Warming commitments

There’s been quite a lot of recent discussion about warming commitments. It started with an article by Bob Berwyn called Net Zero Emissions Would Stabilize Climate Quickly Says UK Scientist, followed soon after by one saying [w]arming already baked in will blow past climate goals, study finds. The first article is (I think) based on a recent multi-model analysis which suggests that the most likely value of Zero Emission Commitment (ZEC) on multi-decadal timescales is close to zero. The second article is reporting on results from another recent paper suggesting that [g]reater committed warming after accounting for the pattern effect.

So, why are we being presented with what appear to be inconsistent results? The simple answers is that we’re not really being careful enough to define what we mean by a warming commitment. The first article, and paper, are considering what would happen when we get emissions to zero. The second article, and paper, are essentially considering what would happen if atmospheric greenhouse gas concentrations remained at today’s levels. These are clearly two different scenarios.

When we get emissions to zero, the first paper indicates that – on multi-decade timescales – the zero emission warming commitment (ZEC) would be close to zero. On the other hand, if atmospheric CO2 concentrations were to remain constant, then we would continue warming to equilibrium. At today’s atmospheric CO2 concentrations, this would lead to additional warming of around 0.5oC or even more, according to the paper being highlighted in the second article above. However, it is important to realise that constant concentrations require continued emission, as illustrated by the second figure in this Steve Easterbrook post.

I should also stress that our understanding that there is little warming commitment associated with zero emissions has been understood for quite some time. The first paper to point this out was probably Matthews and Caldeira (2008), followed by Solomon et al. (2009), and Cao and Caldeira (2010). There’s also a Realclimate post pointing this out in 2010, the Steve Easterbrook post I mentioned above from 2013, and a post I wrote in 2016.

There are, however, a number of important caveats. That the zero emission warming commitment is probably small probably only applies on multi-decade timescales. The models that demonstrate this typically don’t include slower processes (such as ice sheet retreat, sea level rise, permafrost release) that may lead to additional warming on longer timescales.

Also, even though there is probably little commited warming on multi-decade timescale once we get emissions to zero, without negative emissions global surface temperatures will remain at an elevated level (relative to pre-industrial times) for a very long time. It does, however, indicate that our future warming depends mostly on future emssions. We can still influence how much future warming we are likely to experience, even if we can’t turn everything off right now.

So, I think it’s good that there is more recognition that the ZEC is probably small. It does address claims that there’s nothing we can do to avoid a lot of future warming and does illustrates that, in the context of future warming, most of the inertia is societal, rather than inertia in the climate system.


Net Zero Emissions Would Stabilize Climate Quickly Says UK Scientist, article by Bob Berwyn.
Warming already baked in will blow past climate goals, study finds, Associated Press article.
Is there warming in the pipeline? A multi-model analysis of the Zero Emissions Commitment from CO2, MacDougal et al. (2020).
Greater committed warming after accounting for the pattern effect, Zhou et al. (2021).
Stabilizing climate requires near‐zero emissions, Matthews and Caldeira (2008).
Irreversible climate change due to carbon dioxide emissions, Solomon et al. (2009).
Atmospheric carbon dioxide removal: long-term consequences and commitment, Cao and Caldeira (2010).
Climate Change Commitments, Realclimate (2010).
How Big is the Climate Change Deficit?, Steve Easterbrook (2013).
Committed Warming, my post from 2016.

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42 Responses to Warming commitments

  1. There are a couple of other caveats about the zero emission commitment (ZEC) that I wanted to add, but the post was getting a bit long. Even though global surface warming will roughly stabilise when emissions get to zero, this doesn’t mean that there won’t still be changes. Sea level rise will continue for a long time. Also, there will probably be some warming adjustments as the Southen Ocean (which has been warming slowly) continues to warm, while the land may actually cool slightly.

  2. ” These are clearly two different scenarios.”
    Yes, and they rest on two different calculations. I think Berwyn is based on emission calculations. I think Zhou is based on atmospheric accumulation measurements. I think Zhou is a harder, more certain number because it can be checked easily by MLO and other observational data sets. I think Berwyn is a softer, less certain number that is likely to be based at least partly on nation state reports on their emissions. Maybe I am wrong about that?

