It appears that, when Arctic permafrost melts, it can release carbon dioxide gas into the atmosphere much faster than previously thought.
A University of Michigan team studied places in Arctic Alaska where permafrost is melting and is causing the overlying land surface to collapse, forming erosional holes and landslides and exposing long-buried soils to sunlight.
And, they found, sunlight boosts the bacterial conversion of exposed soil carbon into carbon dioxide gas by at least 40 percent.
"Until now, we didn't really know how reactive this ancient permafrost carbon would be - whether it would be converted into heat-trapping gases quickly or not," says University of Michigan ecologist and aquatic biogeochemist George Kling.
"What we can say now is that regardless of how fast the thawing of the Arctic permafrost occurs, the conversion of this soil carbon to carbon dioxide and its release into the atmosphere will be faster than we previously thought. That means permafrost carbon is potentially a huge factor that will help determine how fast the Earth warms."
Enormous stores of organic carbon have been frozen in Arctic permafrost soils for thousands of years. If thawed and released as carbon dioxide, it could double the amount of CO2 in the atmosphere.
Already, the melting of ground ice is causing land-surface subsidence features called thermokarst failures. What's new about this latest study is the discovery that soil carbon won't be thawed and degraded directly in the soil; instead, it will be mixed up and exposed to sunlight as the land surface fails.
Sunlight - especially ultraviolet radiation - can degrade organic soil carbon directly to carbon dioxide gas, and can also alter the carbon to make it a better food for bacteria. When bacteria feed on this carbon, they respire it to carbon dioxide.
"Whether UV light exposure will enhance or retard the conversion of newly exposed carbon from permafrost soils has been, until recently, anybody's guess," says Rose Cory of the University of North Carolina. "In this research, we provide the first evidence that the respiration of previously frozen soil carbon will be amplified by reactions with sunlight and their effects on bacteria."