“I think this idea of tipping points in the earth’s ecosystem is absolutely critical,” said Kyle C. Cavanaugh, a researcher with Brown University and the Smithsonian Environmental Research Center in Edgewater, Md., who led the new paper, released on Monday by the journal Proceedings of the National Academy of Sciences. “The changes in temperature can be pretty small, but once you cross a threshold, you can get rather dramatic changes in the ecosystem.”

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, Dr. 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.

The mangrove forests that fringe shorelines in the tropics are among the earth’s environmental treasures, serving as spawning grounds and nurseries for fish and as habitat for a wide array of organisms. Yet in many places, mangroves are critically endangered by shoreline development and other human activities.

So a climatic change that allows mangroves to thrive in new areas might well be seen as a happy development. Yet as they spread in Florida and elsewhere, the mangroves are displacing salt marshes, which are also ecologically valuable and also under threat from development. Their ecology is markedly different from that of mangroves, raising new questions about what will be lost if marsh grasses are killed off by the invading trees.