    Also, I think it makes sense to keep the CCC – constant composition commitment – in the discussion because it ties the two scenarios together and is a certain point between where we are today and where we will be when we hit the ZEC point at which point we should expect to see the atmospheric accumulation number start to drop as the natural carbon cycle begins to remove carbon from the oceans and atmosphere.

    I think/hope I have that right. I am confident that someone will let me know if I have that wrong.



  3. Everett F Sargent says:

    I project that we will not see ZCE anytime this century. Arguing how much milk is spilt this century is almost a fool’s errand. Perhaps put up reasonable future carbon emissions through say 2050, then put up purported emissions thru to 2100. Oh and AR6 and those SSP’s 2.6 and 1.9 don’t count because but unicorns.

    We will have further committed warming whether you like it or not. All pseudo academic arguments aside. :/

    Oh and two. anyone that understands this stuff at all can see the differences in those two papers. Now let us discuss realistic future carbon emissions

  4. John Hartz says:

    Not directly related to the OP, but certainly topical, here are two important articles I comment you and everyone following this thread to read:

    After the Insurrection: Accountability, Reform, and the Science of Democracy by Andrew Rosenberg, Blogs, Union of Concerned Scientists, Jan 8, 2021

    Capitol Rioters Walked Away. Climate Protesters Saw a Double Standard. by John Schwartz, Climate, New York Times, Jan 7, 2021

  5. Jai Mitchell says:

    We are going to find that our aerosol forcing is more than the models currently hold, that the majority if it will go away when we cut fossil fuels (note we are seeing some of that in 2020 due to covid) and that ECS is greater than 5C. So the fundamental assumptions in the “near zero” warming at ZEC are flawed from the very start.

  6. Jai,
    We’ve probably had this discussion before, but the ECS is almost certainly not above 3C. The ESS may be that high, but the timescales over which the slow feedbacks that are relevant for the ESS are very long and so it’s not entirely clear how the ESS is particularly relevant (in the sense that it doesn’t, IMO, really change the basic picture – start reducing emissions and get to (net) zero as soon as possible).

  7. “ECS is almost certainly not above 3C.”

    Really? Here is what I find when I search that question:

    “While a number of studies were published in earlier decades, the chart highlights the flurry of research since the early 2000s. Most studies have a best estimate of sensitivity between 1.5C and 4.5C, but there are a few very-high or very-low sensitivity studies as well.

    The range of sensitivity across all of these studies has likely narrowed slightly over time, though the average has remained fairly close to 3C.”


    “What is the current best estimate for climate sensitivity?

    The 2013 Intergovernmental Panel on Climate Change (IPCC) fifth assessment report estimates ECS has a ‘likely’ range of 1.5 – 4.5°C. It adds ECS is extremely unlikely to be below 1°C and very unlikely greater than 6°C. The wide range accounts for all evidence. The IPCC estimate TCR has a likely range of 1.0 – 2.5°C and is extremely unlikely more than 3°C.”


    I think it might be accurate to say that ECS is likely to be around 3C. But to say that ECS is almost certainly not above 3C seems wrong to me.

    Do you have links to studies, etc that support this level of certainty? What I see is discussion about how the range of ECS has remained stubbornly wide at the 1.5 to 4.5C range roughly. Were you thinking of TCR?

    from the carbonbrief link again:

    “A remarkably stable range

    In 1979, the Charney Report from the US National Academy of Sciences suggested that ECS was likely somewhere between 1.5C and 4.5C per doubling of CO2. Nearly 40 years later, the best estimate of sensitivity is largely the same. ”


  8. Everett F Sargent says:

    “We’ve probably had this discussion before, but the ECS is almost certainly not above 3C.”

    Did you mean 5C instead of 3C? AR5 also suggested p = 6C, I know there are newer estimates out there (e. g. James Annan’s co-authored paper).

  9. Everett F Sargent says:

    Well that didn’t come out right …
    ECS .ge. 6C for p .le. 0.10

  10. Yes, sorry, that was a typo. I meant the ECS is almost certainly not above 5C.

  11. Smil says models have nothing to do with reality, thank for the eye opener before coffee

  12. Willard says:

    Hi Loki,

    No more “but renewables,” please.

