What level CO2 is “dangerous”?

Posted by William M. Connolley on March 6, 2007
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*I* don’t know. It depends on what you mean. Or perhaps as RP Jr said, “It is a little like saying, would you prefer a poverty rate of 10% or 8%? Well, lower is better, the question is how do you get there? Not by arguing about ideal poverty rates I’d say”. Which is a nice way of phrasing it, although there are problems with the analogy.

But the main reason for this post was to pull out of the comments over at inkstain this between Eli and RP:

ER: “Do you believe that any level of GHG CO2 equivalent mixing ratios would be so dangerous/costly as to be avoided through serious mitigation. If so where (My answer is 550-600 ppm, although to avoid that action will have to be taken almost immediately. In this I differ significantly with Tol and Nordhaus on when we have to start although not the end point.)”

RP: “450, though likely not in the cards. 550-600 also unlikely. As you know I don’t think that this is the best way to frame the problem or think about action”

I find this a bit curious… 450 is a very low value to consider dangerous, and as CO2 equivalent we’re practically there already (where are we at CO2e, anyway? Anyone know?).

[Update: I have two answers for this (which I belatedly realise you can get from the SPM fig 2: 430 (add all the +ve's) or 380 (include the negatives too, since net forcing is just about equal to CO2 forcing). The latter answer is probably better -W]

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  1. #1 Fergus Brown
    2007/03/06

    About 430, I believe.
    As we’re likely to reach 450 in about 5-6 years’ time, this figure may be moot, anyway. is it a coincidence that a reasonable projection for reaching 550 (eq) in about 40 years or so coincides with the estimated extent of oil reserves?

    Regards,

  2. #2 John L. McCormick
    2007/03/06

    Fergus, I agree with your 430 ppm estimate. But, your comment

    [about 40 years or so coincides with the estimated extent of oil reserves?]

    does not account for China now investing in Sasol coal-to-liquids and US Northern Plains coal fields, Canada tar sands and US Rocky Mountain oil shale deposits poised to extend the age of oil for a very long time. That is inevitable in a world of living-in-the-moment consumers. We are not smart monkies.

  3. #3 Adam
    2007/03/06

    “there are problems with the analogy.”

    Well 0ppm CO2 is not particularly desirable, for one.

  4. #4 SteveF
    2007/03/06

    This chap believes that elevated CO2 is dangerous because of increased levels of toxicity. Hmmmm.

    http://www.ias.ac.in/currsci/jun252006/1607.pdf

  5. #5 llewelly
    2007/03/06

    One nit on the 430 CO2 equiv vs 380 – Most of the negative anthro forcing in SPM-2 is in the form of aerosols (-1.2 W/m^2 total, vs -0.25 W/m^2 for strat O3 and land use), which have much shorter atmospheric lifespans than any of CO2, CH4, N2O, or the halocarbons. (I don’t know about the lifespan of tropospheric O3.)

    So the lifespan problems which makes CO2 equivalent a total of things that are really only temporarily equivalent seems to be a bigger problem with the 380 net figure than the 430 positive only figure.

  6. #6 Eli Rabett
    2007/03/06

    The lifespan of trop O3 is essentially that of NOx viz

    NO2 + hv –> O + NO
    O + O2 + M –> O3 + M

    NO cycles back to NO2 by reactions with RO2 principally HO2 or reactions on surfaces.

    I strongly disagree with the statement about the uselessness of targetting poverty rates and GHG mixing ratios. RPJ may have missed the bit about equivalent OTOH he may not have.

  7. #7 Hank Roberts
    2007/03/07

    To paraphrase a political slogan:
    It’s the ocean, stupid ….
    http://www.pnas.org/cgi/content/abstract/104/9/3037?lookupType=volpage&vol=104&fp=3037&view=short

  8. #8 Adam
    2007/03/07

    JFleck links to this paper from his blog (I’ve not noticed a reference to it anywhere else yet): http://tinyurl.com/324g4r

  9. #9 Luboš Motl
    2007/03/07

    I think that at 800 ppm, the normal high end of office concentration, you may start to feel unfresh, trying to open the window.

    Although I am sure that air-conditioning in 2200 when this concentration is reached will include oxygen production, still, it would be good to feel fresh without relying on technology. So it could be a good idea to slow CO2-producing things down when we are approaching 800 ppm around 2200 in the case that we don’t have alternative sources of energy such as nuclear fusion.

    Regulating things below 800 ppm or so is kind of silly. 450 million years ago, we probably had 3000 ppm, as shown by a small number of stomata on fossil leaves. And the leaves were very happy: the Earth was obviously not evaporating. ;-)

    It’s not a good idea to try to regulate great grandsons’ lives because they will surely look at us as some kinds of monkeys, and if we confined their lives, they would surely be upset about our generation.

