Thu Oct 4, 2012, 06:27 PM
muriel_volestrangler (83,048 posts)
Melting permafrost CO2 may cause another 0.25C rise whatever we do now
Studying the permafrost carbon feedback is at once exciting (because it has been left out of climate models for so long) and terrifying (because it has the potential to be a real game-changer). There is about twice as much carbon frozen into permafrost than there is floating around in the entire atmosphere. As high CO2 levels cause the world to warm, some of the permafrost will thaw and release this carbon as more CO2 – causing more warming, and so on. Previous climate model simulations involving permafrost have measured the CO2 released during thaw, but haven’t actually applied it to the atmosphere and allowed it to change the climate. This UVic study is the first to close that feedback loop (in climate model speak we call this “fully coupled”).
The permafrost part of the land component was already in place – it was developed for Chris’s PhD thesis, and implemented in a previous paper. It involves converting the existing single-layer soil model to a multi-layer model where some layers can be frozen year-round. Also, instead of the four RCP scenarios, the authors used DEPs (Diagnosed Emission Pathways): exactly the same as RCPs, except that CO2 emissions, rather than concentrations, are given to the model as input. This was necessary so that extra emissions from permafrost thaw would be taken into account by concentration values calculated at the time.
As a result, permafrost added an extra 44, 104, 185, and 279 ppm of CO2 to the atmosphere for DEP 2.6, 4.5, 6.0, and 8.5 respectively. However, the extra warming by 2100 was about the same for each DEP, with central estimates around 0.25 °C. Interestingly, the logarithmic effect of CO2 on climate (adding 10 ppm to the atmosphere causes more warming when the background concentration is 300 ppm than when it is 400 ppm) managed to cancel out the increasing amounts of permafrost thaw. By 2300, the central estimates of extra warming were more variable, and ranged from 0.13 to 1.69 °C when full uncertainty ranges were taken into account. Altering climate sensitivity (by means of an artificial feedback), in particular, had a large effect.
This paper went in my mental “oh shit” folder, because it made me realize that we are starting to lose control over the climate system. No matter what path we follow – even if we manage slightly negative emissions, i.e. artificially removing CO2 from the atmosphere – this model suggests we’ve got an extra 0.25°C in the pipeline due to permafrost. It doesn’t sound like much, but add that to the 0.8°C we’ve already seen, and take technological inertia into account (it’s simply not feasible to stop all emissions overnight), and we’re coming perilously close to the big nonlinearity (i.e. tipping point) that many argue is between 1.5 and 2°C. Take political inertia into account (most governments are nowhere near even creating a plan to reduce emissions), and we’ve long passed it.
Via Open Mind (who just titled it "Oh Shit").
2 replies, 744 views
Melting permafrost CO2 may cause another 0.25C rise whatever we do now (Original post)
Response to muriel_volestrangler (Original post)
Thu Oct 4, 2012, 07:16 PM
AverageJoe90 (10,745 posts)
1. I filed it in the "Bad News, But Nothing to Wring Hands About", section.
Last edited Fri Oct 5, 2012, 12:57 AM - Edit history (1)
In fact, only +.25 *C is actually far less severe than what I thought it would be, if that pans out(though I don't think it's likely).
And to be honest, the climate system's been wacky for a little while now. But this doesn't mean that we can't still mitigate and even eventually reverse the warming trend; the biggest issues will be political action and what exactly is done.
I can say, though, that the longer we wait, the worst things will get, and we can't really afford to wait much longer, because how well global civilization endures by 2100 depends on what actions are taken.
Edit: Having looked at the graphs, it also seems that only the halting of emissions has been taken into account. It does not account for the possibility of Co2 removal, which would make a major difference.
Response to muriel_volestrangler (Original post)
Thu Oct 4, 2012, 09:54 PM
GliderGuider (20,665 posts)
2. You skipped over the money shot:
As if that weren’t enough, the paper goes on to list a whole bunch of reasons why their values are likely underestimates. For example, they assumed that all emissions from permafrost were CO2, rather than the much stronger CH4 which is easily produced in oxygen-depleted soil; the UVic model is also known to underestimate Arctic amplification of climate change (how much faster the Arctic warms than the rest of the planet). Most of the uncertainties – and there are many – are in the direction we don’t want, suggesting that the problem will be worse than what we see in the model.
And from the review on Skeptical Science:
Alert readers may have already noticed that this article has not yet used the word “methane”. When organic matter in the permafrost is thawed and decomposes it produces mostly CO2 but also small amounts of methane, particularly so in the wetlands that are prevalent in areas of thawing permafrost. Schuur and Abbott (2011) polled 41 experts on permafrost decay who estimated that about 3% of the carbon released from the permafrost will be in the form of methane. Methane has a restricted lifetime in the atmosphere, measured in decades, but while present in the air it has a greenhouse effect some 25 times that of CO2 over a 100-year period and higher values over shorter periods. According to Schuur and Abbott, the small amount of methane is responsible for approximately half of the warming effect from the permafrost emissions.
The UVic model does not simulate methanogenesis. That is to say that it does not model the generation of methane—all of the permafrost carbon that goes into the atmosphere in the model is in the form of CO2. This is a significantly conservative simplification over the time period studied.
Also, their model assumes only purely thermal degradation of the permafrost. Physical erosion, for example at coastlines, is not considered. Their model accounts only for permafrost down to a depth of 3.5 metres and there is plenty of carbon stored below those depths that was excluded from their modelling.
Finally, this study does not consider any contribution of methane from methane hydrates, either from under permafrost or under ice sheets, nor from fossil methane currently trapped under an impermeable seal of continuous permafrost.
Any model at this point that fails to account for methane and underplays the Arctic amplification isn't all that interesting.