A bit more about carbon budgets

I’ve been mostly at home today since we had a power cut at work and the site ended up being closed. So, I thought I would post a few more thoughts about the Millar et al. 1.5oC carbon budget paper. This isn’t intended to be anything all that substantive, so if you want something more thorough, there is a Realclimate post and Dana has a new Guardian article.

In some sense the bottom line from the Millar et al. paper is that we’ve warmed just over 0.9oC and, therefore, have 0.6oC to go before we reach 1.5oC. If you then compute the remaining carbon budget it is bigger than other estimates. One obvious issue with this is that this is based on a late-1800s baseline. If you use an earlier baseline, then we are closer to 1.5oC than they suggest. Also, it is based on a single observational dataset. Other datasets suggest we have warmed more than 0.9oC, relative to their baseline. Hence, we might have less warming to go than they suggest.

However, a key aspect of the Millar et al. analysis is that some climate models suggest that emitting 545Gtc (how much we had emitted by 2015) should have warmed the surface about 0.3oC more than was observed. The models do suggest that this should have happened later than 2015, but let’s ignore that for now. Let’s also ignore that their estimate of how much we’ve warmed may be too low anyway. One interpretation has been that this indicates that the models are running too hot. However, most climate models (but not all) use forcing/concentration pathways as an input. You then have to run a different model to determine the associated emission pathways.

From a physics perspective, what’s of interest is how much we warm for a given change in forcing, not for given amount of emissions. That our observed warming may be associated with higher emissions than was expected does not necessarily indicate that models are running too hot (at least not in terms of climate sensitivity). It may, however, indicate that natural sinks have been taking up more of our emissions than was expected, which is itself an interesting issue. If that is the case, then that might indeed indicate that we could emit more, for a given level of warming, than we had previously thought.

For example, if earlier estimates for a 1.5oC carbon budget (66%) were around 2300GtCO2 (about 630GtC) then the Millar et al. result might suggest that it could be 20% higher (say 2760GtCO2, or about 750GtC). This would give about 120GtC more than previous estimates and, when measured from 2015, sounds like a big change because previous estimates had suggested that there was very little left – a big percentage change in what we had left, even if it a relatively small change in the total budget.

If I have interpreted this correctly (which I may not have) the interesting question then becomes whether or not natural sinks are indeed taking up more of our emissions than expected and, if so, if we would expect this to continue. I don’t know the answer to this, and it would be good to get some clarification.

However, something else I wanted to mention was the issue of committed warming. It is roughly the case that we expect the equilibrium response to a certain level of emission (i.e., the total amount emitted) to match the transient response at the time at which emissions cease. I’ve simplistically demonstrated why in this post but it’s essentially because if we halted all emissions, atmospheric CO2 would drop at a rate that essentially balanced how we would warm if concentrations were fixed at the peak value. The atmospheric concentration would then tend to a long-term value equivalent to 20-30% of our emissions remaining in the atmosphere for millenia. The equilibrium response to this concentration is then expected to be comparable to the transient response to the peak concentration.

However, if we’re underestimating the uptake by natural sinks, then the transient response to a given amount of emissions would be lower than expected. If, then, the equilibrium response to the long-term concentration is to still match this, then our estimates of the long-term atmospheric concentration should also be too high. It’s, however, not obvious that the latter should be the case because this is – I think – set more by the size of the reservoirs than by the rate at which they take up our emissions.

So, if the above is roughly correct, then even if the Millar et al. result is robust, it may still not indicate a larger carbon budget. It would mostly indicate that we’ve slightly under-estimated the committed warming once emissions cease (i.e., the equilibrium response to the long-term concentration would be higher than the transient response at the time at which emissions cease).

Okay, I’m going to stop there. I don’t know if the above are reasonable comments, or not, so treat them with suitable caution. It’s mainly me just pondering this issue, so I will probably end up thinking differently once I’ve thought about this a little more. I’ll aim to clarify things, if necessary. Any comments (mostly) welcome.

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63 Responses to A bit more about carbon budgets

  1. Magma says:

    For what it’s worth, I’ve never been a fan of the ‘carbon budget’ approach. Without a better understanding of atmospheric residence times and draw-down rates of GHGs, with substantial uncertainties remaining in TCS and ECR values, and even in the actual emissions of GHGs vs. bottom-up National Inventories, it seems to me we’re just generating a large number for the sake of providing a single large number. I’d rather see more focus on the concentration of atmospheric GHGs (CO2eq is fine), how this effects surface temperature and ocean heat content, and how quickly CO2eq must level off and diminish before the consequences become both large and irreversible. But maybe I’m in the minority here.

  2. Magma,

    I’d rather see more focus on the concentration of atmospheric GHGs (CO2eq is fine)

    It seems to me that they are all related. A temperature target, or a zero emissions target, or an atmospheric CO2 target all require some kind of carbon budget. However, if we have more confidence in the long-term concentration (once the fast sinks have reached equilibrium) then maybe we could define the budget on the basis of that, rather than on the basis of near-term warming. I suspect, however, that there is enough uncertainty in that, that even that wouldn’t be definitive.

