Monday, April 30, 2018


  1. Lewis and Curry, again

    I should probably say something about the new Lewis & Curry paper. It’s mostly an update to their earlier paper that I’ve discussed before. Bottom line; there are reasons to be cautious. 
    The basic formalism is that one can use an energy balance approach to estimate the transient climate response (TCR) and the equilibrium climate sensitivity (ECS). Given the change in forcing, \Delta F, the change in surface temperature \Delta T, and the change in system heat uptake rate, \Delta Q, we get
    TCR = F_{2 \times CO2} \dfrac{\Delta T}{\Delta F},
    ECS = F_{2 \times CO2}\dfrac{\Delta T}{\Delta F - \Delta Q},
    where F_{2 \times CO2} is the change in forcing due to a doubling of atmospheric CO2.
    If new estimates suggests that the change in forcing is slightly higher, and the change in system heat uptake rate is slightly lower, than previously thought, then the estimates for the TCR and ECS go down, which I think is the main difference between the new paper and the old one.
    As I’ve already mentioned, there are reasons to be cautious about these results
    niclewis says:
    Do you at least accept that internal variability can indeed lead to a range of temperature pathways for a given pathway;
    That doesn’t make sense. I assume that you mean “for a given forcing pathway”? 
    Of course I accept that. If you had read and understood the new Lewis & Curry paper, or the earlier one, you wouldn’t ask such an odd question. Both papers make proper allowance for internal variability in both surface temperature and radiative imbalance/heat uptake.

  2. Nic,
    That doesn’t make sense. I assume that you mean “for a given forcing pathway”?
    Well, yes, that is what I meant. 
    If you had read and understood the new Lewis & Curry paper
    Thanks, I see that your attempts to avoid condescension are failing, or you’re not even trying (the latter, I suspect). 
    Both papers make proper allowance for internal variability in both surface temperature and radiative imbalance/heat uptake.
    I realise that you choose your starting and end points to try to account for variability, but I do not think you can compensate for all possible impacts of internal variability, which is essentially Andrew’s point. 
    So, my question was whether or not you accept Andrew Dessler’s point. If you do, then you should be willing to acknowledge that your results may not be indicative that climate sensitivity is probably lower than other estimates suggest. It may simply be as Andrew suggests; the method you’ve used is imprecise and that the ECS values that you infer from this method could be far from the actual value. 
    If you disagree and think that your results are accurate/reasonable estimates of the system’s true values, then you don’t accept what I asked and should acknowledge this. Ideally, you should explain how the pathway we’ve followed is precisely determined by the forcing pathway and largely insensitive to internal variability.

It is interesting that Judith has, in the past, argued that internal variability could explain a lot of the observed warming, but now authors a paper essentially suggesting it plays no/little role.
Joshua says:
Climateball (IMO) is very frequently about pricing the ignorance if one’s interlocutor.
If what they say “doesn’t make sense” or if they “don’t understand,” one doesn’t have to actually be correct. Someone arguing “nonsense ” is also very useful (Nic hit a twofer above). Someone bring an “advocate” or “alarmist” – or in all fairness, a “denier” – is an effective shortcut. And it’s certainly good that Nic, Judith, and Roy can be proven to not be activists.
Andrew E Dessler says:
I agree with several previous commenters that the terms people use our causing some confusion. We wrote our ACP paper carefully to be as clear as possible, but I have not been as clear in my tweets and in blog comments (because to be honest I don’t spend a lot of time thinking about them before I post). I said several times that the Lewis and Curry method is biased, but I should’ve said it is imprecise. Our ACP paper shows that the method would give us an accurate estimate if we had 100 different realizations of the 20th century. However, The imprecision of the method means that with only one realization (the historical record), it is possible that you could get an answer that is far from true. Several other papers that have come out recently have also suggested that the pattern of warming that we experienced during the late 20th century causes energy balance estimates of ECS to be lower than the climate system’s true value.

Steven Mosher says:
“Dessler’s study is just showing that the observational estimates of ECS are plausibly estimates from the bottom of the spread and that the difference between observational and ensemble based estimates is plausibly the effects of internal variability on the observational estimates.”
I like the plaussibility language much better than other descriptions.
Now if we did the Dessler study with a different model and found the observational estimate in
the top of the spread, what would we conclude?

Andrew E Dessler says:
Note to Mosher: no ensemble study can tell you where the historical climate trajectory would fall within the ensemble. that’s really an ill-posed question. rather, the ensemble tests tell us that the methodology produces imprecise answers.
other papers (e.g., Marvel et al., Zhou et al.) show using different methods that the existing surface pattern is causing energy balance methods to yield too low of an ECS. 
combine all of these results and you arrive at a reasonably robust conclusion that L&C’s ECS estimate (and others derived the same way) is biased low.