Some more about Hansen et al.

I thought I would expand a bit on my previous post about the recent Hansen et al. paper. Something I did like is that the paper highlighted that there is no known paleoclimate analogues for the current anthropogenic forcing pathway. Humans are producing a substantial perturbation to a complex, non-linear system. Even though we have a good understanding of how the system might respond to such a perturbation, we shouldn’t rule out the possibility of surprises.

Additionally, the paper seems to be making a few key points. Firstly, it’s arguing that equilibrium climate sensitivity (ECS) is probably ~4oC, rather than the more standard best estimate of ~3oC. It’s also highlighting that the greenhouse gas (GHG) forcing is already 4 Wm-2, equivalent to a doubling of atmospheric CO2. So, the fast-feedback equilibrium warming (ECS) for today’s GHG forcing is already 4oC. Furthermore, it’s arguing that this implies a relatively large aerosol forcing that is currently masking about ~1oC of warming, implying a large amount of warming in the pipeline if aerosol emissions are substantially reduced.

Although the above arguments could well be correct, there are some other factors to consider. Even though todays’ GHG forcing is indeed probably around 4 Wm-2, it’s a combination of a number of different GHG species. The dominant one is CO2, but methane and some other species, contribute about one-third of this forcing. These species have relatively short atmospheric lifetimes. Hence, if human emissions of these species were also to substantially reduce, this would counteract some of the warming that would result if we were to substantially reduce aerosol emissions, although the timescales wouldn’t be quite the same.

Also, even though CO2 has a long atmospheric lifetime, this doesn’t mean that all the CO2 that has been emitted will remain in the atmosphere for a very long time, it just means that a significant fraction of what has been emitted will remain in the atmosphere for a very long time. So, if CO2 emission go to zero, the CO2 forcing will also decrease slightly. This is why the best estimate for the zero emission committment (how much further warming there will be if emissions go to zero) is very close to zero. In other words, the warming that has been locked in due to emissions to date is very close to the amount of warming that has occured to date.

Essentially, what this implies is that the warming in the pipeline depends mostly on future emissions and isn’t locked in due to past emissions. There’s, of course, no problem with estimating constant forcing/concentration warming committments, but maintaining a constant forcing/concentration does require continued emissions, so I do think it’s worth making clear that future warming depends mostly on future emissions, rather than being locked in due to past emissions. It is still possible to influence future warming, even if it will be very challenging to do so.

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20 Responses to Some more about Hansen et al.

  1. russellseitz says:

    How does the Tonga eruption figure in Hansen’s estimate?

    Nature reports it injected more than a thousand water vapor molecules into the stratosphere for each of SO2, ( ~140 mt H2O to .42 mt SO2)

    Which translates into a short term positive forcing of ~1.5 ˚ C

  2. russellseitz says:

    On closer reading, the Nature piece says instead that the ~ 10 % increase in stratospheric H2O increases the likelihood of exceeding the Paris Agreement AGW bound of 1.5˚ C in the near future ( 2025-2026)
    by ~ 7%.

  3. Russell,
    Interesting. I hadn’t seen that paper.

  4. Andrew Dessler was unpersuaded by that Tonga paper👇.

    But maybe tempered that a bit when pointed to this earlier paper which drew similar magnitude estimates of warming impact of the event.🤷

    But even if those impacts are this large over ~decade or so as the water vapour cycles through the atmosphere, doesn’t most of it ultimately net precipitate out? Leaving the warming from baseline virtually unchanged (save any long-lived ghg feedbacks during the temporary temperature excursion)?🤔

    👆Stated as genuine questions.

  5. Oops.

    Here’s the other paper which I *think* gave Dessler pause over his initial skepticism of the new Tonga paper.

  6. I looked again at AR6 WG1 report (section and this introduction:

    “The zero emissions commitment (ZEC) is the the climate change commitment that would result, in terms of 50 projected GSAT, from setting carbon dioxide (CO2) emissions to zero. It is determined by both inertia in 51 physical climate system components (ocean, cryosphere, land surface) and carbon cycle inertia”

    Many people still believe (judging from social media) that ocean heat equilibration (part of physical inertia) means continued warming after emissions cease, not realising that this is carbon cycle inertia cancels it out (within a 0.3C range of confidence). It is surprising though, albeit seemingly a lucky coincidence (!?)

