Almost one quarter of land-surface area in the northern hemisphere is covered by permafrost. Scientists estimate some 1,600 gigatons – one gigaton is equivalent to 1 billion tons – of carbon are fixed within this frozen ground, primarily in the form of organic matter.
By comparison, the atmosphere now contains around 850 gigatons of the element as carbon dioxide, about half the quantity trapped into permafrost soil.
Many scientists, now, fear permafrost could thaw and melt in a very short – geologically speaking – matter of time, and this huge amount of carbon be released into the atmosphere.
“Permafrost historically has served as a carbon sink, largely isolating carbon from participating in the carbon cycle,” says Andrew Jacobson, a professor of earth and planetary sciences at Northwestern University. His research about permafrost in Alaska is funded by the National Science Foundation (NSF) and the David and Lucile Packard Foundation. “However, global warming could transform the Arctic into a new carbon source by accelerating the rate of permafrost melting. This undoubtedly would have a dramatic effect on the global carbon cycle.”
Jacobson says the key concern is that permafrost carbon will oxidize to carbon dioxide as melting accelerates, causing a positive feedback to global warming. A vicious cycle is created as a warmer climate facilitates more carbon release, which in turn favors more warming.
While a logical first step for modeling global warming is quantifying carbon flow, unresolved complexities surrounding the Arctic carbon cycle make it difficult to create models for that element.
Jacobson and his team take a complementary approach by analyzing naturally occurring isotopes of other elements, such as calcium and strontium, which track permafrost melting and therefore provide insight into carbon release.
Initial data show that rivers and permafrost have distinctly different calcium and strontium isotope compositions.
When permafrost thaws during the summer and melts into rivers, the rivers show calcium and strontium isotope compositions that approach those for permafrost. Jacobson hypothesizes that in a warmer world, the permafrost signature in rivers will be more pronounced for longer periods of time.
Changes in the isotope composition of rivers can relate to changes in the release of carbon. So the calcium and strontium isotope composition of Arctic rivers can track the impact of warming on permafrost stability and carbon dioxide release.