The debate over global warming tends to focus on future perils — scary maps of flooded suburbs, the northward creep of tropical diseases, rich farmland turning into desert.
But some of the effects of global warming have already arrived in South Florida, as coastal cities flood more frequently and overheated corals turn white and die. The region's temperatures have not gone up, however, and many scientists say climate change has had little effect on hurricanes.
While most climate scientists agree the Earth has warmed over the past century, they say it's extremely difficult to assess the impact of slight temperature increases on complex natural systems.
"There is general consensus among scientists that climate change is occurring and that human activities are influencing that," said James W. Jones, director of the Florida Climate Institute at the University of Florida. "A lot of the controversy stems from the fact there's a lot of uncertainty about how much it's changing and how fast it's changing. … We need to go into the middle of this debate and not say nothing is going to happen and not say the sky is falling."
Sea level rise
Storm sewers spew water into the street. Rowboats glide through residential neighborhoods. Water laps over thresholds.
The most immediate, easily measured and incontrovertible impact of global warming on South Florida is a rise in sea levels that has already generated flooding in coastal cities.
The tide station in Miami Beach has registered an increase of seven inches since 1935, according to the National Oceanic and Atmospheric Administration. An older station in Key West has measured an increase of eight inches since 1920.
Higher temperatures raise sea levels because water expands as it warms, although the melting of polar ice packs is expected to accelerate the increase. With sea levels higher, storm surges at high tide have pushed water through storm sewer systems into the streets of eastern Fort Lauderdale, Hollywood and other coastal cities.
"Tides come up through the pipes and explode out of the grates," said Dr. John Golia, who lives in the Hendricks Isle section of Fort Lauderdale.
Although the conditions for increased flooding have existed for years, scientists say it's only in the past years that storms have arrived at the right tidal conditions to push water into neighborhoods.
Although South Florida is considered among the most vulnerable parts of the United States to climate change, the region has so far escaped the most obvious consequence: Higher air temperatures.
Temperatures have risen around most of the globe, according to data from NASA and the National Atmospheric and Oceanic Administration. But in Florida, as well as much of the southeastern United States, some weather stations report higher temperatures, others report lower ones, and most report little change.
"They're all over the map," said Misra Vasu, assistant professor of meteorology at Florida State University's Center for Ocean-Atmospheric Prediction Studies. "Most of the United States is warming slowly but in the southeastern United States there's a cooling trend over the past 50 years."
"That raises complacency that climate change is not affecting us so why should we care?" Misra said. "That's a dangerous attitude to take. Climate change has no borders."
Higher ocean temperatures have killed corals along the garland of reefs that stretch from Martin County to the Dry Tortugas, vital elements of the marine environment and major tourist draws that support fishing, diving and snorkeling.
A major coral die-off in the Keys took place after corals bleached, expelling the algae which give them color and nutrition, said Richard Dodge, director of the National Coral Reef Institute of Nova Southeastern University.
"It appears, from the limited record we have, that bleaching is occurring more and more and that a pretty logical conclusion is that bleaching is occurring because oceanic temperatures are warming up because of global warming," he said.
Also hit was Biscayne National Park, a popular fishing and diving destination that covers vast stands of coral in southern Biscayne Bay.
"We have seen an increase in coral die-offs from diseases and other stressors that come with a rise in water temperatures," said Mark Lewis, the park's superintendent.
The number of major hurricanes — defined as those with sustained winds of 110 mph or more — has more than doubled since 1995.
Warmer oceans produce stronger hurricanes, so this is a pretty clear result of global warming, right?
Maybe not. Chris Landsea, science and operations officer for the National Hurricane Center, said the slight increase in global temperatures is dwarfed by factors such as La Nina, the eastern Pacific cooling that nurtures Atlantic hurricanes.
Global warming has probably added 1 or 2 miles per hour to current hurricane wind speeds, he said. Although some scientists say global