  13. Eventual_Horizon says:

    Touching on Jai and ATTP’s exchange, in a recent pre-release of James Hansen’s new book he addresses aerosols. Quoting a relevant couple sentences:

    That result was obtained under the assumption that the fast feedback climate sensitivity is 3°C for doubled CO2. Up-to-date analyses of paleoclimate data (Chapter 25) imply that the fast feedback climate sensitivity is probably in the range 3 to 4°C for doubled CO2. If the real world fast feedback sensitivity is closer to 4°C, the inferred aerosol forcing may approach -2 W/m

    If it holds true that 1W/m = 0.75C then loss of aerosols due to a cessation of all emissions would lead to a possible 1.5C temp rise. Even if you want to cut that figure in half it’s still really odd to claim that everything levels off nice and smooth if you cease emissions. There seems to be a bit of a head-in-the-sand attitude about aerosols sometimes.

  14. Eventual,
    Yes, but the aerosol forcing is currently also comparable to the forcing due to short-lived GHGs. Admittedly, the timescales are different, but if we were to halt *all* emissions now, then there would be some short-term warming due to the precipitation of aerosols, followed by cooling back to roughly the level we were at when emissions ceased due to the decay of the short-lived GHGs. For example, the yellow line in this figure from the IPCC SR15 report.

  15. Just to expand on ATTP’s point immediately above, there are a number of “zero emission commitment” parameterizations/combinations for the CMIP’s (and other) model experiments – as detailed in that plot.

    And, as the green line above indicates, it is not lost on atmospheric physicists and climate modelers that if we entirely eliminated both the long-lived climate warming forcing warming agents (CO₂, N₂O) and the short-lived climate forcing (SLCF) cooling agents (sulfate aerosols, etc.), but (somehow, however improbably) *not* the SLCF warming agents (NO, NO₂, black carbon, etc.), we would expect a warming commitment. But it is difficult to see how that combination of forcings could occur in practice.

    But I also wanted to briefly mention Hansen’s speculation/claim above that ECS may be closer to 4°C, and that recent paleo studies are supportive.

    It seems to me that the 2020’s “An Assessment of Earth’s Climate Sensitivity Using Multiple Lines of Evidence”, which narrowed the climate sensitivity and was authored by the who’s who in climate sensitivity research, tends to work against Hansen’s conjecture. (And the Sherwood, et al., paper isn’t definitive, but just anecdotally, it was ranked by Science Magazine as runner-up in Breakthrough of the Year (behind the Covid-19 vaccines). Just saying.)

    It’s not the first time that Hansen has been on the other side of what the expert consensus science has been saying about sensitivity and emissions/concentrations targets. I happened to notice I long-ago excerpted and posted Myles Allen challenging Hansen’s approach at the 2009 Four Degrees and Beyond Conference. That critique and assessment of where the balance of opinion lay has largely stood the test of time, I’d say. Just noting.

  16. Hmm, just realizing I cut my excerpt just as Myles Allen was beginning to discuss a better question/approach (his presentation was “What will it take to avoid 2, 3 and 4+ degrees?”). The full presentation is still hosted here.

  17. Ben McMillan says:

    Making the aerosols go away is going to have a massive positive effect on human health (and probably other species too) even if slightly unfortunate for warming. Air pollution in general leads to premature deaths of millions each year. And at least then we would have a better idea of where we are in terms of climate sensitivity.

    If we really need aerosols to keep us cool, better to do it deliberately and put them in the right place.

  18. Ben McMillan says:

    Also, I think what we have here are a bunch of “unrepresentative concentration pathways”.

    Interesting from an understanding-the-dynamics point of view but a bit besides the point if we are talking about Paris targets.

  19. “Making the aerosols go away is going to have a massive positive effect on human health.”

    This is actually a key point, and actually somewhat related to the ZEC warming message.

    A very large part of the climate mitigation challenge is that the (large) costs are overwhelmingly front-loaded – the investment spending unavoidably must be incurred by the current generation if we are going to avoid ~2°C.