  3. FWIW, the best published and peer-reviewed paper on trends of the components in the Carbon Cycle is W. Li, et al, “Reducing uncertainties in decadal variability of the global carbon budget with multiple datasets”, PNAS, 2016. I’m not aware of anything more recent.

    The only indication I have regarding a change in some of the sinks was a personal communication from Glen Peters, one of the co-authors in Li, et al, PNAS 2016, who suggested in a different context that sinks might be saturating in their ability to take up CO2, not in the sense of being depleted, but in the sense that they could only do it so far. There were no technical references offered. So I don’t know if this is based upon an update to Li, et al in the works, or perhaps the update of the Global Carbon Project. Dr Peters certainly publishes a lot in this area. He also has a few things to say regarding CO2 budgets. And there was a post on the question in RealClimate back in 2007 with references. One was Raupach, et al. Does Li, et al indicate that’s not true? Don’t think so. While their constraints on sinks are more precise than traditionally available from data, they don’t use the same terminology or method as do Raupach, et al. Li, et al did not estimate trends. The trend estimate methods of Raupach, et al‘s section C3 are worth raising eyebrows over, beyond whatever unaccounted for noise lives in their data compared to Li, et al.

    I’d say the judgment is still in progress, but I would not be surprised if there was some throttling going on.

  4. Sinks:
    Are safe natural sinks available?
    OK, OK greening of plants and forests.
    But oceans?

  5. But oceans?

    Phytoplankton photosynthesize carbohydrates from CO2 just like land flora.

  6. Everett F Sargent says:

    Here you go …
    http://www.climatechange2013.org/images/report/WG1AR5_AIISM_Datafiles.xlsx
    Emissions …
    http://cdiac.ess-dive.lbl.gov/GCP/carbonbudget/2016/

    Table AII-2-1abc (1PgC = 1 gigatonne C), Use RCP 2.6.

    I’ve digitized Millar (2017) Figures 2a (Blue curve is RCP 2.6) and 3b (Green curve is their RCP 2.6-2017). I also digitized their Figure 3b 33-50% (or the 3rd lowest sextile).

    RCP 2.6 (their or the IPCC blue curve, Figure 2a, 2000-2100 or 100 years) = 380 GtC
    RCP 2.6 (their or the IPCC blue curve, Figure 2a, 2018-2100 or 82 years) = 218 GtC
    RCP 2.6-2017 (their green curve, Figure 3b, 2000-2100 or 100 years) = 506 GtC
    RCP 2.6-2017 (their green curve, Figure 3b, 2018-2100 or 82 years) = 336 GtC

    RCP 2.6-2017 – RCP 2.6 difference (2018-2100) = 336 – 218 = 118 GtC (less excess C emissions from 2000-2017 inclusive (e. g. 18 years)), I SWAG about 176 GtC (2000-2017 C emissions) versus 506-336=170 GtC so 118 GtC – (176 – 170) = 112 GtC.

    I posted a number of 110 GtC in a previous thread/post. If Millar (2017) is off by even one sextile (Figure 3b, 33-50% area = 116 GtC) then we would HAVE to follow RCP 2.6 (cumulatively speaking).

    That means, 380 – 218 = 162 GtC versus 176 GtC C emissions to date (2000-2017 inclusive).

    In other words, we could already be 16 GtC in the hole, if humanity needs to do RCP 2.6 to stay below 1.5 degrees Centigrade.

    IMHO, Millar, et. al. need to publish a correction for some of their very misleading numbers (e. g. RCP 2.6 at 218 GtC and current CO2 emissions is more like two decades not several years). 😦

    Note to Self: This is just for C emissions used in the CMIP5 scenarios, for other GHG see above spreadsheet and assumptionjs made for those other GHG’s).

  7. Everett F Sargent says:

    “In other words, we could already be 16 GtC in the hole, if humanity needs to do RCP 2.6 to stay below 1.5 degrees Centigrade.”

    … should be …

    “In other words, we could already be 14 GtC in the hole, if humanity needs to do RCP 2.6 to stay below 1.5 degrees Centigrade.”

    Too many numbers, even for me. 😉

  8. Everett F Sargent says:

    “(e. g. RCP 2.6 at 218 GtC and current CO2 emissions is more like two decades not several years)”

    … should be …

    “(e. g. RCP 2.6 at 218 GtC and current C emissions is more like two decades not several years)”

    Strike two. 😦

  9. Steven Mosher says:

    Open question.

    when the 2C boundary was set, what was the relavant pre industrial period.

    I recall seeing some really cool charts that show the bad things that happen when we cross 2C
    what was the pre industrial figure for those?

