Much of the Florida shoreline was once too cold for the tropical trees called mangroves, but the plants are now spreading northward at a rapid clip, scientists say. That finding is the latest indication that global warming, though in its early stages, already is leading to ecological changes so large they can be seen from space.
Along a 50-mile stretch of the central Florida coast south of St. Augustine, the amount of mangrove forest doubled between 1984 and 2011, the scientists found after analyzing satellite images. They said the hard winter freezes that once kept mangroves in check had essentially disappeared in that region, allowing the plants to displace marsh grasses that are more tolerant of cold weather.
In one respect, the situation resembles the change in climate that has allowed beetles to ravage millions of acres of pine trees in the U.S. West and Canada, and more recently to gain a foothold in New Jersey.
In the beetle and mangrove cases, scientists have found that it is not the small rise in average temperatures that matters, nor the increase in heat waves. Rather, it is the disappearance of bitter winter nights that once controlled the growth of cold-sensitive organisms.
“I think this idea of tipping points in the Earth’s ecosystem is absolutely critical,” said Kyle C. Cavanaugh, a researcher at the Smithsonian Environmental Research Center in Edgewater, Md., who led the research for the new paper, released by the journal Proceedings of the National Academy of Sciences.
Though scientists have long warned of the potential environmental consequences of unchecked global warming, the pace and scale of some recent developments have surprised them, given that the Earth has warmed by only about 1.5 degrees Fahrenheit since the 19th century. It is expected to warm substantially more than that over the coming century.
Yet already, Cavanaugh said, “the changes are happening faster than we expected.”
The northward spread of mangroves poses a more complicated set of ecological questions, however, than some other changes linked to global warming, such as the deaths of pine forests or coral reefs.
“We can’t put a price tag or a value on what is happening,” said Daniel S. Gruner, a biologist at the University of Maryland who took part in the research. “We’re not saying it’s good or bad. It’s just what the data show.”
