The effect of permafrost on the ZEC

I’ve written a number of posts about warming commitments, in particular the zero emission commitment (ZEC). In other words, how much additional warming will there be after emissions get to zero. The answer is that on multi-decadal timescales [the ZEC] is close to zero. A common response to this is what about permafrost? It is true that the estimates typically don’t include the potential impact of release from permafrost, so it is a reasonable question.

However, the lead author (Andrew MacDougall) of the recent paper that estimated the ZEC has used one of the models to try and assess the effect of the permafrost carbon feedback on the zero emissions commitment. They carried out a perturbed parameter experiment (i.e., run the same model many times, but vary the parameters), and considered scenarios where 1000 PgC is emitted before emissions go to zero, and another where 2000 PgC is emitted (for context, total emissions to date is about 600 PgC).

Credit: MacDougall (2021)

There were quite a lot of details that I won’t go into, but the main result is presented in the above figure, which shows how permafrost infuences the ZEC in the 1000 PgC runs (left-hand panel) and 2000 PgC runs (right-hand panel). In other words, how much extra warming is there after emissions go to zero when compared to equivalent runs that don’t consider permafrost.

The basic result (see Table below) is that on multi-decade timescales, the effect is small (probably less than 0.1oC) but that it will increase with time, possibly adding a few tenths of a oC after 500 years. For reference, the temperature increase in the 1000 PgC run when emissions go to zero is about 1.5oC, so after a few hundred years we could have warmed by ~2oC. This isn’t great, but this additional warming would be relatively slow and it’s still considerably smaller than the baseline warming (i.e., most of the warming we’re likely to experience is due to our emissions, not due to these feedbacks).

Credit: MacDougall (2021)

The analysis also suggests that the impact doesn’t depend strongly on cumulative emissions; the effect is similar for the 1000 PgC and 2000 PgC runs. This is mostly because the linear relationship between emissions and warming breaks down in these models when cumulative emissions are high. There are, however, indications that this may not be a robust results and, hence, it is possible that the effect will continue to increase with increasing emissions.

Also, even this study doesn’t consider abrupt thaw, which could accelerate permafrost processes over the next few centuries. This could further increase the permafrost, but probably by 10s of percent. This probably won’t change the basic result. The effect of permafrost on the zero emission commitment (ZEC) is probably small on multi-decade timescales. Hence, this doesn’t change that the ZEC is probably close to zero on these timescales.

On longer timescales (centuries) the effect of permafrost will probably increase, but is probably still going to be much smaller than the warming due to direct anthropogenic emissions. It could increase warming from 1.5oC to ~2oC over a period of a few centuries, but it seems unlikely that it will commit us to much more than 2oC if we are able to get emissions to zero well before we cross that warming threshold. This isn’t necessarily great news, but it still indicates that the dominant factor in determining how much we will warm is how much we end up emitting.


Warming commitments – Posts I’ve written about our warming commitments.
Is there warming in the pipeline? A multi-model analysis of the Zero Emissions Commitment from CO2 – MacDougall et al. (2020) paper that discusses the zero emission commitment (ZEC).
Estimated effect of the permafrost carbon feedback on the zero emissions commitment to climate change – MacDougall (2021) paper that estimates how permafrost effects the ZEC.

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36 Responses to The effect of permafrost on the ZEC

  1. I’d add – first on the positive side (see tweet figures below), :
    • all the IPCC AR6 WGI >50% probability remaining carbon budgets for less than 2.0°C aim for less than 1,000 GtC (3,667 GtCO₂) in MacDougall’s first experiment (except the 50%/2.0°C combo, which is is ~1,000 GtCO₂. So, we *could* see even less permafrost carbon feedback, if we get to zero emissions quickly.
    • although the post-100 yr cumulative permafrost carbon emissions are projected at ~100 GtC, this needs to be contrasted to potential deliberate anthropogenic negative emissions over the next several centuries. The ZECMIP model runs anticipate a fairly sharp drawdown of atmospheric CO₂ in the first century after zero-emissions of about 80-110 ppm, but this is only from natural (mostly ocean) processes. Any deliberate human interventions of reforestation, soil carbon restoration, direct air capture would be additional and could go to offset the permafrost carbon feedback.

