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The prospect that human-driven warming is poised to push Earth past dangerous tipping points is now a cornerstone of many environmental campaigns. But what tipping points are well established and which ones remain what Stephen W. Pacala of Princeton University has called "the monsters behind the door"? I have a piece in the Week in Review section exploring these concerns. Given the limits on space in print, I thought it worthwhile to add some additional voices here and encourage further discussion. The bottom line? A growing effort to clarify such risks has yielded what amounts to the same message climate experts have been conveying for more than two decades: More emissions of greenhouse gases raise the odds of trouble.

Field said that if the tropics became dry enough for fires to break out, tropical forests would pass a "tipping point" from absorbing carbon dioxide from the atmosphere to releasing it. "Tropical forests are essentially inflammable. You couldn't get a fire to burn there if you tried. But if they dry out just a little, the result can be very large and destructive wildfires. It is increasingly clear that as you produce a warmer world, lots of forested areas that had been acting as carbon sinks could be converted to carbon sources," he said. The result could lead to runaway warming.

Holdren said temperatures should be kept from rising more than 3.6 degrees. To get there, he said the U.S. and other industrial nations have to begin permanent dramatic cuts in carbon dioxide pollution by 2015, with developing countries following suit within a decade. Those efforts are racing against three tipping points he cited: Earth could be as close as six years away from the loss of Arctic summer sea ice, he said, and that has the potential of altering the climate in unforeseen ways. Other elements that could dramatically speed up climate change include the release of frozen methane from thawing permafrost in Siberia, and more and bigger wildfires worldwide.

The world has only six years left to limit greenhouse gas emissions, Pachauri warns. "It's essential that we take action by which we allow emissions to peak no later than 2015," he says, to limit the world's temperature increase to 2C. Beyond that and we reach a tipping point when the world's poorest communities will suffer the most. "They are the ones who are the most vulnerable" due to a much greater scarcity of water, a decline in agricultural lands, and the danger of sea level rises, as spelt out in the fourth assessment report by the UN Intergovernmental Panel on Climate Change.

Without decisive action, global warming in the 21st century is likely to accelerate at a much faster pace and cause more environmental damage than predicted, according to a leading member of the Nobel Prize-winning Intergovernmental Panel on Climate Change.