    But the *climate* benefits to such mitigation activities – primarily in the form of avoided damages – are similarly almost all going to be experienced many, many decades into the future.

    The ZEC means that some of the benefits begin accrue quite quickly. So this is unambiguously good news if you are trying to sell the cost-benefit of mitigation, particularly in the medium-term.

    On the other hand, *while* we are reducing emissions to zero, the relative *climate* benefits of inaction vs action are almost indistinguishable. This is a function of final warming being a function of cumulative emissions and the bulk of emissions having already been emitted.

    Cumulative CO₂ emissions by 2030 in an aggressive 50% reduction vs “unchanged emissions from 2020” are roughly 2594 vs 2684 GtCO₂.

    So, a roughly expected 3% temperature difference in 2030 for a 50% emissions reduction, and about a 0.05°C delta against some counterfactual “might have been temperature”.

    Math which does not excite most people.

    So anything that co-benefits like “health due to reduced pollution” can bring forward in time is going to be crucial (although, notably, our “revealed preference” has not been to value those benefits on their own to pay for those either.)

    Problematically, this temporal mismatch in costs and benefits may mean that we have to find ways to sweeten the pot financially for the current generation in order to get them to act at sufficient scale. Think “cut taxes, massive government economic stimulus” for the historical emitters just so they are incented sufficiently to make deep enough cuts in the present so that future benefits.

    It’s possible it’s all just an intergenerational win-win – MMT over delivers, etc. – and happily-ever-after narratives are easier sells, for sure.

    But the more that near-term co-benefits like health can be realized and communicated, the better.

  20. Also, I think what we have here are a bunch of “unrepresentative concentration pathways”.

    The zero emissions commitment experiments aren’t supposed to be suggestive of mitigation pathways.

    They’re informative of what warming to expect if/when we get to zero emissions. It’s really got nothing much to say about how much we’ve emitted when we’d eventually get to that point, or when that date might be.

    Just that if we got to zero emissions – someday! – we would expect little warming from that point forward.

    I rather like the plot* below, because it shows the ~linear temperature increase with carbon (or CO₂) emissions.

    One thing I like about it is that there are no dates on it, which I think helps people “get away” from “but we’re so far away from zero emissions, it doesn’t matter!” and similar non sequiturs. The climate system essentially does not care about when emissions occur or don’t occur, or reach zero or not. It only “cares” about what our cumulative emissions are. Whenever. Approximately a 2°C increase relative to pre-industrial for every 1,000 GtC cumulative emissions.

    If we keep emitting, we keep moving up and to the right. But if we stop emitting (i.e. zero emissions!), we effectively stop moving on the plot (barring negative emissions).

    *plot is IPCC AR5 WG1 SPM Figure SPM.10

  21. One thing I like about it is that there are no dates on it

    Aaargh! Wrong figure! That one superimposed the RCP’s and associated dates.

    Let me find the right one.

  22. IPCC AR5 SYR SPM Figure SPM.5 (b)

  23. Eventual_Horizon says:

    The argument is that aerosol cooling is roughly equivalent to warming caused by short-lived greenhouse gases. Stop emissions and they’ll conveniently cancel each other out. Even if you think Hansen is off with a <= 1.5C latent warming there's more recent research supporting a floor of 0.5C and a possible upper end of 1.1C. See https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL076079

    I'm having a really hard time thinking of which SLCF agents could possibly be responsible for so much current forcing. The biggest, CH4, is predominantly produced from natural sources and agriculture so it's hard to see it being eliminated especially with much of the tundra turning into an effective wetland for increasing months of the year. (Speaking of CH4, take a gander at methanelevels.org and appreciate that we've had effectively zero net drawdown in emissions this year. Something new and unwelcome.)

    That leaves tropospheric ozone and some lesser gases. Doesn't really add up.

    @rust – "But the more that near-term co-benefits like health can be realized and communicated, the better."

    Yes indeed. People still remain irrationally fearful of nuclear even though it produces the least deaths per GwH of any energy source by far. I'll keep my fingers crossed that MMT saves us all.

