Using satellite data, land-cover change history, computer models, and weather records, the researchers found a link between the losses of wetlands and more severe freezes in some agricultural areas of south Florida.

In other areas of the state, changes in land use resulted in slightly warmer conditions. They concluded, based on the study, the conversion of wetlands by itself may be enough of a trigger to enhance damage inflicted upon agriculture in these areas of south Florida during freezes events.

The Landsat 5 satellite was constructed and launched by NASA, and its data are processed and distributed by the USGS. The researchers studied three freeze events and simulated the conditions with a computer climate model, using weather and land-cover change records. The freezes took place on December 26, 1983, December 25, 1989, and January 19, 1997.

The study, authored by Curtis Marshall and Roger Pielke of Colorado State University (CSU), Fort Collins, Colo., and Louis Steyaert of the USGS and NASA's Goddard Space Flight Center, Greenbelt, Md. appeared in a recent issue of the American Meteorological Society's Monthly Weather Review.

The researchers found a strong tie between areas that were converted from wetlands to agriculture use during the 20th century, and those that experienced colder minimum and longer duration subfreezing temperatures in the current land-use scenario.

Water typically doesn't cool as quickly as the land at night, which may explain why when wetlands are converted to croplands the area freezes faster and more severely.

"The conversion of the wetlands to agriculture use itself could have resulted in or enhanced the severity of recent freezes in some of the agricultural lands of south Florida," Marshall said. He noted some other areas also experienced warming from land changes.

The study focused on "radiation freeze events" that occur at night, frequently under calm wind conditions and when there is little or no cloud cover. At night much of the warmth absorbed by the land during the day escapes into the atmosphere, cooling the ground.

This study of wetland changes is very important to Florida and the rest of the country. Over the past 150 years, the citrus industry has been moving further south.

As a result more wetlands are being transformed into agricultural lands, and these changes are causing temperatures to change. Ironically, as the industry moves further south to avoid freezes, the land changes create the conditions the industry is trying to avoid.

The scientists analyzed land-cover changes over the past 100 years in Florida based on reconstructed pre-1900 natural vegetation data and land-cover data derived from early 1990s Landsat images. They input land-cover and weather record data into the CSU Regional Atmospheric Modeling System and re- created the conditions for the three freezes.

In all three cases, the most densely cultivated areas were colder and also experienced subfreezing conditions for a longer period of time. Those areas include south and southwest of Lake Okeechobee and other key agricultural areas in the Kissimmee River valley.

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