Climate change commitments

There is an interesting letter in Nature Geoscience this month on what climate changes we have actually already committed ourselves to. The letter, by Mathews and Weaver (sub. reqd.), makes the valid point that there are both climatic and societal inertias to consider.

Their figure neatly demonstrates the different issues:

The upper line is often what is referred to as the ‘climate change commitment’ (for instance Wigley, 2005). This is the warming you get if we keep CO2 (and other GHG and pollutant levels) constant at today’s values. (Technically, the figure shows the case staying at year 2000 values). In such a scenario, the planet still has a radiative imbalance, and the warming will continue until the oceans have warmed sufficiently to equalise the situation – giving an additional 0.3 to 0.8ºC warming over the 21st Century. Thus the conclusion has been that because of climate inertia, further warming is inevitable.

However, constant concentrations of CO2 imply a change in emissions – specifically an immediate cut of around 60 to 70% globally and continued further cuts over time. Matthews and Weaver make the point that this is a little arbitrary and that the true impact of climate inertia would be seen only with emissions cut to zero. That is, if we define the commitment as the consequence only of past emissions, then you should set future emissions to zero before you calculate it. This is a valid point, and the consequence of that is seen in the lower lines in the figure.

CO2 concentrations would start to fall immediately since the ocean and terrestrial biosphere would continue to absorb more carbon than they release as long as the CO2 level in the atmosphere is higher than pre-industrial levels (approximately). And subsequent temperatures (depending slightly on the model you are using) would either be flat or slightly decreasing. With this definition then, there is no climate change commitment because of climate inertia. Instead, the reason for the likely continuation of the warming is that we can’t get to zero emissions any time soon because of societal, economic or technological inertia.

That is an interesting reframing of an issue that comes up all the time in discussions of adaptation and mitigation. This is because it demonstrates that adaptation (over and above what is necessary to reduce vulnerabilities to current climate conditions) is unnecessary if mitigation is dramatic enough.

However, the practical implication of this reframing is small. We are clearly not going to get to zero emissions any time soon, and even the 60-70% cuts required to stabilise concentrations initially seem a long way off. Thus as a practical matter, it doesn’t really matter whether the inertia is climatic or societal or technological or economic because the globe will continue to warm under all realistic scenarios (what we do have a possible control over is the magnitude of that warming). Thus further adaptation measures will still be needed.

212 comments on this post.
  1. flxible:

    Gilles – “(…) probably we could fit also data with an influence of major wars as well.”
    Or maybe to fossil fuel production?

    I’m amazed at the improvement in your grasp of english latley, but your “debate” continues to hover at the same level of simple contrarianism.

  2. Len Ornstein:

    Gavin: about 185 and your comment there:

    At the moment of a switch to “zero emissions” (“at THAT moment”) there would be about 0.6ºC of ‘surface warming’ still in the pipeline”.

    Do you mean to imply that the subsequent decrease of CO2 in (“so absent any human emissions, there would be a net decrease of CO2”) would cancel the temperature rise that would otherwise result from the warming still in the pipeline so that M & W’s “zero emission” plot would immediately go flat?

    That’s very hard to believe – and if so, certainly should have required explication by M & W – or you!

  3. David B. Benson:

    Gilles (199) — Tamino’s to box model follows from the known physics. Read Gavin Schmidt (with co-authors) about ModelE and also

    I have an even simplier conceptual model based on the known physics:

  4. Gilles:

    Ray : “Gilles, if you were in fact a scientist”

    thais only proves your low capability of discerning the truth, at least as far as I am concerned.

    , you would know the difference between doing a fit to an arbitrary putative forcing and a KNOWN forcing like volacnism.
    Tamino isn’t trying to fit the short-term response. You should understand that by the fact that he is doing a 30-year smooth. He’s interested in the response of the oceans to the mean level of volcanism. That seems to me to be a very reasonable question AND it gives what is actually quite a good match.

    KNOWN forcing ? let’s read the AR4, chapter :

    “The considerable difficulties in calculating hemispheric and regional volcanic forcing changes (Robock and Free, 1995; Robertson et al., 2001; Crowley et al., 2003) result from sensitivity to the choice of which ice cores are considered,
    assumptions as to the extent of stratosphere penetration by eruption products, and the radiative properties of different volcanic aerosols and their residence time in the stratosphere. Even after producing some record of volcanic activity, there are major differences in the way models implement this. Some use a direct reduction in global radiative forcing with no spatial discrimination (von Storch et al., 2004), while other models prescribe geographical changes in radiative forcing (Crowley et al., 2003; Goosse et al., 2005a; Stendel et al., 2006). Models with more sophisticated radiative schemes are able to incorporate prescribed aerosol optical depth changes, and interactively calculate the perturbed (longwave and shortwave) radiation budgets (Tett et al., 2007). The effective level of (prescribed or diagnosed) volcanic forcing therefore varies considerably between the simulations (Figure 6.13a).”