  10. Probably the AR3 “burning embers”. Don’t recall the baseline.

  11. Ha, ha, it was called TAR then… how time does fly?

  12. Everett F Sargent says:

    SM,

    Manabe (1967) …
    Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity
    http://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281967%29024%3C0241%3ATEOTAW%3E2.0.CO%3B2

    … and …
    William Nordhaus (1975) …
    https://www.carbonbrief.org/two-degrees-the-history-of-climate-changes-speed-limit

    PI is something you all are still trying to figure out, but from the emissions spreadsheet I’d swag before 1850 or 1750 (from Historical Budget sheet). IANACS

  13. angech says:

    Is it possible to tie the OHC to a global temperature?
    I know that this is unreasonable on a yearly basis due to natural variatino.
    If so a yearly surface temperature reflective of the OHC should give rise to a global temp value.
    The interest here is how much higher a 1C rise in SST equates to an atmospheric temperature rise.
    Would it be huge like 5C or eqivalentish like just a C hotter?
    My guess would be the first.

  14. Steven,

    I recall seeing some really cool charts that show the bad things that happen when we cross 2C
    what was the pre industrial figure for those?

    Maybe this figure, which seems to show that the 2C is relative to actual pre-industry (mid-1700s).

    angech,

    Is it possible to tie the OHC to a global temperature?

    In some sense we can, because we know the upper ocean (well-mixed layer) will be in equilibrium with the atmosphere. Also, the OHC change of the upper 300m is about half of the OHC change in the upper 2000m, so one could probably do a basic calculation to relate OHC changes to surface temperature changes.

  15. angech,
    I missed this.

    The interest here is how much higher a 1C rise in SST equates to an atmospheric temperature rise.

    A 1C rise in SST (sea surface temperature) would be associated with a 1C rise in surface temperature. If you mean average ocean temperature, then that is currently a lot smaller than the rise in surface temperature, but that’s mostly because most of the energy is going into the upper ocean and when you average of the whole ocean, you get a small number. My understanding is that as we approach equilibrium, the deeper ocean will warm more and more so that, eventually, the change in ocean temperature is comparable to the change in surface temperature.

  16. verytallguy says:

    That’s a very informative chart, nicely showing major impacts, including, notably that 2C is ~the midpoint of where we expect a tipping point for Greenland.

    Do you have a reference for the chart?

    Also, anyone know of a reference for the Greenland tipping point that feeds into the chart? (I think there was a section in the AR4 for that, but I’d guess there is newer science by now)

  17. vtg,
    It’s from this paper by Hans Joachim Schellnhuber, Stefan Rahmstorf & Ricarda Winkelmann (Why the right climate target was agreed in Paris) but which – for some reason – the Nature site can no longer find (I even searched their own site and it still gave an error when I tried to access the paper).

  18. verytallguy says:

    Thanks AT.

    Angech’s confusion on the relationship between SST and surface temperature raises some interesting points.

    My understanding:
    As measured SST is a lower bound on surface temperature anomaly as temperatures rise. That’s because SST is measured as a proxy for air temperature 2m above the surface, which is what is measured on land. Land temperatures rise much faster than air temperatures, but the difference is not many degrees as Angech seems to think:

    So far so simple. My understanding is then that one of the issues comparing model data to measured data is that model data is typically reported as the 2m air temperature, whereas the measured data assumes that is the same as SST, which in fact it isn’t quite. Please, someone correct me if I have this wrong. There’s then the issue of masking, ie only comparing model data from regions which we have reliable surface data for.

    Angech finally seems to be confusing total ocean temperature with SST. Here I don’t think there can be no simple relationship. The difference between SST and average ocean temperature will depend surely on the rate of overturning (mixing) in the ocean and the rate of surface warming.

    I’m not sure if

    My understanding is that as we approach equilibrium, the deeper ocean will warm more and more so that, eventually, the change in ocean temperature is comparable to the change in surface temperature.

    is correct or not.

    Is it the case that ocean overturning is driven by cold water sinking at the poles (which is why, counter to the situation on land, deep water is cold)?

    If that is true, then even in a much warmer world, the temperature of downwelling water is limited by the freezing point, so we’ll still have very cold deep water independent of the average surface temperature, and the average ocean temperature will therefore never rise by as much as the average surface temperature. The rate of downwelling is also expected to be reduced by global warming, though I’m not sure if that is expected to be a transient effect as ice sheets melt or permanent.

    I could, of course, be wrong and haven’t been able to quickly find any reference to back up my speculations.

  19. vtg,

    As measured SST is a lower bound on surface temperature anomaly as temperatures rise. That’s because SST is measured as a proxy for air temperature 2m above the surface, which is what is measured on land.

    My understanding is that the SST versus 2m issue is to do with the rate at which they warm. Hence, it seems that once we’ve returned to equilibrium, it should all have – approximately – warmed by the same amount.