    In Figure SPM.7 it illustrates the reduced carbon takeup in sinks (land and ocean) for higher emissions scenarios. And indeed in ‘Remaining Carbon Budget’ it states:

    “For climate stabilisation beyond the 21st century this confidence would decline to very low confidence due to uncertainties in Earth system feedbacks and the ZEC.”

    Which is a long-winded way of me asking: should we be careful not to elevate the ZEC as a rule to be relied on without adding appropriate qualifiers?

  7. Richard,

    Which is a long-winded way of me asking: should we be careful not to elevate the ZEC as a rule to be relied on without adding appropriate qualifiers?

    Good point. I think I do often suggest that it’s likely to be the that the ZEC will be small if we get to (net) zero soon, but that if emit a lot more before doing so that carbon cycle feedbacks could become sufficiently significant that it may no longer be the case.

  8. Greg Robie says:

    After the first post I started reading the paper. I did not find it said what was critiqued. This post is better, but it ignores that the latent heat of fossil cold is disappearing at something to the tune of 8 x 10^20 BTU/yr. The references ‘no committed warming from The Special Report on 1.5°C is based on the role of the fossil cold of the cryosphere not existing when it still is and is significant.

    Furthermore annyalized stored cold in frozen temporate soils is not being generated in winter as much as was once the case. This means things warm up perceptibly faster in the [final] spring when it arrives than would otherwise have been the case. Here in the mid Hudson Valley we currently have no frost in the ground. Code puts the frost line at 36”.

    I did construction professionally. A generation ago mid-December was the latest you could safely pour foundations. Last Thursday tree trimming crews working for the power company near my home were trimming growth with swelling buds (i.e., the next winter this winter will likely – & like last year – kill these buds requiring the trees to again rebud). That taxes trees and opens them up to infections they would otherwise fight off.

    I have not taken the time to confirm this, but apparently our oceans are taking up 4% less CO2, and terrestrial systems are doing 17% less. And don’t forget the the six ‘clean’ hydrogen pages of the IRA law exempt fractional distillation of oil emissions when making hydrogen with that process.

    Just saying.


  9. pjie2 says:

    How does the warming relate to sea level rise? If warming stopped today, would sea levels keep rising? If so, for how long, and by how much?

  10. Climate models show that Global Mean Surface Temperature (GMST) decouples from CO2 concentration for centuries after emissions decrease or cease, GMST remaining constant or decreasing very slowly. This is the case for many other related parameters such as sea level rise. Solomon, et al (2009) were perhaps the first to make this point explicitly. Chapter 4 of the IPCC WG I Report (Lee, et al. (2022)) Section 4.7.1, Fig. 4.39 Table 4.8 reports for ensembles of ESMs that while there is a significant initial drawdown of atmospheric CO2 concentrations after emissions cease, the GMST is stable. More recently this is to be seen in MacDougall, et al.

    The basic reason is that there is a lot of heat and CO2 coming out of the oceans

    Solomon, S., G.-K. Plattner, R. Knutti, and P. Friedlingstein (2009), Irreversible climate change due to carbon dioxide emissions, Proc Natl Acad Sci USA, 106, 1704-1709, doi:10.1073/pnas.0812721106.

    Lee, J.-Y., J. Marotzke, G. Bala, L. Cao, S. Corti, J.P. Dunne, F. Engelbrecht, E. Fischer, J.C. Fyfe, C. Jones, A. Maycock, J. Mutemi, O. Ndiaye, S. Panickal, and T. Zhou, 2021: Future Global Climate: Scenario-Based Projections and NearTerm Information. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 553–672, doi:10.1017/9781009157896.006.

    MacDougall, A.H. et al., 2020. Is there warming in the pipeline? A multi-model analysis of the Zero Emissions Commitment from CO2. Biogeosciences, 17(11), pp.2987-3016.

  11. 👆

    The basic reason is that there is a lot of heat and CO2 coming out of the oceans

    The opposite, no?🤔

    As in: there’s a lot heat and CO₂ **going into** the oceans.

    Atmospheric CO₂ falls *because* it’s going into the oceans.

    And, although the already existing energy imbalance/radiative forcing begins to fall as a result, it is still more than enough to keep adding heat to earth system. All else equal, the surface temperature would continue to warm. But enough heat continues to invade the ocean *from* the atmosphere that atm./surface temps ~stabilize (at approximately net-zero emissions.)

    That’s my understanding, anyway. 🤷

    By the way, Myles Allen is delivering the 3rd of his 6 “Why Net Zero” lectures on January 31. This one titled “The Ocean Physics Behind Net Zero”.