    On the “negative” side, I think this now means that 3 or the 18 ESM and EMIC models in the earlier ZEC model intercomparison explicitly show little additional warming due to permafrost carbon after zero-emissions. But the other 15 are still sort of black boxes on this feedback. But looking good so far!

  2. Rust,
    Good points, thanks.

    However, I still think this isn’t quite what we will experience.

    The ZECMIP model runs anticipate a fairly sharp drawdown of atmospheric CO₂ in the first century after zero-emissions of about 80-110 ppm

    The reason being that – in reality – some of this has already been realised. The long-term airborne fraction is about 25%. Given that we’ve emitted about 600GtC, 25% would be 150GtC, which is about 70 ppm. So, if we were to stop emitting now, atmospheric CO2 would be drawn down from ~420ppm to about 350ppm on a timescale of ~200 years. I think the big drop in atmospheric CO2 you’re referring to is based on a scenario where the emissions have been fast enough that little has been taken up while emissions are occuring (i.e., the AF is close to 100% when emissions cease).

  3. My point about the Table 2* estimates of permafrost carbon feedback of up to 346 GtC over 500 yrs (in the A1 experiment) was just that this could be offset at a much more manageable anthropogenic negative emissions rate (especially for nature-based solutions, which are limited by the annual biologic carbon fixing, etc.)

    *I may have missed or cross-posted, but it is also in Ken’s main post.

  4. Rust,
    Yes, that is a good point. Emissions of ~1GtC per year from permafrost will probably be easier to manage through negative emission technologies (NETs) than if we get to ~20GtC of anthropogenic emissions per year and expect a substantial fraction of that to be removed using NETs.

  5. Rust,
    I should have acknowledged that I added the table after seeing your tweet 🙂

  6. Well, on the atmospheric drawdown, I am actually estimating that from the plots (a, c) from the 2020 MacDougall, et al., ZECMIP paper.

    I am quite possibly mistaken, but they seem to be for the first 100 years following a 1000 GtC cumulative carbon emissions perturbation. But the annual emissions are increased by 1%/yr until 1000GtC is achieved. So this presumably would already be including a significant historical equilibriation (decay) of the atmospheric CO₂.

    Now, yes, they drop emissions to zero overnight, so our mitigation pathways to 1.5-2.0°C – nevertheless very steep – would already account for some decay when zero-emissions are achieved. But I don’t think a 1%/yr emissions increase experiment would yield an AF (airborne fraction) of anywhere near 100? Would it?

    I’d welcome being corrected here.

  7. Rust,
    Okay, I had forgotten that those are 1000 GtC scenarios, so the overall drawdown would be > 70 ppm. It would still be interesting to know what the airborne fraction is when emissions cease. Since they’re 1% runs, the emissions take place over a period of 70 years, so it may well be higher than the 0.45 we currently have, but may not be as high as the ~100% I suggested above.

    If we do get to 1000 GtC with an AF of 0.45, then atmospheric CO2 would be about 490 ppm. If the long-term AF is 0.25, then if emissions stop when 1000 GtC has been emitted, then 490 ppm would drop down to about 400 ppm over a timescale of ~200 years. Okay, so maybe the 80 – 110 ppm over a timescale of a century, or so, is reasonable.

  8. Greg Robie says:

    This focus on emissions as what is of interest/pertinent seems to be overlooking what the loss of the latent heat of ice means, and this in our rapidly evolving climate dynamics. In the long term (modeling strengths, the emission questions are valuable. That value is discounted by the assumptive weakness of current models: they don’t – & can’t – do tipping.

    I think of this as a Nordhaus v Weitzman thing on the economic side of this matter … and which got the Nobel (& when!). What might be the substance of this analysis if the scientific boundary regarding climate policy matters was still 1°C?