  10. #10 Eli Rabett
    2007/03/07

    pH lubos.

  11. #11 Adam
    2007/03/08

    OT but EU plans for 20% of 1990 levels reduction?

    http://news.bbc.co.uk/2/hi/europe/6429097.stm

  12. #12 guthrie
    2007/03/08

    IS Lubos actually a scientist? His continual lack of scientific rigour makes me laugh.

    For example:
    “Regulating things below 800 ppm or so is kind of silly. 450 million years ago, we probably had 3000 ppm, as shown by a small number of stomata on fossil leaves. And the leaves were very happy: the Earth was obviously not evaporating. ;-)”

    So, on the basis of fossilised remains of plants which, 450 million years ago were rather different to modern plants, and that inhabited completely different ecosystems in which the modern ancestors of mammals were barely even noticeable, Lubos proclaims that a much higher CO2 levels than current are not really a problem.

  13. #13 Eli Rabett
    2007/03/08

    Took the discussion over to my place. Roger and I came to another agreement, that adaptation alone is not sufficient.

  14. #14 Adam
    2007/03/08

    I always thought that as adaptation would become a de facto policy without any need for lobbying, it’s a bit pointless for anyone to spend their time calling for it – except in those areas where the effects are subtle or only uncovered by research, and then that would be down to people in the field (one example was a discussion by conservationists working on a recreation of the Great Fen in Norfolk on sea-water incursion).

    That would mean that the main thrust of any discussions (especially by policy wonks), would be on mitigation…even if you’re not even aiming for a point at which to stop.

    As for whether or not to have targets, I tend to think of it like the old cliché: working to a specification is a bit like walking on water, it’s a lot easier if it’s frozen.

    You can then aim for a more desirable one once you’ve achieved your first cut. I think Hansen’s alternative scenario was along these lines?

  15. #15 Hank Roberts
    2007/03/08

    Will this help make the numbers fit what’s happening to ocean pH?

    “A refined approach to define anthropogenic CO2 is introduced that has a stronger thermodynamic orientation …. is discussed. Here likely changes in the CO2 solubility pump are a thermodynamic property of this definition, whereas it is a separate phenomenon in the mechanistic definition. …”

    http://www.blackwell-synergy.com/doi/abs/10.1111/j.1600-0889.2005.00173.x

  16. #16 llewelly
    2007/03/08

    I always thought that as adaptation would become a de facto policy without any need for lobbying, …

    High quality studies showing the New Orleans levees to be inadequate in the face of a major hurricane landfall, were published every few years, from about the mid-1980s until the present. Little has been done. This is only one case of many, which shoes that in some parts of the world, it is indeed necessary to lobby for adaptation to conditions which have been present for at least decades, and probably centuries , let alone conditions forecast to appear due to global warming.

  17. #17 llewelly
    2007/03/08

    Correction to my previous post – I intended to say, ‘and probably millenia’ rather than ‘and probably centuries’ .

  18. #18 Adam
    2007/03/08

    That’s true, and the same can be said of examples like the high winter death rate in the UK as well as death rates from vehicle pollution I suppose.

    Humans have tended to adapt on the reactionary rather than precautionary principle. That is, planes bugs get fixed after a crash, cure rather than preventative medicine, moving away from areas after repeated flooding, etc.

    Larger scale, people will need to adapt agricultural practices etc. I suppose the big question is predictive adaptation vs reactive adaptation? We’ll probably get both even if there’s many calls for the former. That fact that we’re still adapting to centuries & millennia scale issues shows that and that adaptation is the de facto policy. ;)

    Meantime, that issue is of a smaller scale given decent mitigation.

  19. #19 Adam
    2007/03/09

    EU agrees to cut ghg emissions to 20% below 1990 levels.

    http://www.guardian.co.uk/eu/story/0,,2030142,00.html

    According to the 2006 inventory report (http://reports.eea.europa.eu/technical_report_2006_6/en) the emissions at 2004 were at 95.2% of 1990 levels for the EU-25 and the EU-15 are at 99% of 1990 levels. The 2012 target for the EU-15 was 92%.

    Taking EU-15 for the moment, the emissions total in 1990 was 4,265.7 million tonnes, so 80% of that is 3412.56 million tonnes. The EU-25 emitted 5370.7 million tonnes of which 80% is 4296.56 so a reduction of about 100million tonnes a year would be required. Note this is CO2 equivalent and ignoring land use, land use change and forestry.

    The 30% reduction if other countries join in is a nice idea…but I’d have thought that the quickest way to reduce Chinese emissions would be to stop buying Chinese manufactured goods. ;)