Target CO2

We’ve often gone over the Charney sensitivity constraint for the Last Glacial Maximum. There is information about the greenhouse gases (CO2, CH4 and N2O), reconstructions of the ice sheets and vegetation change, and estimates of the dust forcing. A recent estimate of the magnitude of these forcings is around 8 +/- 2 W/m2 (Schneider von Deimling et al, 2006). This implicitly includes other aerosol changes or atmospheric chemistry changes in with the sensitivity (or equivalently, assumes that their changes are negligible). So given a temperature change of about 5 to 6ºC, this gives a Charney sensitivity of around 3ºC (ranging from 1.5 to 6 if you do the uncertainty sums).

Hansen suggests that the dust changes should be considered a fast feedback as well (as could the CH4 changes?) and that certainly makes sense if vegetation changes are included on the feedback side of the equation. Since all of these LGM forcings are the same sign (i.e. they are all positive feedbacks for the long term temperature change), that implies that the Earth System sensitivity must be larger than the Charney sensitivity on these timescales (and for this current geologic period). So far so good.

Hansen’s first estimate of the Earth System sensitivity is based on an assumption that GHG changes over the long term control the amount of ice. That gives a scaling of 6ºC for a doubling of CO2. This is however problematic for two reasons; first most of the power of this relationship is derived from when there were large N. American and European ice sheets. It is quite conceivable that, now that we are left with only Greenland and Antarctica, the sensitivity of the temperature to the ice sheets is less. Secondly, it subsumes the very special nature of orbital forcing – extreme regional and seasonal impacts but very little impact on the global mean radiation. Hansen’s estimate assumes that an overall cooling of the same magnitude of the LGM would produce the same extent of ice sheets that was seen then. It may be the case, but it is not a priori obvious that it must be. Hansen rightly acknowledges these issues, and suggests a second constraint based on longer term changes.

Unfortunately, prior to the ice core record, our knowledge of CO2 changes is much poorer. Thus while it seems likely that CO2 decreased from the Eocene (~50 million years ago) to the Quaternary through variations related to tectonics, the exact magnitude is uncertain. For reasonable values based on the various estimates, Hansen estimates a ~10 W/m2 forcing change over the Cenozoic from this alone (including a temperature-related CH4 change). The calculation in the paper is however a little more subtle. Hansen posits that the long term trend in the deep ocean temperature in the early Cenozoic period (before there was substantial ice) was purely due to CO2 (using the Charney sensitivity). He then plays around with the value of the CO2 concentration at the initiation of the Antarctic ice sheets (around 34 million years ago) to get the best fit with the CO2 reconstructions over the whole period. What he ends up with is a critical value of ~425 ppm for initiation of glaciation. To be sure, this is fraught with uncertainties – in the temperature records, the CO2 reconstructions and the reasonable (but unproven) assumption concerning the dominance of CO2. However, bottom line is that you really don’t need a big change in CO2 to end up with a big change in ice sheet extent, and that hence the Earth System sensitivity is high.

So what does this mean for the future? In the short term, not much. Even if this is all correct, these effects are for eventual changes – that might take centuries or millennia to realise. However, even with the (substantial) uncertainties in the calculations and underlying assumptions, the conclusion that the Earth System sensitivity is greater than the Charney sensitivity is probably robust. And that is a concern for any policy based on a stabilization scenario significantly above where we are now.

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157 comments on this post.
  1. Hank Roberts:

    Ocean Acidification’s Effects on Marine Ecosystems and Biogeochemistry – EOS meeting report
    The U.S. Ocean Carbon and Biogeochemistry Program sponsored a scoping workshop on ocean acidification research from 9-11 October 2007. The workshop brought together 93 scientists to address present and future acidification impacts, and to reach consensus on research priorities.

    ReCaptcha: “WATER injured”

  2. Mark:

    Chris #138:

    “there exists an unequivocal lead in CO2 concentrations over temperature increase (which we can contrast with the well-established paleoclimate temperature lead over CO2 levels)”

    But the paleoclimate record showing a temp lead over CO2 does not apply in this case. Mostly because Triceratops didn’t have an SUV. Tyrannosours didn’t dig oil wells. Humans do.

    “Death always follows heart failure”. That’s fine, but if you’re talking about someone whose head has been chopped off, I would say that citing this genuine fact doesn’t explain when death occurs IN THIS INSTANCE.

  3. David B. Benson:

    A fairly low cost route to removing carbon dioxide from the atmosphere is via enhanced mineral weathering (enhanced carbonate formation). Here are some links.

    Olivine weathering:

    See references 7, 8 and 9 in

    Peridotite weathering:

    Mine tailings:

    My cost estimates always come up about the same, or less, than the most optimistic of the usual CCS proposals. These have a cost similar to deeply burying biochar. So olivine, etc., weathering appears to be the best currently available solution; it has the further advantage of releasing micro-nutrients into a biologically available form, being then a soil amendment.

  4. Magnus Westerstrand:

    David I almost got involved in a project similar to what you are proposing for LKAB in Sweden… however it is on ice atm… As I can see from your links the problems seams to be the same as when LKAB thought about it, transport cost and kinetics… It would be interesting to go further on any how and they are still talking about it. Mainly about using the apatite rich mine tailings…

  5. Hank Roberts:

    > weathering

    So we need just one small asteroid hit, that imacts in exactly the right geological strata to throw up a lot of one of these materials ….

  6. David B. Benson:

    Here is the Tech R3eview article on peridotite weathering. If this works, carbon dioxide can be removed from the atmosphere for a very low cost per tonne:

    (Thanks to Micael Tobis for noticing this article.)

  7. Mark Singer:

    Responding to makale (No. 150) regarding: “(it is a preprint, not a paper)” and “We don’t know what it will be once it is a paper. Hansen has a habit of being right, but there may be some flaw in the analysis that a referee catches.”

    The situation has changed. Originally, on 7 April 2008, when this forum was started, the Hansen et al. article was a preprint (version 1). But the article (version 3) was published in The Open Atmospheric Science Journal, 2008, Volume 2, pp. 217-231, URL

    This journal states:
    “Aims & Scope
    “The Open Atmospheric Science Journal is an Open Access online journal, which publishes research articles, reviews, and letters in all areas of climate research and atmospheric science.

    “The Open Atmospheric Science Journal, a peer reviewed journal, aims to provide the most complete and reliable source of information on current developments in the field. The emphasis will be on publishing quality papers rapidly and freely available to researchers worldwide.” (See URL

    The paper as published states (p. 231) that it was received May 22, 2008, revised August 19, 2008, and accepted September 23, 2008.

    . . .