Frosty Florida: spread of agriculture may promote freezes.

Sunny southern Florida seems like a perfect place to grow fruits and vegetables, even in the winter. But the 20th-century transformation of what had been wetlands into croplands might have had unintended consequences. The shift has made the area more susceptible to crop-damaging freezes, researchers suspect.

In the early 1900s, much of Florida's citrus industry moved south to areas of the state that seemed the least prone to ruinous freezes. To make room for orchards and farms, people drained wetlands and diverted rivers, actions that affected the local climate, say Roger A. Pielke Sr. and Curtis H. Marshall of Colorado State University in Fort Collins and Louis T. Steyaert of the U.S. Geological Survey in Greenbelt, Md.

The scientists mathematically reconstructed south Florida's pre-1900 plant cover and mapped the area's current vegetation. By plugging these data into a weather-modeling program that Pielke and another colleague developed, he and Marshall could predict the temperatures before and after the draining of the wetlands.

They found an overall cooling trend of a few degrees Celsius where wetlands were replaced by farmlands. The greatest change appeared in the area south of Lake Okeechobee, where farmers now grow citrus fruits, sugarcane, and winter vegetables. Also, the temperatures were likely to stay below freezing for a few hours longer per episode than they did when wetlands were prevalent. The researchers report their findings in the Nov. 6 Nature.

"With the current landscape, there's a different input of heat and moisture into the atmosphere," says Pielke. The wetlands used to provide a buffer that absorbed the heat during the day and released it at night, he says. "The area is more susceptible now to freezes than it would have been in the late 1800s."

Others have studied the effects of major weather systems such as El Nino on Florida crop freezes, but the simulations provide a first look at the consequences of local land-use changes, says Pielke.

"It seems obvious that changing the surface properties really can change the local climate," says Eugenia Kalnay of the University of Maryland at College Park. "What's difficult is to quantify it." The new model enables researchers to do just that, she says.

Climatologist Jim O'Brien of Florida State University in Tallahassee contends that even if the encroachment of farmlands made local freezes slightly more likely, it's the big weather patterns bringing cold Canadian air to Florida that have the most severe impacts on crops.

Even if that is so, Pielke and Marshall say they hope forecasters realize that weather conditions may cause more-frequent and long-lasting cold spells than they did a century ago. Knowing this, they say, farmers could be better prepared to protect their crops from freezes.