    Anyway, the sea ice version of the lost latent heat of ice seems to be better understood than the land-based permafrost warming and loss one. Sea bed permafrost seems (pun intended) glacial in it role as a forcing.

    Setting aside the emission contribution of permafrost decline, it is the seasonal impacts of the annual reserve of cool these declines constitute that are yet to be robustly incorporated into the models. This is like the Inuit observations of seasonally increased Arctic refration that are unmodelable.

    Is it possible that the few models that do not confirm that “so far so good” punchline has pertinent … but for motivated reasoning? The joke this is the punchline to is the answer given an interviewer by a free falling person about how their 100 story plummet feels as they pass the 70th floor.

    Just asking (actually saying – AGAIN!).


    sNAILmALEnotHAIL …but pace’n myself

    life is for learning so all my failures must mean that I’m wicked smart


  9. Greg,
    I don’t really see why the latent heat of ice is that relevant. Once it’s been released, it gets radiated away. The more relevant would be reductions in albedo due to the sea being darker than the ice.

  10. Greg Robie says:

    Perhaps this can change the perception? Imagine that 50% of the last ice ages’ crysophere was still part of today’s Anthropocene’s climate system. Wouldn’t we still be in the Holocene?

    Don’t our trusted models sort of “think” like this? Isn’t the thing they are ‘best’ at not getting right are our planet’s polar crysopheres? If so, then I would think it intellectually honest to be exceptionally vigilant about what the models get wrong relative to the cryosphere and observations.

    Also, just saying …

    …and it is not to0 late to call for a virtual academic conference to develop a zero fossil carbon business plan for Academia during COP 26. At this point I would suggest doing so using Open Space Technology.

    sNAILmALEnotHAIL …but pace’n myself

    life is for learning so all my failures must mean that I’m wicked smart


  11. Willard says:

    Enough peddling, Greg.

  12. Ben McMillan says:

    I guess the climate pessimists are worried about whether permafrost and other things could act as a feedback mechanism that makes the system locally unstable at high levels of forcing.

    Looking at the article, that doesn’t seem to be a concern for permafrost because the warming commitment is not dependent on how much carbon humans emit. It sounds like it is because things are so deep in the nonlinear regime that even if the permafrost carbon emissions double when human emissions double, the temperature rise due to the permafrost ends up being the same, as the climate sensitivity is lower in the 2000PgC world. (although they suggest a full earth system model would actually be more linear)

    But also, A1 and A3 have the same emissions pathway until 2067 so it isn’t that surprising that they don’t differ very much after 100 years.

    This paper explicitly excludes methane release, and abrupt thaw mechanisms, so I guess it isn’t really addressing the concerns of the “methane bomb” worriers.

  13. Ben,
    Even the author suggests that we should treat the result that the permafrost effect is similar for the 1000 PgC and 2000 PgC scenarios with some caution. In their model the linear relationship between emissions and warming breaks down and the logarithmic dependence of the forcing on changes in CO2 dominates. However, more complex models suggest that the linear relationship holds to much higher emissions levels. If so, then I think the permafrost effect for the 2000 PgC scenario would be about twice that of the 1000 PgC scenario.

  14. Ben,

    This paper explicitly excludes methane release, and abrupt thaw mechanisms, so I guess it isn’t really addressing the concerns of the “methane bomb” worriers.

    I don’t think this is quite the methane bomb scenarios.

    It does say

    To date no Earth system model (McGuire et al., 2018) accounts for abrupt thaw processes in permafrost systems. These processes, including thermokarst production, active hill slope erosion, and coastal erosion, could accelerate thaw processes by 40 % over the next few centuries (Turetsky et al., 2020).

  15. Ben McMillan says:

    I’m assuming that as usual abrupt processes are subject to very high levels of uncertainty, so it all goes in the category of ‘poorly understood tail risk’. i.e. it is a bit like sea level rise…

    “Permafrost suddenly thaws and releases a lot of methane” is one of the things that gets thrown around as a “nightmare scenario” (not that there is good evidence that this is likely to happen), and as a reason that we might have a large and rapid warming commitment.

