Sunlight stimulates carbon dioxide release from melting Arctic ice, study finds
|February 11, 2013|
Over the last decade, arctic warming has been a hot topic. Passionate advocates on both sides of the argument debate whether and how dramatically arctic climate change will affect global temperatures and climate.
New evidence from a study led by Rose Cory, PhD, assistant professor of environmental sciences and engineering at Gillings School of Global Public Health, suggests that arctic warming is an even bigger global problem than previously thought. Cory has measured the effect of sunlight on the carbon long locked away in the frozen soils. Conversion of soil organic carbon to carbon dioxide gas has potential to double the amount of greenhouse gas in the earth’s atmosphere.
Cory’s study, “Surface exposure to sunlight stimulates CO2 release from permafrost soil carbon in the Arctic,” was reported online Feb. 11 in the Proceedings of the National Academy of Sciences [pdf].
Stores of organic carbon have been frozen for thousands of years in perennially frozen subsoils, called permafrost. If thawed and released as carbon dioxide gas, the effect could be similar to effects now seen from human use of fossil fuels. The likelihood of that doubling is great, given the circularity of the relationship: as the Earth warms from human release of heat-trapping gases into the atmosphere, the frozen arctic soils also warm, thaw and release more carbon dioxide. The release of carbon dioxide accelerates the Earth’s warming, which further accelerates the thawing of arctic soils and the release of even more CO2.
Permafrost soils may not thaw quietly, Cory says. The melting of ground ice is causing land-surface subsidence, or sinking, known as “thermokarst failures.” A thermokarst failure is generated when ice-rich, permanently-frozen soils are warmed and thawed, the ice melts, and the soil collapses and either creates an erosional hole in the tundra or, if the slope is sufficient, a landslide.
Thermokarst failures change the trajectory of the debate on the role of the Arctic in global climate. For example, the new, unanticipated outcome is that soil carbon will not be thawed and degraded directly in the soils, but instead, the carbon will be mixed upward from the depth of the ice and exposed to sunlight as the land surface fails. Sunlight – especially ultra-violet sunlight, the wavelengths that cause sunburn – can degrade the organic carbon directly into carbon dioxide gas, and sunlight can also alter the carbon to make it a better food for bacteria.
When bacteria feed on the carbon, they respire it into carbon dioxide, in much the same way people respire carbon in food and exhale carbon dioxide as a by-product. The gas released into the atmosphere from soils and bacteria traps heat near the Earth’s surface. Whether UV light exposure will enhance or retard the conversion of newly exposed carbon from permafrost soils has been until recently anybody’s guess.
“In this research,” Cory says, “we provide the first evidence that the respiration of previously frozen soil carbon will be amplified by reactions with sunlight (photochemical processes) and their effects on bacteria.”
Cory studied seven thermokarst failures in arctic Alaska and found that sunlight enhances the bacterial respiration of permafrost soil carbon by at least 40 percent, compared to carbon remaining in the dark. In contrast, sunlight photo-transformations of organic carbon already exposed in lakes and streams inhibited bacterial degradation.
These contrasting effects were attributed to light exposure history. Organic carbon released from deep permafrost has had little prior exposure to sunlight, while the carbon exposed to UV light in surface waters already has been degraded. Although scientists do not know how much permafrost carbon will be thawed or how fast it will be released, Cory’s study shows that this ancient soil carbon has an extremely high sensitivity to light and could be converted and released into the atmosphere as carbon dioxide gas at much faster rates than previously thought.
Cory’s fellow researchers and co-authors of the PNAS article are Byron C. Crump, PhD, associate professor in the Center for Environmental Science’s Horn Point Laboratory at the University of Maryland at Cambridge, and George W. Kling, PhD, Robert G. Wetzel Professor, and Jason Dobkowski, master’s student and researcher from Kling’s laboratory, both from the Department of Ecology and Evolutionary Biology at the University of Michigan at Ann Arbor.