  24. Eventual,
    Below is the AR5 radiative forcing diagram. In fact, the best estimate for the total change in forcing is actually slightly bigger than that due to CO2 alone, which would suggest that the other positive forcings are slightly bigger than that due to aerosols. However, as you can see, the aerosol forcing is quite uncertain.

  25. Ben McMillan says:

    I guess my general issue with “committed warming” is that it is a game that you can play a whole bunch of different ways, and not necessarily much to do with what would happen in a realistic rapid decarbonisation. Which is fine if you want a thought experiment, of course. But a bit dodgy if it ends up being turned into a newspaper headline.

    More plausible rapid decarbonisation scenarios give peak warming right about the time you hit zero emissions even if you are a bit pessimistic about non-CO2 forcings.

  26. Ben,

    More plausible rapid decarbonisation scenarios give peak warming right about the time you hit zero emissions even if you are a bit pessimistic about non-CO2 forcings.

    Yes, exactly. This, in my view, is a key point. There isn’t much warming in the pipeline when we get emissions to zero under realistic future pathways.

  27. Eventual_Horizon says:

    I’m not sure I follow. Adding up everything in the AR5 diagram for “short lived gases and aerosols” gives -0.64. That’s about a 0.5C boost in warming if everything there were to disappear with a snap of a finger. If we include CH4 as a short-lived greenhouse gas and assume we can entirely eliminate it then we end up with net cooling.

    I did some digging in SR15 and found they cited a paper (https://acp.copernicus.org/articles/17/2709/2017/acp-17-2709-2017.pdf) as “largely supporting the AR5 best estimate and uncertainty range of aerosol forcing.” But the paper itself concludes “average total, multi-model ozone and aerosol forcing over the period 1990 to 2015 is almost +0.2 W m” Subtracting their mean forcing for ozone (+0.06) and you still have a net positive for aerosols which is definitely outside the bounds of AR5.

  28. Eventual,

    I’m not sure I follow. Adding up everything in the AR5 diagram for “short lived gases and aerosols” gives -0.64.

    Yes, but the CH4 in well-mixed greenhouse gases roughly cancels that. So, roughly, the net change in forcing is very similar to the change in forcing due to the long-lived GHGs (CO2, N20 and the halo-carbons). It’s not exact, but the short-lived GHGs roughly cancel the aerosols (but with different decay times).

  29. David B Benson says:

    I suppose is related. In any case, suggests urgency.

  30. James Charles says:

    ‘We’ have ten years?
    “ . . . our best estimate is that the net energy
    33:33 per barrel available for the global
    33:36 economy was about eight percent
    33:38 and that in over the next few years it
    33:42 will go down to zero percent
    33:44 uh best estimate at the moment is that
    33:46 actually the
    33:47 per average barrel of sweet crude
    33:51 uh we had the zero percent around 2022
    33:56 but there are ways and means of
    33:58 extending that so to be on the safe side
    34:00 here on our diagram
    34:02 we say that zero percent is definitely
    34:05 around 2030 . . .
    34:43 need net energy from oil and [if] it goes
    34:46 down to zero
    34:48 uh well we have collapsed not just
    34:50 collapse of the oil industry
    34:52 we have collapsed globally of the global
    34:54 industrial civilization this is what we
    34:56 are looking at at the moment . . . “

  31. https://advances.sciencemag.org/content/7/3/eaay1052?utm_campaign=Carbon%20Brief%20Daily%20Briefing&utm_content=20210114&utm_medium=email&utm_source=Revue%20Daily

    Recent study from Duffy et al about crossing a tipping point with land carbon cycle. How does “warming in the pipeline” look if we don’t reach net zero before we cross this tipping point?

    Have Berwyn and Mann et al considered continued warming that might occur if we were to achieve net zero emissions after we cross a tipping point like the one Duffy discusses?



  32. Mike,
    As I understand it, the weakening of the land sink is well understand and is incorporated into the models. In fact, the roughly linear relationship between warming and emissions – which underpins the carbon budget approach – emerges when you consider both radiative climate feedbacks and carbon cycle feedbacks.

  33. “As I understand it, the weakening of the land sink is well understand and is incorporated into the models.”