    and look at the Figure 6.13a before concluding that the physics is well known. There is obviously considerable uncertainty in the effect of volcanic (and all types of as well ) aerosols.And again the match with data is not that good, because no characteristic feature of volcano eruptions is really visible in the instrumental curve, besides the very short term (which goes hardly out of the natural fluctuations anyway) : the date of the break doesn’t match, the instantaneous slopes don’t match, the amplitude of the sharp decreases don’t match either.

  5. Lawrence Coleman:

    Re: Gilles, the information you are clingling to seems to me a little out of date, sure there was a temporary slowing of the CH4 curve a few years back but as we speak its on the upward move again.
    Also as CH4 released becomes oxidised in the atmosphere in about 7 years (7 years is the tip of the bell curve for CH4 transformation rates) it turns into CO2..even more CO2. So another way of thnking about all the clathrates beneath the ocean is as potential CO2!
    If you say ” I think we are still very far from a catastrophic runaway” it shows how unimaginative you must be..can’t you visualise anything at all? If you happen to know the solution to prevent the oceans warming any further..then and only then can you smugly rest on your laurels.

  6. Gilles:

    “Re: Gilles, the information you are clingling to seems to me a little out of date, sure there was a temporary slowing of the CH4 curve a few years back but as we speak its on the upward move again.”

    The “upward move” is less significant than the flat temperature curve since 2000, but I didn’t claim that CH4 wasn’t to rise anymore in the future, I said that there is no hint of non linear acceleration due to retroaction. The fact that more methane is released when temperature rises is a normal linear response, probably associated with a growing number of warm days in Arctic (which is probably partly due to a high AMO oscillation) , but I see no sign of a feedback on methane curve.

  7. Completely Fed Up:

    Gilles:”The “upward move” is less significant than the flat temperature curve since 2000,”

    1) a curve is not flat.
    2) the temperature graph since 2000 is increasing
    3) it is far too short to ascribe it to any climate trend

  8. Hank Roberts:

  9. Dr Nick Bone:

    For info. I’ve done some updates to my little spreadsheet here showing the combined impact of Earth system sensitivity (ESS) and CO2 response to temperature (gamma).

    I originally put this sheet together after the article “Good news for the Earth’s climate system?”: see
    one of my comments.

    The latest sheet and table look at the case of a drop to zero emissions immediately after a peak level of CO2 is reached. The “Residual (persistent) CO2” row is the level of CO2 at Stage 2 of my 5-stage scenario, as detailed in comment 62 above. The rows “Ultimate CO2” and “Ultimate warming” indicate the situation reached at Stage 4 after all the slow feedbacks have played out.

    There are columns for peak CO2 of 4 x pre-industrial, 2 x pre-industrial, 1.5 x pre-industrial and present levels. The final column indicates what might have happened if we’d stopped emitting back in the 1980s when Global Warming first became a political issue.

    The sheet only considers CO2 and CO2 feedbacks: to add in other greenhouse feedbacks (from methane etc.), a rough approximation is just to scale up “gamma” slightly e.g move to the 20ppm column.

    There isn’t a lot of “good news” anywhere in this analysis. However, it is important to keep up hope and keep civilization going. If our civilization collapses this century, it will only limit the short-term problem (Stages 1 and 2) while the long-term outlook (Stages 3 and 4) still looks bleak. It seems our descendants will need to try and clear up our mess, i.e. mop up the residual CO2 somehow, and I doubt they’ll be able to manage that if they’re back living in caves.

  10. Ken Peterson:

    Having a beer with a group of retired Engineers, one of the guys was holding forth on a solution he had just developed for some arcane world problem. The guy who had gone for the round, returned and said, “You have to first determine if it’s an engineering problem or a political problem. It it’s a political problem, engineers will never solve it.”

  11. Ray Ladbury:

    That is why, looking at the average effect smoothed over time is a better way to do it.

    Afraid of uncertainties, Gilles?

  12. walter crain:

    hi gavin,
    they’re talking about you over at Capital Weather Gang:

    and from a commenter,

    “Finally, scientists need to…provide hard scientifc and verifiable proof that this consensus indeed exists. I’ve heard tons of claims, but have been able to find precious little evidence to support this…”

    PROJECT JIM, anyone?