  16. is the McDougal et al paper consistent or contradictory with regard to the 2020 Turetsky et al paper? Turetsky says “The permafrost zone is expected to be a substantial carbon source to the atmosphere, yet large-scale models currently only simulate gradual changes in seasonally thawed soil. Abrupt thaw will probably occur in <20% of the permafrost zone but could affect half of permafrost carbon through collapsing ground, rapid erosion and landslides… After considering abrupt thaw stabilization, lake drainage and soil carbon uptake by vegetation regrowth, we conclude that models considering only gradual permafrost thaw are substantially underestimating carbon emissions from thawing permafrost."



  17. Did you *read* MacDougall’s paper, Mike? Where he talks specifically about Turetsky’s work on abrupt thaw?

    Anyway, maybe an allowance of an additional 40% due to unmodelled abrupt permafrost thaw.

    Which makes sense, because in Turetsky’s paper – which I’m *sure* you’ve read! – they look at a range of “net ecosystem carbon balance” loss of ~25-80 GtC over the next 300 years (twitter pic below). Compare to the MacDougall A1 and A3 estimates for 100 and 500 years.

    Turetsky specifically says “As abrupt thaw is not simulated in any Earth system model, it remains an unresolved Earth system feedback to climate change from a climate policy perspective. Our results suggest that abrupt thaw over the twenty-first century will lead to a CO2 feedback of 3.1 PgC per °C global temperature increase and a CH4 feedback of 1,180 TgC per °C global temperature increase under RCP8.5. Over the longer period to 2300, we estimate abrupt thaw feedbacks of 7.2 PgC CO2 per °C increase and 1,970 TgC CH4 per °C increase. These estimates suggest that the CO2 feedback from abrupt thaw is modest but strengthens beyond the twenty-first century. In contrast, our estimates of the abrupt thaw CH4 feedback are more substantial and vary less over time…”

  18. I skimmed MacDougall, but it’s not written for a lay person like myself. I can’t make heads or tails of all the acronyms and formulas. I am interested in the content and meaning, but not all of us have the math and science background to unpack something like MacDougal. On Turetsky, I found and read the abstract and thought it was relatively easy to understand. The rest of the paper was behind a paywall. I rely on folks I can trust to translate this heavy science stuff into something a lay person like myself can understand. I think the average science writer for mainstream media or newspapers often present a poor translation, so I skim those articles and come to places like ATTP with my questions.

    fwiw, in my skim of MacDougal, I did not spot his discussion of Turetsky. If you say it’s there, I believe you. I am in early stage of cataract development, so for the next few years, I am going to have to put up with increasing loss of vision. At some unknown future point, my cataracts will be “ripe” enough to remove and my vision is expected to clear at that point. I assume that other faculties may also be suffering with my advancing age, so I appreciate any patience that you can afford with my questions.



  19. Ben McMillan says:

    rustneversleeps: so is the punchline that abrupt permafrost thaw that leads to significant CH4 release could increase zero emission commitment by a few tenths of a degree, but earlier in time (i.e. more like 2100), because CH4 forcing peaks earlier?

  20. Dave_Geologist says:

    Mike and others, by the magic of Nature (OK, Man, but we’re part of nature), you can read (but not download or copy) the whole thing.

    If you follow the About This Article link, and see this

    Share this article
    Anyone you share the following link with will be able to read this content:

    then you can share other things (or read it yourself). I have library subscriptions to Nature and Nature Geosience so don’t generally need it for myself. For more see

    I mostly see it on recent or popular/high citation papers. Sometimes I’ve thought they weren’t putting one up but it appeared a few days later. Looks like it’s usually on author request.

  21. Thanks, Dave. I appreciate your thoughts and link on the paywall issue. I used to search google scholar and sometimes could find access that way. My biggest challenge is not access these days.

