New evidence suggests the carbon cycle has a natural release valve -- a built-in upper limit to the concentration of CO2 in the atmosphere.

Some 56 million years ago, millions of tons of carbon were dumped into the air and ocean during what's known as the Paleocene-Eocene Thermal Maximum. Scientists believe it's the closest natural precedent for current global warming.

But the ancient warming didn't last forever. That's because the geologic and atmospheric cycles that dictate Earth's climate have incorporated a thermostat of sorts.

Analysis of ocean floor sediments collected off the coast of Newfoundland suggest ancient rocks absorbed massive amounts of CO2 in the wake of Thermal Maximum -- evidence of the kind of carbon release valve scientists have theorized.

"It's long been thought that when the planet warms, as it did during the PETM, the rate of rock weathering on land, which absorbs CO2 from the atmosphere, increases," Donald E. Penman, a postdoctoral fellow and geologist at Yale University, said in a news release. "This draws down CO2 and cools the planet back down again."

Penman says this weathering process wasn't complete until excess carbon became buried in the ground, the evidence of which was found in sediment cores drilled from the bottom of the ocean.

"What our paper details is a pulse of carbonate burial in the aftermath of the PETM," Penman said. "We analyzed a sediment core in which, before the PETM, there is no carbonate at all, and then in the recovery phase of the event, it has lots of carbonate."

Penman is the first author of a new paper on the discovery, published this week in the journal Nature Geoscience.

For those worried about global warming, the discovery may sound like good news. While Penman and his colleagues believe man-made climate change will trigger a similar weathering-and-CO2-absorption cycle, the process won't happen quickly or soon enough to stave off the more devastating consequences of global warming.

"If the PETM is any guide, it will take tens of thousands of years," Penman said.