent.
It has also been found that the precipitation rate is proportional to the difference in pressures between the center and the outside of the hurricane. This is due to the fact that precipitation usually occurs when moist air rises, cools through adiabatic expansion, and supersaturates, producing water vapor condensation. This link between pressure differences and precipitation rates indicates that a doubling of atmospheric carbon dioxide also could cause a precipitation increase of forty percent in the most severe hurricanes. (Gutowski et al., 1994)
It is also possible that the increase in atmospheric carbon dioxide could have an effect on the frequency of hurricanes as well as the intensity. In this study, though, the authors assumed that the frequency of hurricanes stayed the same as it is today because it is difficult to predict how global warming will affect the frequency of hurricanes. The intensities of hurricanes could increase in a doubled carbon dioxide climate, making the probability that a strong hurricane could come along, for example, once every fifty years instead of every one hundred years, as in the present climate. Hurricane frequency would likely increase due to the increase in sea surface temperature. This is because in order for hurricanes to develop, ocean water of at least 26 degrees Celsius is needed in the tropics. An increase in the SST would allow the tropical ocean to remain above 26 degrees for about three quarters of the year, compared with only about half the year at the present time. This increased period of warm ocean water could possibly increase hurricane frequency.
The authors of this article use the example of a once in a hundred year, three day rainfall design to examine the effects of hurricanes on the flooding of an area of southeastern Florida. The area being studied is the C-11 Western basin located along the Atlantic Ocean between Fort Lauderdale and Hollywood, Florida. The land is low-lying, relatively flat, and is comprised of rural and wetland areas, citrus groves, as well as residential and commercial areas. The soil in this area can hold very little moisture, forcing a large percentage of the precipitation to become runoff. The amount that runs off and the rate at which is does so is greatly influenced by the intensity and volume of precipitation. This means that any increase in precipitation rate and amount would likely result in additional runoff. The results from this study indicate that the precipitation amount for a three day event could increase by approximately 11% on the first day, 16% on the second, and 73% on the third. Because the ground can hold such little moisture, this increased runoff could threaten both life and property.
Scientists are now beginning to determine the effects of climate change on many natural processes, including hurricanes. It is difficult to accurately determine the effects of doubled carbon dioxide on hurricanes because the climate models presently used do not have fine enough resolution to accurately predict such small scale atmospheric disturbances. There are many other ways that these estimates for the effects of climate change on hurricanes could be improved. The linkage between the global climate model and the flood basin model is relatively simple and more complex approaches may give more accurate results. Another simplification that was made was that the frequency of hurricanes would remain unchanged in a doubled carbon dioxide atmosphere, which may or may not be the case. Future developments in climate models may make it easier to predict what degree of impact a doubling of carbon dioxide will have on hurricanes and may save many lives and dollars in the process.
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