  12. Now. But tale CO2 emissions to zero and both will emerge

  13. russellseitz says:

    A lot can happen to CO2 chemically and physically between its exiting the seabed and reaching the sea surface and the shallow Tonga eruption was a rare exception to the abyssal norm

  14. pjie2,

    How does the warming relate to sea level rise? If warming stopped today, would sea levels keep rising? If so, for how long, and by how much?

    Good question. Sea level rise would continue. For how long and how much is tricky to answer, but I think that even for today’s level of warming we may well have already locked in > 2m of sea level rise. However, this would probably take centuries to materialise.

  15. russellseitz says:

    Two Meters over centuries seems realistic, but orogeny never sleeps , and the Anthropocene remains to date embedded in the present interglacial.

    As surely as all politics is local, we should recall that a lot of the land on maps of the EU was not there 500 years ago. The Norfolk Broads, like much of the Netherlands were built and drained after the late 14th century coastal storms that ushered the Little Ice Age and necessitated dutch advances in coastal engineering by creating the Zuider Zee.

    Sea level rise was successfully resisted centuries before the industrial revolution, and it seems unlikely that those living in river deltas today will meekly submit to centimeter a year rise. The literal downside is that will clamor for increased steel and cement production to defend their land.

  16. Geoff Henderson says:

    “future warming depends mostly on future emissions, rather than being locked in due to past emissions”? Really? I have three problems with that statement:
    a) I don’t believe it is consistent with Hansen’s arguments so it’s misleading to draw such a different conclusion without making it clear it’s your view, not Hansen’s
    b) if by “warming”, you mean temperature rise (units degC), you are missing Hansen’s point that aerosol forcing is a big unknown and this is related to estimates of ocean mixing of thermal energy (also a big unknown). Until these are quantified with certainty the “baked-in” temperature rise and the time required to achieve equilibrium is also uncertain
    c) if by “warming”, you mean Earth’s energy imbalance (EEI, units of W), the time constants of the ice sheets become very important (again surprises are very possible). You concede this in acknowledging that sea level rise will continue. But the surprises can include rapid disintegrations.

    I believe 5 m seal level rise this century is possible. I started an MSc to work on this, but have been sucked back into the wind industry (I’m a mech eng). I wasn’t able to convince myself it is highly probable, but nor could I convince myself it is totally improbable.

    Who is the banker who will underwrite the losses in this casino?

  17. Geoff,
    a) I’m not quite sure what you’re getting at here. I wasn’t implying that it was consistent with Hansen’s view. I was pointing out what I believe the current consensus position to be.

    b) By “warming” I mean global mean surface temperatures. I’m not missing Hansen’s point. He could well be right, but others have assessed this and the best estimate is that the aerosol forcing is largely balanced by the forcing from short-lived GHGs. Hence, they should roughly cancel if (when?) we get emissions to zero. There are, of course, uncertainties, so this may not be the case, but it is what many regard as mostly likely given current evidence.

    c) Yes, I agree that there are potential surprises. So, I’m certainly not suggesting that we should dismiss these possibility, but we should be cautious of claiming that they’re more likely than they probably are. IMO, at least.

  18. Geoff Henderson says:

    Hmm – the “precautionary principle” (which the nations of the world signed up to in 1992) literally means we should be cautious about underestimating probabilities of catastrophe for our civilisation, not overestimating. Note that I am worried about our civilisation, not the planet. The planet has the 3 riders (war, pestilence and famine) on her side. Our “civilisation” is the antithesis of relying on the 3 riders, especially in the age of nuclear weapons. But equilibrium will be restored one way or t’other. Hansen thinks we need to get to 350 ppm, and I don’t think we should be betting against him on that issue, as you seem to be happy to. What value of ppm do you think we can live with?

  19. Geoff,

    Hansen thinks we need to get to 350 ppm, and I don’t think we should be betting against him on that issue, as you seem to be happy to.

    No, I’m not happy about this.

    What value of ppm do you think we can live with?

    I don’t think there’s an easy answer to this. I think we should be aiming to cut emissions as fast as possible, while recognising that some parts of the world should do so faster than others and that we should aim to do so in a way that doesn’t disadvantage those regions that have contributed little to creating this problem. In other words, I think it’s complicated.

  20. David B Benson says:

    Anything above about 280 ppm for CO2 will continuer to make it too hot.

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