    As far as the overall change in ocean temperature is concerned, I’m not sure myself. All else being equal, we would probably expect the new equilibrium to be such that everything has warmed by about the same amount, otherwise there would be net energy flows that would still be warming parts of the system. However, if we do see some kind of big change in some of the ocean cycles, then it’s possible that some parts will end up warming more/less than others. However, I still think that – approximately – it will all eventually warm by a similar amount.

    The reason there is currently a big difference between the change in temperature of the whole ocean and the change in temperature of the surface is not (I think) because we expect the change in surface temperature to be much greater than the change in overall ocean temperature, but because the deeper parts of the ocean take longer to warm.

  20. vtg,
    I should have read this

    If that is true, then even in a much warmer world, the temperature of downwelling water is limited by the freezing point, so we’ll still have very cold deep water independent of the average surface temperature, and the average ocean temperature will therefore never rise by as much as the average surface temperature.

    Yes, this is a good point. The deep ocean temperature is probably set by the freezing point of water (or that water at about 4oC is densest) so these regions may not warm by an amount comparable to the surface.

  21. Okay, vtg’s comment has made me very unsure as to how much we expect the deeper parts of the ocean to warm. It may well be that there will still be quite a substantial difference between the change in average ocean temperature and the change in surface temperature (because the deeper parts of the ocean will be set by the temperature of the densest water – about 4oC). However, I still think that the main reason there is such a large difference now between the average change in total ocean temperature and surface temperature is mostly because the deeper parts of the ocean have warmed little and warm slowly.

  22. Marco says:

    ATTP, there is a problem with the Nature site at the moment. Most links, including that to any journal, give a server error.

  23. Marco,
    Okay, thanks (I seem to often have a problem accessing their site, so I wondered if it was just me).

  24. Phil says:

    @ATTP

    The deep ocean temperature is probably set by the freezing point of water (or that water at about 4oC is densest) so these regions may not warm by an amount comparable to the surface.

    The freezing point of water depends on the pressure, and the interesting/unique property of water is that with increasing pressure the freezing point gets less (Damn those hydrogen bonds! 🙂 ). This is best phase diagram I could find online. Clearly the deep ocean will be at greater pressures than the surface.

    The other point is that, of course this a phase diagram for pure water, not saline ocean water, you may need to look at Raoult’s Law.

    I’m not clear at all whether the temperature of the maximum density of liquid water changes with pressure, but my intuition suggests it would.

    That’s probably just made you even more unsure, but I thought I should mention it …

  25. Phil,
    Thanks.

    I’m not clear at all whether the temperature of the maximum density of liquid water changes with pressure, but my intuition suggests it would.

    Not sure either, but probably. However, it’s probably the case that the temperature of the water in the deepest parts of the ocean is set by this, and therefore won’t warm much as the surface warms.

  26. Andrew Dodds says:

    If memory serves, the whole most densest-above-freezing-thing doesn’t work for salt water, cf

    The deep ocean temperature will be set by the temperature of deep water formed off of Antarctica – I’d guess that this is unlikely to change much(?) unless the whole deep water circulation system breaks down. After all, there is no particular reason why you would expect average ocean temperatures to be much different from surface temperatures, unless you knew in advance that the circulation was driven by descending cold water at the poles.

    Which leads to an interesting thought in itself; if we managed to switch the oceans to a circulation driven by the descent of warm, highly saline water formed from evaporation in the tropics, the whole ocean could heat up by ~20K for the same surface radiation budget. Unlikely this millennium though.

  27. Andrew Dodds says:

    Hmmm link from https://en.wikipedia.org/wiki/Seawater didn’t appear.. trying

  28. Here’s a comment by Ray Pierrehumbert.

    Regarding the temperature of the deep ocean:

    There is no reason the deep ocean would keep getting colder and colder. Liquid water is so opaque in the infrared, it really has no way of losing heat. Vertical heat transport is fairly small, but insofar as it’s there, the vertical heat transport is downward. And (small though the effect is), heat is coming OUT of the hot interior of the earth into the ocean, not going the other way.

    Since the densest water sinks, the temperature of the deep ocean is set by the temperature of the water at the surface that becomes dense enough to sink. This is not a columnwise 1D process (unlike the tropical atmosphere, which really does behave in a sort of 1D way). The temperature of the deep ocean pretty much follows the polar temperatures, with some allowance for salinity effects.

    As others have noted, the equation of state for water does not come close to the liquid/solid transition even at the large pressures of the bottom of the ocean. On a very water-rich exoplanet, however, with a much deeper ocean, you can actually form Ice VI, which unlike Ice 1, stays at the bottom of the ocean. Very interesting oceanography. But Earth is nowhere near that regime.