    If this is well understood, doesn’t it make sense to include the 20 to 30 year time to achieve net zero in general discussion and presentation of this topic? Isn’t it sensible to mention and maybe focus on the cost-effectiveness and misery-reduction that accrues if we achieve net zero before we cross the tipping point that Duffy discusses?

    A 20 to 30 year time frame to achieve net zero is much more easily understood than a time frame like “as fast as we reasonably can”?



  34. Mike,
    I’m suggesting that this is already taken into account.

  35. Yes, I understand what you are saying. I am asking you why you don’t routinely discuss the time frame to get to net zero as 20 to 30 years instead of using language like “as fast as we reasonably can”? If you believe this is true and that things start to slide away from us if we have not reached a net zero status by 2040 (on the safe side?) instead of using vague time frames that are nothing like saying “by 2040, or 2050 at the latest”?

    Can you find an instance on your site where you have said, “hey, we need to reach net zero status by 2040 or 2050 at the outside.” I am not finding anything that specific here from you. Maybe I missed it? It seems like a good meme to absorb and repeat, does it not?



  36. The “land sink” paper suggests that it might be *reduced* in 20+ years, mike, not that it stops or reverses. If we have gotten our act together and substantially reduced emissions by then, this will have less impact.

    The reduction itself here is proposed to be (primarily) mediated by temperature rather than emissions. We actually can’t do much about temperature increase in the next ~20 yrs (and, no, this is not contrary to the “zero emissions commitment” discussions, but reflective of the fact emissions occur en route to zero), but we minimize the impact of a weakened sink at that time if emissions are dramatically reduced by then.

    As ATTP alludes, the understanding of an expected roughly linear proportional temperature response to cumulative CO₂ emissions occurs in the context of temperature being roughly logarithmic with atmospheric CO₂ concentrations (“ΔF = 5.35 ln (C/C₀)”). Part of squaring those two different CO₂ relationships to temperature is an expected changing “airborne fraction” of current emissions through time – i.e. a modeled weakening of the land sink through time as cumulative emissions and concentrations increase.

  37. John Hartz says:

    The graphics embedded in this article are quite impressive…

    A Global Tour of a Record-Hot Year by Henry Fountain, Blacki Migliozzi & Nadja Popovich, Climate & Environment, New York Times, Jan. 14, 2021


  38. Nearing the Climate Cliff: Study Warns Plants Are Decades Away From Absorbing Less Carbon

    “The main thing that comes out of this study is that the speed with which we will reach dangerous levels of climate change is most likely an underestimate,” said study co-author Christopher Schwalm, a senior scientist at Woodwell Climate Research Center.

    “As things continue to warm, the photosynthesis will be overwhelmed by the amount of respiration,” Schwalm said. “The net balance of vegetation will shift from being climate positive —the slowing effect— to climate negative, which means it’ll accelerate climate change.”

    The big finding: if humans continue using fossil fuels at current rates, plants’ ability to absorb carbon dioxide will be reduced by almost 50 percent as soon as 2040.


    I think Schwalm is saying that the weakening of the land sink may not be adequately understood and that it has likely been underestimated to the extent the land sink has been incorporated in the models.

    The kicker here is global temperature and how much/fast that rises. The global temp rise is strongly liked to the extent to which we continue to use fossil fuels. China’s 2020 coal output rose to the highest level since 2015. Seems like ought to be going the other direction. We should encourage the Chinese to get their act together and reduce their coal output as fast as they reasonably can.


  39. who knew?


    ” Future warming projections come from complex climate models, which combine historic data, current observations, equations that encompass current understandings of the bio-geo-physical processes, and some assumptions about processes where direct observation or modelling is more difficult….

    Now comes the crunch: recent research indicates that the current assumptions are both too optimistic, and the future emissions will cause faster warming than the models currently project.”

    I have been told that the changes in the carbon sinks that are discussed in this article have all been basically accounted for in the projections. I think David Spratt did not get that memo.



  40. Pingback: ‘Net zero’ | …and Then There's Physics

  41. Pingback: CO2 emission reductions | …and Then There's Physics

  42. Pingback: The effect of permafrost on the ZEC | …and Then There's Physics

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