Call it the gooey broccoli theory of climate change.

Buried in the ground across the Far North are billions of tons of plant matter deposited over thousands of years since the last Ice Age. All that organic stuff won't decay and release its greenhouse carbon into the air as long as it remains frozen within the region's extensive permafrost, the frozen layer of earth that sometimes reaches 1,000 feet deep on Alaska's North Slope.

Think of all that prehistoric litter as so much broccoli stored safely inside the freezer, says University of Colorado scientist Kevin Schaefer. Tasty stuff -- as long as it's not taken out, de-thawed and forgotten about.

"As long as it stays frozen, it stays stable for many years," Schaefer said. "But you take it out of the freezer and it will thaw out and decay."

The Arctic's permafrost and its vast store of frozen vegetable matter have now begun to melt in response to the warming climate, Schaeffer said. "And, just like the broccoli, the biomass will thaw and decay, releasing carbon into the atmosphere like any other decomposing plant material."

In a study that calculates what will happen if Arctic warming trends continue as predicted, Schaeffer and his co-authors estimate that almost two-thirds of the Earth's permafrost could melt by 2200, releasing vast stores of carbon-based greenhouse gases into the air, almost certainly accelerating climate warming in the Arctic and across the globe. The study was published in February in the journal Tellus by researchers at the Cooperative Institute for Research in Environmental Sciences and the National Snow and Ice Data Center, and received extensive news coverage across the globe.

It's as though an Arctic carbon bomb -- built from organic matter locked up in frozen ground over millennia -- is now ticking and about to detonate, the scientists explained in this summary.

The coming meltdown will dramatically accelerate the accumulation of greenhouse gases like carbon dioxide and methane.

"The amount of carbon released is equivalent to half the amount of carbon that has been released into the atmosphere since the dawn of the industrial age," Schaefer said. "That is a lot of carbon."

The take-home lesson? The danger posed by thawing permafrost must trigger much more aggressive international efforts to reduce human emissions of greenhouse gases before it's too late.

"If we want to hit a target carbon concentration, then we have to reduce fossil fuel emissions that much lower than previously calculated to account for this additional carbon from the permafrost," Schaefer said. "Otherwise we will end up with a warmer Earth than we want."

The latest updates on the status of Arctic climate don't bode well for that goal. Arctic sea ice covered only about 5.54 million square miles in February -- tying with 2005 for the smallest ice pack observed in that month since satellite monitoring began in the late 1970s.

Last month's ice cover was about 500,000 square miles below the 1979-2003 average, according to a March 2 update by the snow and ice data center. As of this winter, an area almost as large as Alaska has disappeared from the ice pack.

With such climate change continuing to impact the Arctic, Schaeffer and his coauthors calculated what would happen to permafrost under various predictions of future warming by the Intergovernmental Panel on Climate Change.

They found that permafrost areas will shrink by 29 to 59 percent over the next two centuries, and the layer of permafrost that seasonally thaws will expand 20 to 38 inches deeper into the ground. As all this material turns to muck and begins to rot, more carbon will ooze into the air, which will trigger yet more warming due the greenhouse effect -- and in turn, trigger even more thawing.

"The thaw and release of carbon currently frozen in permafrost will increase atmospheric CO2 concentrations and amplify surface warming to initiate a positive permafrost carbon feedback on climate," they wrote in the paper's abstract. "We predict that (this feedback) will change the Arctic from a carbon sink to a source after the mid-2020s."

The Arctic will then be spewing so much carbon dioxide and methane into the air that it will cancel out much of the rest of the globe's natural carbon uptake.

"The thaw and decay of permafrost carbon is irreversible and accounting for the (permafrost carbon feedback) will require larger reductions in fossil fuel emissions to reach a target atmospheric CO2 concentration," they wrote.

The amount of carbon that might be released is enormous. The scientists calculated that the coming permafrost meltdown could deliver an extra 190 gigatons -- plus or minus 64 gigatons -- into the air before the end of the century.

To put a billion tons of carbon into perspective, consider this description by Dutch climate scientist Pavel Kabat. "A single gigaton of emissions is roughly equivalent to putting 142,857,142 African elephants into the atmosphere every year (or enough to stretch elephants from the earth to the moon and halfway back again)," wrote to the Salzburg Global Seminar blog. "It's also greater than the weight of every human being on the planet. All 6-plus-billion of us."

During the next 190 years, this colossal amount of additional greenhouse gas will equal about one-fifth of all the carbon in the atmosphere now, and half again as much as all the carbon released by human activity since the beginning of the Industrial Age two centuries ago.

"It means the problem is getting more and more difficult all the time," Schaeffer said. "It is hard enough to reduce the emissions in any case, but now we're saying that we have to reduce it even more."