    Fourier noted that the temperature goes down with depth in the ocean, opposite to the behavior in the atmosphere. He had some attempt at explaining that, but was pretty much off the mark, as he was for the atmosphere as well (though his atmospheric idea would be sort of on the right track for an atmosphere with a radiative-equilibrium rather than convective troposphere). Right about lotsa stuff, tho’

  29. Overturning of stratification occurs in both the ocean and freshwater lakes. In the ocean it leads to the thermohaline circulation. In high latitude cold-water lakes, it’s extreme because of ice formation
    https://www.nationalgeographic.org/media/lake-turnover/

    What I am interested in are the impulses that seem to occur every mid-November in the Pacific Ocean.

  30. I tried posting this several times yesterday – but WordPress. 2 or 3 versions are probably stuck in moderation.

    Ed Hawkins, Climate Lab Book, Defining pre-industrial, January 25, 2017:

    The UN Paris Agreement on climate change aims to ensure increases in global temperature are less than 2°C above ‘pre-industrial’ levels, with an aspirational 1.5°C limit. However, the ‘starting line’ of the pre-industrial era is not defined by the UN agreements, or by the Intergovernmental Panel on Climate Change (IPCC).”

    and later in the same post which I assume is taken from cce’s reference (Estimating changes in global temperature since the pre-industrial period, Ed Hawkins*, Pablo Ortega, and Emma Suckling)

    We suggest that the earlier period of 1720-1800 is a better choice for this baseline. This is because the major natural factors that also affect Earth’s climate – the levels of solar and volcanic activity – were both at similar levels to today (see Figure 1).”

  31. In some sense we can, because we know the upper ocean (well-mixed layer) will be in equilibrium with the atmosphere. Also, the OHC change of the upper 300m is about half of the OHC change in the upper 2000m, so one could probably do a basic calculation to relate OHC changes to surface temperature changes.

    Regarding that “well mixed layer”, it’s well mixed after a fashion, and averaging over the surface of all the oceans. At any particular spot it’s not that well mixed, and often requires storm turbulence to do the mixing. Moreover between 30S and 40N, incoming solar radiation (Qs) is on average greater than loss due to:
    * Qb, net rate of heat loss by sea as longwave radiation back to atmosphere and some to space,
    * Qh, rate of heat loss or gain through sea surface by conduction (“sensible heat flux”)
    * Qe, rate of heat loss or gain due to evaporation or condensation (“latest heat flux”)

    The balance then needs to be achieved by:
    * Qv, rate of heat loss or gain by a water body due to currents (“advection”).

    Indeed, Qv is what drives the ocean currents. It is also estimated to be the primary vehicle for carrying heat energy to depth.

  32. Is it the case that ocean overturning is driven by cold water sinking at the poles (which is why, counter to the situation on land, deep water is cold)?

    Not so simple. Reasons why are detailed in (e.g.) Talley, Pickard, Emery, and Swift’s Chapter S7, “Dynamical Processes for Ocean Circulation”, and illustrated below:

    There’s also a controllable visualization of surface warming projections for places as a function of atmospheric CO2. See
    [video src="http://www.gfdl.noaa.gov/video/cm26_gulf_of_maine_v4.mp4" /]
    And there’s a nice NASA movie here:

  33. Authors have another response. I think it makes the point that I was trying to make here about the difference between emissions and concentrations. Makes another interesting argument (if I understand it right) which is essentially that GMST is defined in terms of the HadCRUT4 dataset and essentially that the 1.5oC limit is when HadCRUT4 would reach 1.5oC. Therefore, under this definition we have about 0.6oC to go.

    I don’t necessarily agree with that, but it does illustrate that one of the issues seems to be that we haven’t properly defined the baseline, or what we mean when we say 1.5oC. Does it mean an actual globally averaged warming of 1.5oC or 1.5oC as determined by one of the commonly used temperature datasets.

  34. Steven Mosher says:

    ATTP

    THANKS! That was the chart

  35. Steven Mosher says:

    On A related Issue.

    Maybe gavin can help

    It would be great for CMIP 6

    Standard OUTPUT required for Submission to the Archive.

    A NetCDF of

    A) SST&SAT Unmasked
    B) SST & SAT Masked for HADCRUT

  36. Okay, I’ve learned something today. The density of seawater increases with decreasing temperature for all temperatures above freezing point if the salinity is above 24.7. So, there is (if I understand this correctly) no temperature above freezing at which seawater has a maximum density (apart from inifinitesimally close to freezing).

  37. Magma says:

    “The density of seawater…” — ATTP

    From Everett’s third link:
    “A single algorithm for seawater density (the 75-term computationally-efficient expression ṽ (Sₐ ,Θ, p)) can now be used for ocean modelling, for observational oceanography, and for theoretical studies.”

    Yet another case of assumptions that many of us may have extrapolated from simple facts or principles learned early that lead to error. I’m going to play with the MATLAB version of that toolbox and generate plots for density vs. various combinations of pressure, temperature and salinity.

