 The Green Swamp is prone to fog in winter.In the pre-dawn hours of January 8, 2008, smoke from a prescribed burn contributed to especially dense fog on Interstate 4 that caused a major pileup involving 70 vehicles in ten separate crashes that resulted in five deaths.A 2017 study by the Climate Impact Lab determined that among the 131 U.S. counties with more than 500,000 residents, Polk County would be the most economically impacted (as a percentage of GDP) by climate change by the end of the twenty-first century.