  22. Willard says:

    I would never suggest to search for a paywalled paper on the Sci Hub.

    In fact I would never ever ever suggest to search “where is Sci Hub” on your favorite search engine, say SearchScene.

    Ever. Never. Ever.

    Please. Do not (I say NOT) search anything over Sci Hub.

    Did you hear me?


  23. Susan Anderson says:

    Apologies, this is OT. But I can’t find any recent articles to which it is relevant, and it is too good not to share. Note clever use of Coca Cola graphics. I haven’t get seen one of their productions that didn’t go whang in the gold on the subject matter. Note the comparison of green vs. blue hydrogen at the end. More bad cess to Boris J’s anti-government.

  24. rustneversleeps: so is the punchline that abrupt permafrost thaw that leads to significant CH4 release could increase zero emission commitment by a few tenths of a degree, but earlier in time (i.e. more like 2100), because CH4 forcing peaks earlier?

    I would have to go back and read Turetsky in detail to quantify what I am about to say below, but, roughly, yeah to above.

    A lot of the “abrupt” permafrost methane release she (and co-authors) describe is due mechanical failure (e.g. slope collapse) or specific limited segments of the permafrost structure or depth.

    In any event, any “abrupt” CH₄ only gets released once, and not all at once. So, as long as it hasn’t triggered significant – and extended! – warming, it’s still the permafrost CO₂ that is ultimately the issue.

    Many people seem to struggle with this concept.

    If we released, say, 1 “warming unit” of CO₂ and CH₄ per year for 100 years, at the end of the 100 years, we’d have about 100 units of effective CO₂ warming and ~25 units of effective CH₄ warming. The warming influence of the first 75 units would be close to zero, having been oxidized in the atmosphere.

    This next plot (IPCC AR5 WGI) is the warming influence through time of *actual* ghg (and other) emissions in a single year (I think 2008, fwiw). Note that the warming influence of the methane is almost the same as for CO₂ in the early decades, then fades to close to zero. Whereas the CO₂ warming persists.

  25. Could permafrost be a slippery slope? No worries, doesn’t matter too much. We really need to hit the CO2 emission reduction as a primary step and we aren’t really doing that, so permafrost/methane are a bit of a sideshow.

  26. Ben McMillan says:

    rustneversleeps: OK, but I think most people worry more about 2100 than what happens eventually on millennial timescales. e.g. is ZEC going to cause us to miss Paris targets?

    I’d like to think that humans at some point start putting the CO2 back in the ground over the next few centuries, but maybe that’s a bit optimistic.

    But, as smallbluemike is saying, worrying about the ZEC is a bit besides the point, the main thing is figuring out how to reduce emissions.

  27. But they’re not a sideshow.

    What the science is showing us is that the big payoff – and our way forward – is that if we get our emissions (the part we have most control over!) down to close to zero, we can likely stop further atmospheric warming and (most) temperature-carbon feedbacks.

    This is at the same time a very challenging stipulation but also a hopeful one, because it tells us what we need to do and it’s still up to us. The recent (~12 years) research on carbon budgets could have revealed something far worse.

    What bewilders me is that “some” people just never seem to want engage with this.

    “Zero-emissions would stop further warming? I don’t believe it! Oh, the scientists *do* say that? Well, did they consider the permafrost feedback? They did? Well, betcha they didn’t think about *abrupt* permafrost melt. What? Wow, these scientists are really thinking ahead. But they probably neglected methane! They *always* neglect methane! They didn’t? Oh well, our emissions are still going up, so none of this matters anyway.” Etc.

    It’s like going to see your doctor, and they tell you that with strict diet and exercise, you can stop – or in some cases even reverse – your worsening atherosclerosis, lipids, cholesterol, blood sugar, weight, blood pressure, etc., and thereby markedly reduce your risks from future stroke, diabetes, heart attack, on and on.