  38. Everett F Sargent says:

    Well that got translated wrong, so let’s try again …

    From Millar (2017) …
    “Long-term anthropogenic warming is determined primarily by cumulative emissions of CO2(refs 7 10): the IPCC Fifth Assessment Report (IPCC-AR5) found that cumulative CO2 emissions from 1870 had to remain below 615 GtC for total anthropogenic warming to remain below 1.5 C in more than 66% of members of the 5th Coupled Model Intercomparison Project (CMIP5) ensemble of Earth system models (ESMs)(11)

    Reference (11) citation in Millar (2017) …
    11. IPCC Climate Change 2014: Synthesis Report (eds Pachauri, R. K. & Meyer, L. A.) (Cambridge Univ. Press, 2014).

    … here …
    So I found Millar (2017) carbon number of 615 GtC by 2100 in their reference (11) …
    http://www.ipcc.ch/pdf/assessment-report/ar5/syr/SYR_AR5_FINAL_full_wcover.pdf

    … page 64 …

    “Table 2.2 | Cumulative carbon dioxide (CO2) emission consistent with limiting warming to less than stated temperature limits at different levels of probability, based on different
    lines of evidence. (WGI 12.5.4, WGIII 6)”

    less than 1.5°C column
    Likely 66% sub-column
    Complex models, RCP scenarios only (see footnote c) row …

    2250 GtCO2 = 12*2250/44 = 614 GtC (or ~615 GtC)

    Now, let’s read Footnote c … shall we …

    “Cumulative CO2 emissions at the time the temperature threshold is exceeded that are required for 66%, 50% or 33% of the Coupled Model Intercomparison Project Phase 5 (CMIP5) complex models Earth System Model (ESM) and Earth System Models of Intermediate Complexity (EMIC) simulations, assuming non-CO2 forcing follows the RCP8.5 scenario. Similar cumulative emissions are implied by other RCP scenarios. For most scenario–threshold combinations, emissions and warming continue after the threshold is exceeded. Nevertheless, because of the cumulative nature of CO2 emissions, these figures provide an indication of the cumulative CO2 emissions implied by the CMIP5 model simulations under RCP-like scenarios. Values are rounded to the nearest 50.”

    This part “assuming non-CO2 forcing follows the RCP8.5 scenario” IMHO is kind of IMPORTANT.

    So, for all other GHG’s (and other stuff) they used RCP 8.5 to come up with their very bogus 615 GtC number.

    I see deniers do this all the time, cherry pick a low ball figure to enhance their overall conclusion. Bad! Sad! 😦 😦 😦

    I’ve got another one in the pipeline, if I am allowed to do so?

  39. Everett F Sargent says:

    OK, the 2nd try got through, so please delete my 1st try, if you don’t mind. TIA 🙂

  40. Phil says:

    The density of seawater increases with decreasing temperature for all temperatures above freezing point if the salinity is above 24.7

    Interesting. This, of course, appears to contradict the fact that sea ice floats. However, if my memory serves correctly, as sea water freezes it deposits out most of the salt. So sea ice is less saline than the water it freezes from, which also (presumably) makes the ice less dense. If I’m correct, then there are two distinct reasons why ice floats – depending on whether you’re talking about pure water (the density argument) or salt water. This may explain Magma’s observation about extrapolating from simple facts.

  41. Everett F Sargent says:

    Freshwater rho ~1
    Seawater rho ~ 1.025
    Ice (freshwater) rho ~ 0.92
    Ice (seawater) ~ 0.92*1.025/1 ~ 0.943 (just a swag, don’t quote me on this please)

    As you say by the time it’s 2nd year ice most of the salt has left the building, so to speak.

  42. However, if my memory serves correctly, as sea water freezes it deposits out most of the salt. So sea ice is less saline than the water it freezes from …

    Yes, there is a brine rejection process which is varied, and can result in brine cells (especially with fast freezing) but sea ice retains a salinity as high as 15 psu for new ice, but more typically 4-10 psu. (Old ice has its brine cells sorted out.) Generally pure ice is about 920 kg/m^3m but measurements have found 857-924 kg/m^3, with the lower values being because of trapped gas bubbles. The mechanics of sea ice freezing are involved, requiring a depth of water to all achieve freezing temperature at the salinity concentration in order for any to form. Sea ice tends to form along coasts where the water is shallow and brackish. 24.7 psu is a “magic number” because it’s the concentration where maximum density is achieved at sea water’s freezing point and then density decreases as temperatures decrease. Rem freshwater max density is achieved about 4C, above the freezing point.

    See Talley, Pickard, Emery, Swift, Descriptive Physical Oceanography, An Introduction, 6th edition, 2011, Chapter 3 and references for further details.

  43. verytallguy says:

    Hyper, thanks for the background and links. ‘re. “Not so simple.” Sure, nothing ever is!

    But is that the most basic explanation of oceanic circulation, or would you put it differently?

