While most attention has been given to carbon dioxide, it isn’t the only greenhouse gas that scientists are worried about. Carbon dioxide is the most important human-emitted greenhouse gas, but methane has also increased in the atmosphere and it adds to our concerns.
While methane is not currently as important as carbon dioxide, it has a hidden danger. Molecule for molecule, methane traps more heat than carbon dioxide; approximately 30 times more, depending on the time frame under consideration. However, because methane is present in much smaller concentrations (compared to carbon dioxide), its aggregate effect is less.
But what has scientists focusing on methane is the way it is released into the atmosphere. Unlike carbon dioxide, which is emitted primarily through burning of fossil fuels, methane has a large natural emission component. This natural emission is from warming permafrost in the northern latitudes. Permafrost is permanently frozen ground. Much of the permafrost is undisturbed by bacterial decomposition.
As the Earth warms, and the Arctic warms especially fast, the permafrost melts and soil decomposition accelerates. Consequently, an initial warming leads to more emission, leading to more warming and more emission. It is a vicious cycle and there may be a tipping point where this self-reinforcing cycle takes over.
Recently, a policy briefing from the world-leading Woods Hole Research Center has moved our understanding of this risk further through a clearly-written summary. The briefing cites two recent papers (here and here) that study the so-called permafrost carbon feedback.
One of these studies makes use of projections from the most recent IPCC report to estimate that up to 205 gigatons equivalent of carbon dioxide could be released due to melting permafrost. This would cause up to 0.5°C (up to 0.9°F) extra warming. Just as bad, the permafrost melting would continue after 2100 which would lock us into even more warming. Under this scenario, meeting a 2°C limit would be harder than anticipated. The current IPCC targets do not adequately account for this feedback.
To put this in perspective, permafrost contains almost twice as much carbon as is present in the atmosphere. In the rapidly warming Arctic (warming twice as fast as the globe as a whole), the upper layers of this frozen soil begin to thaw, allowing deposited organic material to decompose. The plant material, which has accumulated over thousands of years, is concentrated in to upper layers (half of it is in the top 10 feet). There is a network of monitoring stations that are measuring ground temperatures have detected a significant heating trend over the past few decades and so has the active layer thickness.
I communicated with Woods Hole expert Robert Max Holmes, who told me,
Posted by John Abraham on Tuesday, 13 October, 2015
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