For a while now, we’ve been hearing that greenhouse gas emissions are still off the charts, which is to say increasing beyond the U.N.’s worst-case scenario for global warming. Now a Stanford researcher has laid out some specific scenarios–and they’re not pretty.

Chris Field, who is working on the next IPCC report, said “There is a real risk that human-caused climate change will accelerate the release of carbon dioxide from forest and tundra ecosystems, which have been storing a lot of carbon for thousands of years.”

Field, a professor of biology and of environmental Earth system science at Stanford, and a senior fellow at the Woods Institute for the Environment, issued a warning for members of the American Association for the Advancement of Science (AAAS) in Chicago today: “We don’t want to cross a critical threshold where this massive release of carbon starts to run on autopilot.”

And yet, that would appear to be path that we’re on. As Field told the AAAS symposium, “We now have data showing that from 2000 to 2007, greenhouse gas emissions increased far more rapidly than we expected, primarily because developing countries, like China and India, saw a huge upsurge in electric power generation, almost all of it based on coal.”

So what would some of the consequences be? “Tropical forests are essentially inflammable,” Field said. “You couldn’t get a fire to burn there if you tried. But if they dry out just a little bit, 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. Essentially we could see a forest-carbon feedback that acts like a foot on the accelerator pedal for atmospheric CO2.”

The loss of functioning forests worldwide is already estimated to account for about 20% of carbon emissions. But field also warns of another carbon burst from decomposed plants that have been locked in permafrost for tens of thousands of years. As if all that weren’t plenty, Field says the accelerated forest destruction and melting permafrost could combine to create a “vicious cycle” of accelerated carbon emissions.

Field sums up by saying: “We now know that, without effective action, climate change is going to be larger and more difficult to deal with than we thought.”

The Chicago symposium is being held to address new developments since the last IPCC interim report, in 2007. A formal update is due out next year. Field is co-chair of the IPCC’s Working Group 2, which is assessing the impacts of climate change on social, economic and natural systems.