  44. Everett F Sargent says:

    A review of sea ice density
    https://www.researchgate.net/publication/44072998_A_review_of_sea_ice_density
    A review of the engineering properties of sea ice
    https://www.researchgate.net/profile/Wilford_Weeks/publication/259212916_Engineering_Properties_of_Sea_Ice/links/549deed50cf2d6581ab64313/Engineering-Properties-of-Sea-Ice.pdf

    My 1st real job was one summer at the USACE CRREL in 1975. Later on, I did quite a bit of OTEC scale modeling at Cornell (buoyancy or stratified flows using both heated water (surface) and saltwater (bottom flows) in the late 70’s (into the early 80’s)).

  45. But is that the most basic explanation of oceanic circulation, or would you put it differently?

    The “most basic“ explanation is, simply, that in the equatorial region, more energy goes into the oceans, expressed as heat, than comes back out in that region. Accordingly, and like the atmosphere, that energy needs to equilibrate, so it heads towards either polar region, in the form of currents. But complexity then comes into the picture, because ocean waters do not mix well … (I believe you can think of that as resulting from their greater viscosity in comparison to air.) They remain in rivers differentiated by density, mostly salt, some by temperature, but, also, they tend to remain intact simply because once they start moving, there’s a streamflow effect. And they subduct in places. Adorn the amazing stuff that’s water with its vastly greater thermal capacity, and because of viscosity, the spatial scale on which currents operate tends to be in the 40 km to 60 km range, rather than hundreds of km like atmospheric systems, and you’ve got a really interesting and critical part of climate. It’s one which, incidentally is not terribly well understood and, so, it’s why I’m a strong financial supporter and big fan of Woods Hole Oceanographic Institution. I’m continually blown away when I learn about ocean systems. Even their units are astonishing, like the Sverdrup. 1.2 Sverdrups is about the flow of all the rivers on Earth. The AMOC, of which the Gulf Stream is a part, has historically carried at 20 Sverdrups. (Recall a cubic meter is about a tonne of mass, and a Sverdrup is a million cubic meters flowing per second.) The long term trend of the AMOC flow is decreasing, but it’s variable. See Tal Ezer, 2015, “Detecting changes in the transport of the Gulf Stream and overturning circulation from coastal sea level data: The in 2009–2010 and estimated variations for 1935–2012”, Global and Planetary Change.

  46. John Hartz says:

    Speaking of natural sinks…

    A surprising scientific study released Thursday presents troubling news about the enormous forests of the planet’s tropical midsection — suggesting that they are releasing hundreds of millions of tons of carbon to the atmosphere, rather than storing it up in the trunks of trees and other vegetation.

    The new results, published in the journal Science, contradict prior work in suggesting that these forests — including the Amazon rain forest but also huge tropical forests in Indonesia, Congo and elsewhere — have become another net addition to the climate change problem. However, the new accounting also implies that if the current losses could be reversed, the forests could also rapidly transform into a powerful climate change solution.

    “The losses due to deforestation and degradation are actually emitting more CO2 to the atmosphere, compared with how much the existing forest is able to absorb,” said Alessandro Baccini, the lead author of the study and a researcher at the Woods Hole Research Center. He conducted the study with fellow scientists from Woods Hole and Boston University.

    Tropical forests used to protect us from climate change. A new study says they’re now making it worse. by Chris Mooney, Mooney, Energy & Envronment, Washington Post, Sep 28, 2017

  47. Yeah, but if that’s so, how come it isn’t showing up in the Keeling Curve? From Graven, Keeling, Piper, Patra, Stephens, Wofsy, Welp, Sweeney, and Tans (2013):

    I mean, there is an enhancement of the uptake and an enhancement of the release, but not much change otherwise. Am I missing something?

  48. hyper,
    According to Scott Denning (on Twitter) this implies that there must be more uptake in the non-Tropical regions. The problem, he says, is that these will probably saturate soon, which could be an issue if the Tropics are less of a net sink than we had expected.

  49. angech says:

    …and Then There’s Physics says:
    “A 1C rise in SST (sea surface temperature) would be associated with a 1C rise in surface temperature. ”

    Ta . Appreciated. The stuff about ice and density and sinking really gets tough. As an aside if the deep sea ice stays cold and dense how can the surface very cold water get to the bottom?
    It is definitely not as dense as the water on the bottom which is already very cold and denser as it is under so much more pressure? And a comment was made as to how impervious to heat loss it is, RPH. I guess if it is more dense from having a higher concentration of salt in it derived from the salt that did not get taken up in the ice formation and that it is able to lose more heat by conduction through the ice from under the ice that overall it is a heavier, colder package and as it sinks it will become more dense.

    Re your comment “essentially that GMST is defined in terms of the HadCRUT4 dataset and essentially that the 1.5oC limit is when HadCRUT4 would reach 1.5oC. Therefore, under this definition we have about 0.6oC to go.”

    What starting point time is this? 1850?