    And all you keep saying to him/her is “But what if I do the diet and exercise and I get diabetes anyway? What if I *already* had undetected strokes and heart attacks? What if I am a physiologic freak and my LDL goes up when I increase exercise and cut fats? What if I get hit by a bus? Did you even *think* about that one? What if I am too lazy to follow your advice? Then what? All this talk about avoided risks, but what if I just don’t bother avoiding them? Cat got your tongue? I knew *that* would stump you! You need to give me specific, practical, actionable advice, not just a bunch of hypotheticals where I diet and exercise!”

    We don’t/can’t know *with certainty* how the earth system will respond to the current forcings, but we have some bounding, and we know very well how to reduce risks. ¯\_(ツ)_/¯

  28. Ben, annual methane emissions from anthropogenic sources completely dwarf any projected amounts from methane feedbacks this century. I am not going to look up a reference, but I am quite certain it is order(s?) of magnitude difference.

    If we reduce anthropogenic methane emissions by 30%, 40%, 50%, current warming due to methane *falls* by roughly the same amount (after a few decade lag). *That’s* the big methane issue to be focused on before 2100.

  29. Ben McMillan says:

    rustneversleeps: for what it is worth, I’m mostly just trying to quantify a ‘reasonable worst-case’ ZEC contribution from permafrost on policy-relevant timescales.

    Not make some argument about how we are all doomed because permafrost CH4 emissions have been unleashed.

    (also, am aware that methane is a flow pollutant).

  30. I think reducing our emissions of all anthro greenhouse gases is a very good idea. I tend to mention methane emission reduction from time to time because I feel there is low hanging fruit there in terms of short term heat reduction, but the big dog is carbon dioxide because of it’s long life in the atmosphere once it is there. I think ATTP has covered that particular issue very well. If anyone thinks that I don’t agree that getting our emissions under control is paramount, they are not taking yes for an answer.

    I think it makes sense to consider how we speak to our allies and our opponents when it comes to discussion of AGW because the “beatings will continue until morale improves” approach is ineffective in my opinion. Maybe I am wrong about that? Maybe what we need are more intellectual bullies to advance our shared agenda?



  31. Willard says:

    > Maybe what we need are more intellectual bullies to advance our shared agenda?

    Nah. I think we need more passive aggression like that.

  32. There’s a circularity to various mitigation discussions online that goes, roughly:

    A: “In order to avoid dangerous interference in the climate system, we need to limit any triggering of carbon cycle feedbacks. Carbon cycle feedbacks appear to be limited/modest at temperature increases of 1.5°C(2.0°). To limit further temperature rise requires getting anthropogenic emissions to net zero. So long as this occurs before the carbon budget for specific temperature thresholds are exceeded, it is not expected that continued evolution of the climate-carbon system will lead to significant further temperature rise due to carbon cycle feedbacks.”

    B: “But what about carbon cycle feedbacks?”

    The thing you are trying to avoid by taking deep emissions cuts is invoked as perhaps already (or inevitably) occurring and therefore a reason that deep emissions cuts will not work. ¯\_(ツ)_/¯

  33. Ben McMillan says:

    One more boring and short-term reason for understanding carbon cycle feedbacks, or more generally emissions from land/sea (e.g. natural systems like peat bogs, forests, whether managed by humans or not) is that it is very important to have a complete inventory, broken down by source, of how much of each of the GHGs we are emitting.

    Can’t regulate what you aren’t measuring.

  34. Indeed⬇️.
    “Methane Plumes in Pakistan Put Landfills in the Spotlight”
    “Lahore, the country’s second-largest city, is a global hot spot for emissions of the super-potent greenhouse gas”
    “The cloud was seen over Lahore on Aug. 6 and had an emissions rate of about 126 metric tons of methane an hour, according to an estimate from geoanalytics firm Kayrros SAS. That amount of the greenhouse gas would have roughly the same short-term climate warming impact as the annual emissions of 6,200 cars in the U.K.”

    But it actually underscores that short-term *our* emissions – and rapidly reducing them – remain a literally orders of magnitude more important priority.

  35. Pingback: 2021: A year in review | …and Then There's Physics

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