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.