  50. Thorsten Mauritsen says:

    ATTP, concerning already committed warming, I don’t think that is so simple that temperature stays roughly constant at the level it is at the time at which emissions stop. Burning fossil fuels also emits aerosol and other short-lived greenhouse gases, which together most likely have a cooling effect. Once emissions of these constituents stop, there will therefore likely be some warming on top of the already realized warming. Robert Pincus and I wrote a paper about this recently:

    http://www.nature.com/nclimate/journal/v7/n9/full/nclimate3357.html

  51. Thorsten,

    I don’t think that is so simple that temperature stays roughly constant at the level it is at the time at which emissions stop. Burning fossil fuels also emits aerosol and other short-lived greenhouse gases, which together most likely have a cooling effect

    Thanks. I was aware that aerosols and non-CO2 GHGs were complicating factors, but wasn’t sure to what extent. I’ll have a look at the paper.

  52. This is an interesting comment in the abstract of Thorsten’s paper

    However, when assuming that ocean carbon uptake cancels remnant greenhouse gas-induced warming on centennial timescales, committed warming is reduced to 1.1 K (0.7–1.8). In the latter case there is a 13% risk that committed warming already exceeds the 1.5 K target set in Paris15.

  53. Everett F Sargent says:

    ATTP,

    I just found the Thorsten paper two days ago, didn’t know if I should post it here though. Nonetheless, it’s good to see it posted here by the lead author.

    I’m sort of backing away from the Millar (2017) paper as they appear to have mixed CO2/C budgets with CO2e budgets (or do I blame the IPCC).

  54. Re: The Scott Denning thing. I’m not on Twitter, or Facebook, for that matter, so there’s no opportunity to engage. However I think there needs to be some clarity on what “saturate” means. Does it mean the reservoir is full, because depletion rates are so flow and it has comparatively finite capacity? Or does it mean, as with some other Carbon sinks, that there is a limit to how much can be accepted into the reservoir per unit time, and that is being approached or exceeded? Perhaps it doesn’t matter from a predictive and practical perspective, but it might. For instance, if something structurally changed, the throttle might be forced larger. On the other hand, if the reservoir is full, well, then that’s it for the reservoir for the foreseeable future.

    In any case, it’s entirely possible we’re bumping up against these kinds of limits. When we do, we should see it in the Keeling Curve, unless there are offsets.

  55. Here’s the paper Denning must be talking about:

    A. Baccini, W. Walker, L. Carvalho, M. Farina, D. Sulla-Menashe, R. A. Houghton, “Tropical forests are a net carbon source based on aboveground measurements of gain and loss”, Science, 28 September 2017.

    I grabbed it but I’m not likely to read it in full unless there’s some special statistical interest. I’m sure it’s valuable, but I have a stack of technical articles, mostly statistical, some climate, to wade through, and I’m not likely to emerge from that for weeks.

  56. Eli Rabett says:

    Hyper, it is obvious that at some point as temperatures increase decay will overwhelm CO2 fertilization, but that it has turned that corner is extremely worrying. Add that to the CAGW pile.

  57. Pingback: The work of Alec Bogdanoff and Carol Anne Clayson on the ocean surface boundary layer | Hypergeometric

  58. Thanks, Eli. Yes, agree on worry. Glen Peters made his comment to me about concerns regarding overwhelming of natural sinks in the context of a discussion about the cost of doing at-scale clear air capture of CO2. He properly corrected me on two points, on the need to be able to roll this out, and then, once a substantial amount of drawdown was achieved, that the reservoir that needs to be drained isn’t just CO2 in atmosphere, but these natural reservoirs as well. That increases the amount of work by a lot and, it means, essentially, that we need to take out all that’s been emitted since fossil fuel burning began. The implications of this upon cost estimates for real drawdown are daunting, and, so, the worry is squared.

  59. izen says:

    It seems unlikely we could exceed the capacity of the Carbon cycle to remove our additions. It coped with the larger PETM, eventually.

    The Keeling curve indicates we have not exceeded the rate at which it can absorb extra Carbon. With an increasing level and increasingly large additions it has shown stability in the ratio of Carbon in the atmosphere and geochemical/biological land and ocean reservoirs.

    If the Keeling curve does start to indicate that a greater percentage of our FF emissions remain in the atmosphere it would indicate a significant disruption of the biological part of the cycle. The partition ratio would have to change because of a shrinking of the ocean biological cycle.

    That can be driven by an anoxic ocean event. But paleo evidence indicates we need to warm around 12 deg C for that to be a risk.

  60. Pingback: A bit more about committed warming | …and Then There's Physics

  61. @izen,

    Sure, it will remove emissions eventually, but that is not on a time scale which is meaningful to civilizations which by all accounts can’t seem to think on the scale of a single century, let alone hundreds of them. ATTP is addressing some of this in his next post, and I have just download Thorsten Mauritsen and Robert Pincus’ paper for a read, prodded by ATTP’s direction. Looking forward to it.

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