I’ve been mostly at home today since we had a power cut at work and the site ended up being closed. So, I thought I would post a few more thoughts about the Millar et al. 1.5oC carbon budget paper. This isn’t intended to be anything all that substantive, so if you want something more thorough, there is a Realclimate post and Dana has a new Guardian article.
In some sense the bottom line from the Millar et al. paper is that we’ve warmed just over 0.9oC and, therefore, have 0.6oC to go before we reach 1.5oC. If you then compute the remaining carbon budget it is bigger than other estimates. One obvious issue with this is that this is based on a late-1800s baseline. If you use an earlier baseline, then we are closer to 1.5oC than they suggest. Also, it is based on a single observational dataset. Other datasets suggest we have warmed more than 0.9oC, relative to their baseline. Hence, we might have less warming to go than they suggest.
However, a key aspect of the Millar et al. analysis is that some climate models suggest that emitting 545Gtc (how much we had emitted by 2015) should have warmed the surface about 0.3oC more than was observed. The models do suggest that this should have happened later than 2015, but let’s ignore that for now. Let’s also ignore that their estimate of how much we’ve warmed may be too low anyway. One interpretation has been that this indicates that the models are running too hot. However, most climate models (but not all) use forcing/concentration pathways as an input. You then have to run a different model to determine the associated emission pathways.
From a physics perspective, what’s of interest is how much we warm for a given change in forcing, not for given amount of emissions. That our observed warming may be associated with higher emissions than was expected does not necessarily indicate that models are running too hot (at least not in terms of climate sensitivity). It may, however, indicate that natural sinks have been taking up more of our emissions than was expected, which is itself an interesting issue. If that is the case, then that might indeed indicate that we could emit more, for a given level of warming, than we had previously thought.
For example, if earlier estimates for a 1.5oC carbon budget (66%) were around 2300GtCO2 (about 630GtC) then the Millar et al. result might suggest that it could be 20% higher (say 2760GtCO2, or about 750GtC). This would give about 120GtC more than previous estimates and, when measured from 2015, sounds like a big change because previous estimates had suggested that there was very little left – a big percentage change in what we had left, even if it a relatively small change in the total budget.
If I have interpreted this correctly (which I may not have) the interesting question then becomes whether or not natural sinks are indeed taking up more of our emissions than expected and, if so, if we would expect this to continue. I don’t know the answer to this, and it would be good to get some clarification.
However, something else I wanted to mention was the issue of committed warming. It is roughly the case that we expect the equilibrium response to a certain level of emission (i.e., the total amount emitted) to match the transient response at the time at which emissions cease. I’ve simplistically demonstrated why in this post but it’s essentially because if we halted all emissions, atmospheric CO2 would drop at a rate that essentially balanced how we would warm if concentrations were fixed at the peak value. The atmospheric concentration would then tend to a long-term value equivalent to 20-30% of our emissions remaining in the atmosphere for millenia. The equilibrium response to this concentration is then expected to be comparable to the transient response to the peak concentration.
However, if we’re underestimating the uptake by natural sinks, then the transient response to a given amount of emissions would be lower than expected. If, then, the equilibrium response to the long-term concentration is to still match this, then our estimates of the long-term atmospheric concentration should also be too high. It’s, however, not obvious that the latter should be the case because this is – I think – set more by the size of the reservoirs than by the rate at which they take up our emissions.
So, if the above is roughly correct, then even if the Millar et al. result is robust, it may still not indicate a larger carbon budget. It would mostly indicate that we’ve slightly under-estimated the committed warming once emissions cease (i.e., the equilibrium response to the long-term concentration would be higher than the transient response at the time at which emissions cease).
Okay, I’m going to stop there. I don’t know if the above are reasonable comments, or not, so treat them with suitable caution. It’s mainly me just pondering this issue, so I will probably end up thinking differently once I’ve thought about this a little more. I’ll aim to clarify things, if necessary. Any comments (mostly) welcome.