The Alaska permafrost is melting. Although you may not have heard of permafrost – hydrated soil that has remained below freezing 0°C (32°F) for two or more years – it is in actual fact a vanguard against one of the most potent greenhouse gases: methane. Huge amounts of organic carbon are locked up within this soil, and although microbes are keen to decompose it and trigger a release of methane gas, the normally frigid conditions prevent them from being able to do so.
We live in an increasingly warm world, though, and this methane is now sadly beginning to seep skywards. Now, scientists have shed some light on another major effect of the permafrost melting.
A new study published in the Proceedings of the National Academy of Sciences reveals that the organic material released during the melting isn't gradually converted into carbon dioxide, but immediately, which clearly has implications for how we understand climate change.
The melting soil contains organic material plant and animal residues, which microbes use to obtain energy through decomposition. During the energy conversion process, the microbes release a range of compounds back into the environment, including phosphorus and nitrogen. Normally, oxygen levels are low, and methane is released. Under relatively oxygen-rich conditions, carbon is also released in the form of carbon dioxide, which plants can normally use during photosynthesis, but much of which is released into the atmosphere.
The Northern permafrost soils are vast stores of carbon, holding twice that of the atmosphere. This study, led by researchers at Florida State University, is the first time that the rate of conversion of Alaskan permafrost-trapped organic carbon into carbon dioxide emissions has been directly measured and quantified.
The 35,800-year-old permafrost samples analyzed in the study were allowed to thaw under controlled laboratory settings mimicking current environmental conditions. After just 200 hours, over half of the trapped organic carbon was gone, decomposed and consumed by microbes, releasing the stored carbon back into the air as carbon dioxide gas.
“It's like feeding them chocolate,” said Robert Spencer, a Florida State University Assistant Professor in Earth, Ocean and Atmospheric Sciences in a statement. “You are giving them a food source that they really enjoy and is high in energy.”
The authors estimate that the Yedoma permafrosts will, under current thawing projections, release up to 10 million tonnes (roughly 11 million tons) of organic carbon every year, much of which will be converted into carbon dioxide.
This source of carbon dioxide emissions is occurring alongside the global spike in methane gas release. Methane doesn’t spend anywhere near as long a time in the atmosphere as carbon dioxide; however, while it is there, it is able to absorb and re-emit far more thermal radiation, which makes it incredibly dangerous.
As more methane and carbon dioxide are pumped into the atmosphere by the melting of the permafrost, the world will warm, precipitating even greater melting of the permafrosts. It's not clear if this will become a runaway effect, a so-called positive feedback cycle, but it's not exactly the sort of planetary-scale experiment we want to run to find out.