Arguments
Methane release from melting permafrost could trigger dangerous global warming
Posted on 13 October 2015 by John Abraham
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,
John,
The headline is misleading, since the permafrost warming effect is largely one coming from carbon released as CO2, not methane. As the soil thaws more methane can be formed, but also more methane will be consumed in the topsoil layer by methanotrophs. The text also does not consider the additional carbon taken up in permafrost regions as a result of changing vegetation, which is believed to counteract the soil carbon release due to increasing LAI and length of growing season.
Furthermore, the text
is also misleading because the natural CO2 sources to the atmosphere are also much larger than teh fossil fuel source. Lay people will misinterpret this. What you actually meant is that excess CO2 on top of the natural carbon cycle is driven by fossil fuel emissions (roughly 90%), while excess methane (over the preindustrial methane cycle) has a larger variability of sources.
I agree that the headline is misleading. The permafrost feedback is mostly a CO2 story. Methanotrophs may well gobble up some or most of the methane in some soils, but in saturated, anoxic areas, methane will (and currently does) make it into the atmosphere. Increased vegetation growth rates probably make the Arctic a carbon sink for now, but most models do not see the vegetation keeping up once the permafrost starts to thaw in earnest. See my SkS piece on the recent Schuur et al review paper for more details.
I agree also that the text quoted by gws is potentially misleading. Anthropogenic methane emissions are about the same magnitude as natural biochemical sources. Fossil fuel emissions, in the US at least, account for about 40% of the anthropogenic methane emissions, whereas fossil fuels make up about 94% of the CO2 emissions. EPA
I do not agree about the comments mentioning a misleading "headline". "Methane release from melting permafrost could trigger dangerous global warming" whatever is the cause of the release, seems real. Perhaps "trigger" might have be changed to "enhance" or "contribute". But my English is not good enough to conclude.
I do not understand precisely "Lay people will misinterpret this" but I think this paper only deals with the potential increase of GHG emissions and the stock of CH4 in permafrost. As well as it would not be wise to extract all coal, gas and oil, or to emit all stocks of CH4 captured in the permafrost.
The author is quite right for the following reasons:
1. When permafrost melts on flat land it causes a waterlogged landscape because underlying, unmelted permafrost prevents drainage and the result is anoxic conditions. In those conditions methanogens thrive on decaying biota and produce CH4, which passes through a shallow water column or vents directly to the atmosphere with little or no oxidation.
2. Methanogens are active in sub-zero conditions and are able to produce CH4 from surface and subsurface biota prior to permafrost thawing and this gas may accumulate in frozen soils until they thaw when CH4 is released, again with little or no oxidation. Methanotrophs may be present but in the absence of sphagnum moss are likely to be active on the surface rather than in the soil.
3. The article refers to permafrost melting in the north which presumably includes the Arctic continental shelf – a vast offshore area mostly covered by seawater <50m deep and a water column too short for oxidation of methane produced from decaying biota washed onto the seabed surface by the great Siberian rivers. The result is CH4 supersaturated seawater and significant venting to the atmosphere.
4. Biota and CH4 gas are trapped in and under the permafrost cap which covers most of the continental shelf seabed and this is decaying, resulting in the release of CH4 from depths, at present in excess of 50 metres beneath the seabed surface where silts are thawing more rapidly due to salt content. Warming of waters covering the Arctic continental shelf have been shown to be warming at ~1°C/decade since 1980, a process likely to accelerate, together with the rate of CH4 emissions, due to Arctic Ocean warming.
The threat from Arctic CH4 emissions associated with permafrost decay may not be large or abrupt – yet – but it is unquestionably a positive feedback. It has the potential to result in dangerous levels of CH4 accelerating Arctic amplification and global warming. What should be of equal concern is that this process has been initiated by anthropogenic greenhouse gas emissions but, once initiated is beyond human control, other than by untested and possibly dangerous geo-engineering.
Where Dr Abraham and I differ is in regard to the power of CH4, stated to be 30 times greater than CO2. I assume this refers to the GWP of CH4 over a 100 year period. However CH4 only has a life of ~12 years in the atmosphere - though this is increasing. The point I make is that in conditions where the presence of CH4 is increasing (it has increased by over 250% since 1800) it is more appropriate to refer to its GWP over a 20 year period which is now 85.
It adds some gravity to the problem posed by Arctic CH4 emission growth don’t you think?
Riduna, I do not question the "potential" (possibility) for a large methane release. There certainly is enough carbon making a potentially large emission of methane with a short time frame (10s-100s of years) possible. The question is, how likely is that? Current knowledge, AFAIK, suggests not very. We have yet to see representative measurements that would suggest that current or upcoming warming is likely to lead to this scenario. Is there is evidence for large outgassing from continental shelves, or evidence of large outgassing from lakes? If so, it has not yet affected atmospheric levels.
So far, atmospheric methane is increasing again in midlatitudes, not high latitudes. You said it yourself, "The threat from Arctic CH4 emissions associated with permafrost decay may not be large or abrupt ...". I agree. I also agree that the potential is worrying, but it is one of those worst case, low probability, high impact scenarios. We do not want it to happen, but we should also not present it as something that is very likely to happen. Few people like to be scared by unlikely scenarios.
We do need to work toward minimizing that worst potential. The "fat tail" as it is sometimes called, is not unimportant. It can be useful for certain audiences to work the fat tail (e.g. in insurance calculations), but I think it is generally more useful in working with people to consider the more likely scenarios.