Much Ado About CO2

Following a presentation in London by Dr. Murry Salby, there has been much discussion at several sites: No Tricks Zone, Climate Etc. and WUWT. These threads are always a challenge for a reader because there are exchanges debating various issues between highly convinced people who are seldom explicit about the assumptions underlying their relative positions.

Interesting in this case are the reactions to Salby’s assertion that the observed rise in atmospheric CO2 is caused by effects of rising temperatures upon natural sources/sinks, and not by rising fossil fuel emissions. (Leaving aside for today the whole other issue of climate sensitivity to changes in CO2).

Attacks have been mounted both by supporters of IPCC, and also by skeptics of IPCC alarms who nevertheless accept the notion that all or most of the measured rise in CO2 is from humans, fossil fuels and cement in particular. Still others find flaws in Salby’s argument, but are not convinced by the alternative.

I recently posted a review of Salby’s textbook which touched on this topic. Firstly, I agree with those who say you cannot use static calculations on a dynamic and open system like the atmosphere. That is, both inputs and outputs are interactive and vary in response to each other. The most obvious example is increasing CO2 causing plant growth which in turn consumes more CO2. Thus algebra can mislead us, since it is the differentials over time that accumulate the object of interest at changing rates.

Secondly, it seems to me that the atmosphere itself is too small a subsystem to draw any meaningful conclusions. The ocean and land sources/sinks are orders of magnitude larger than the amount in the atmosphere, and the errors in estimating those flows far exceed the man-made emissions (which are also estimates with larger uncertainties than is usually admitted).

Rather than thinking of the air as a reservoir of CO2, it is more like a tidal pool. Imagine a scientist concerned that this tidal pool is changing volume because of water (unpolluted) leaking from a nearby landfill. So a measuring cup sample is taken periodically and tested. All the while, the pool is repeatedly drenched and drained by waves, currents and tides, along with occasional rains and storms. Whatever the test results, the effect of additional water from the landfill can not be discerned in the absence of markers distinguishing it from ocean and rainwater.

I don’t say Salby has all the answers. I agree with him that at the current state of information, atmospheric CO2 from human sources can not be identified apart from much larger natural fluxes of CO2.



  1. Hifast · August 14, 2016

    Reblogged this on Climate Collections.


    • Ron Clutz · August 14, 2016

      Thanks Hifast for picking up on this. I appreciate your reblogging these posts for a wider audience.


  2. joekano76 · August 14, 2016

    Reblogged this on TheFlippinTruth.


  3. Frederick Colbourne · August 15, 2016

    I have studies Dr Salby’s textbook for a couple of years and am much impressed.I have also downloaded and watched his online lectures.

    I don’t feel qualified to assess the overall validity of the theory Dr Salby set out in his most recent lecture. But there is one section that seems to me to be unclear.

    If the total mass of CO2 in the atmosphere is 750 Gt and the flux one way is 150 Gt, that means turnover is short, about 5 years as Dr Salby explains and supports by three arguments.

    But this refers only to a typical molecule of CO2 and tells us nothing about the increase or decrease of CO2 in the atmosphere.

    What seems to me to be unclear is why does Dr Salby expend such effort to make this point?

    How does this fit into the overall thesis presented?

    Does he mean by this (as I thought) that stopping anthropogenic production altogether would means that all excess CO2 above the pre-industrial (or some other definable level) level would be sequestered by sinks in 5 years?

    Does this follow from the logic? And can it be tested empirically?

    I feel that I owe it to Dr Salby to understand fully what he is arguing before attempting to assess the validity of his theory.


    • Ron Clutz · August 15, 2016

      Thanks Frederick, as usual you make a thoughtful comment and raise interesting questions. I will go back to read my notes on Salby’s textbook again, but my first reaction is to agree with you: He does not directly address the questions you raise. Here is what I think about that.

      These Carbon Budgets (like the one heading this post) are not transactional models that allow us to answer your questions. The budget is only good to depict the relative scales of stocks and flows of CO2. And from current (still gross) estimates, we see that man-made emissions are about 1/20 of the natural flux. And as you point out, atmospheric CO2 is replenished over ~five years.

      Salby’s main point is that the whole amount of man-made CO2 is well within the error range of natural flux estimates. Fossil fuel emissions are calculated from transactional information: documented changes in fossil fuel inventories. Even so, the estimates have at best an error range of +/- 15%. Good enough to confirm the order of magnitude, but not much more.

      Estimates of natural fluxes are much cruder, lacking such global transactional measurements. Clearly 15 to 20% of 200 GT is way larger than 10 GT or so of man-made CO2. So I believe the correct answer is: We don’t know. Theoretically, a pulse of CO2 (from a major Volcano, say) should work its way through the atmosphere in 5 years, but we might not notice.

      There is the additional issue from discovering that CO2 is not well-mixed in the atmosphere, as has been assumed. Not only is the concentration variable from place to place, it appears that there are major flows from equator to poles. Thus, samples of CO2 changes in specific locations may not be representative of global conditions.

      Still, if we take at face value the trends as measured, Salby’s point is: The monotonic rise of atmospheric CO2 correlates well with the secular rise of temperatures since LIA, and correlates poorly with the more exponential rise of fossil fuel usage. This is consistent with ice core records and other more short-term measures showing changes in CO2 following temperature changes, and not the other way around. Salby concludes that natural sources/sinks respond to rising temperatures by emitting more CO2.


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