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FORT COLLINS – Significant land-use changes in southern Florida may be leading to increased numbers of agriculturally damaging freezes in the region. According to a study reported in the Nov. 6 issue of the journal Nature, Colorado State University researchers show that the movement of agriculture from northern areas to southern Florida throughout the last century to escape risks of damaging winter weather has altered the local landscape, resulting in an increased frequency and severity of destructive crop freezes.
"We have shown that the likelihood of agriculturally damaging freezes in southern Florida has increased as a result of the conversion of its natural wetlands to agriculture, which is ironic considering that this move was instigated partly to avoid the freezes that occur farther to the north," said Roger Pielke Sr., Colorado State professor of atmospheric science, leader of the research project and immediate past president of the American Association of State Climatologists. "These results provide another example of the potential for humans to perturb the climate system in ways that can have socioeconomic consequences by altering the land surface."
Throughout the 20th century, southern Florida experienced significant land-use change as substantial areas of wetlands were drained for the cultivation of winter vegetable crops and sugar cane. During this time, much of the citrus industry also was relocated to those areas to escape the risk of freeze farther to the north. However, catastrophic cold-weather losses have continued to impact Florida agriculture in recent years despite improvements in freeze protection technologies and continued southward migration of production to less freeze-prone regions. Colorado State researchers are helping to explain why.
"The purpose of this research was to investigate whether anthropogenic landscape change in south Florida, and in particular the conversion of natural wetlands to agricultural land, could have an impact on the frequency and/or severity of damaging freeze events in that region," said Curtis Marshall, research scientist and lead author of the paper. "Our results suggest that the conversion of wetlands alone may have been a sufficient mechanism to exacerbate or even cause the damage inflicted upon agricultural production in south Florida during recent freeze events."
Researchers used the Regional Atmospheric Modeling System, or RAMS, developed at Colorado State to conduct the case study to determine inadvertent effects of draining wetlands in south Florida. RAMS is a comprehensive, proven meteorological modeling system that includes a sophisticated land-surface model scheme to represent the effects of surface properties on atmospheric processes. RAMS is used to investigate the impact on weather and climate of human-induced changes in land use.
The study focused in part on a January 1997 rare freeze that inflicted severe damage in agricultural areas of southern Florida that were once natural wetlands. This event resulted in losses in the fresh-vegetable and sugar-cane sectors that exceeded $300 million. Furthermore, nearly 100,000 migrant farm workers were displaced or unemployed as a result of the freeze.
For this and two other recent Florida freezes, a pair of simulations was conducted using RAMS: one that examined land surface properties of pre-1900s (near natural) land cover; the other that examined current land-use patterns. These two different land cover datasets detailed the impact of conversion of wetlands to agriculture in south Florida during the 20th century.
The RAMS results show that in key agricultural areas that once were natural wetlands, particularly the high-density areas used for cultivation of winter vegetables, sugar cane and citrus fruits to the south and southwest of Lake Okeechobee, minimum temperatures were colder and were below freezing for a longer period than compared to the area’s natural land coverage. Results revealed that the wetlands served as a barrier where the water held heat from the day, often keeping area temperatures above freezing throughout the night.
In other agricultural areas of southern Florida that once were natural wetlands, such as portions of the Kissimmee Valley north of Lake Okeechobee, RAMS indicated that a freeze could occur regardless of whether the wetlands had been converted to agricultural fields or not. However, even in these areas, temperatures were colder and were below 0 degrees Celsius for a longer period when converted to agricultural areas. The duration of exposure to subzero temperatures is critical in determining the amount of crop damage that occurs during a freeze.
"Our results indicate that, even in areas where a freeze would have occurred irrespective of land-surface properties, the agricultural damage may have been worse than it would have been if natural wetlands were still present in those areas," said Marshall.
Classes of land use used to specify model properties for the simulation that prescribed land-surface conditions before the 1900s (a) and in 1993 (b).
Difference between the model?s simulated minimum temperatures (in .C) near ground level on 19 January 1997; differences were determined as the values derived from 1993 usage minus those from pre-1900s usage. Locations inside the zero contour experienced colder minimum temperatures when 1993 land use was used in the model.
Difference in the duration (in min) of subzero-temperature periods for the two different model simulations; differences were determined as in c. Areas inside the zero contour experienced freezing temperatures for longer when 1993 land use was used in the model.