On a number of occasions I’ve pointed out that our warming committment is not actually the equilibrium temperature to our current atmospheric concentration because, if we halted all emissions, atmospheric CO2 would drop as the natural sinks took up more and more of our emissions. I should stress, though, that the long-term atmospheric concentration is limited by the Revelle factor, which I discuss here and here – we would expect about 20% to 30% of our emissions to remain in the atmosphere for thousands of years.
As a rough approximation (which I try to illustrate in this post) the committed warming is comparable to the transient response at the time when we stop all emissions (i.e., the transient response to the peak atmospheric CO2 concentration, or – equivalently – the peak change in forcing). This is also discussed in this Realclimate post, this Steve Easterbrook post and in this paper.
However, it is more complicated than suggested by the above, mainly because we’re not simply emitting CO2; there are also aerosols and short-lived greenhouse gases. In this comment Thorsten Mauritsen mentions that he and Robert Pincus have recently published a paper that looks at this. The paper is called Committed warming inferred from observations.
They essentially use an energy balance (or, observationally-based) approach to quantify the equilibrium climate sensitivity (ECS) and the transient climate response (TCR) and then use these to estimate the committed warming under a number of different scenarios. The two main equations they use are
In the above is the current planetary energy imbalance, is the current aerosol forcing (which is negative), is the current forcing due to short-lived greenhouse gases, and is the small change in forcing between the period considered in the study (centred on 2010) and about now (2016).
The 5 scenarios they consider are, equilibrium warming with everything remaining constant at today’s values (i.e., and both 0 in the top equation), equilibrium warming but taking into account that aerosols will precipitate quite quickly (i.e., include non-zero in top equation), equilibrium warming taking into account both the removal of the aerosol forcing and that due to short-lived GHGs (i.e., include both non-zero and in top equation), equilibrium warming this century (replace with in the top equation), and committed warming taking into account continued ocean uptake of CO2 (bottom equation with all terms non-zero).The results are shown in the figure on the right. On long enough timescales, we would expect something close to the result represented by . This takes into account continued CO2 uptake by the oceans, precipitation of aerosols, and the decay of short-lived GHGs, but allows for a small amount of committed warming mainly due to the forcing adjustment from emissions between the middle of their time period (2010) and about now (2016). However, we might expect aerosols to precipitate faster than the decay of the slow-lived GHGs, which would then produce some short-term warming that then decays towards the value represented by (i.e., tending towards something between and and then tending towards ). The key point, though, is that our committed warming is smaller than would be inferred from the equilibrium response to current concentrations, even if you take aerosols and short-lived GHGs into account.
A few additional comments. The range of warming is mainly due to the quite large uncertainty in the aerosol forcing (which impacts the observationally-based estimates for the ECS and TCR). However, it is becoming increasingly clear that large aerosol forcings (very negative) are probably unlikely. Hence, the range may be smaller than indicated in the figure. On the other hand, there is increasing evidence that observationally-based climate sensitivity estimates are probably lower than the Earth’s true sensitivity. This is partly because the temperature datasets tends to suffer from coverage bias and miss some of the warming at high latitudes. There’s also the issue that combining sea surface temperatures and air temperatures may slightly under-estimate the warming, that forcings may have different efficacies, that the pattern of sea surface warming can impact the forced response, and that feedbacks may increase slightly as we approach equilibrium (although this may not be too big an issue if the committed warming is quite low).
This may indicate that these results slightly under-estimate the committed warming, but that the higher ends of the range may be less likely that these results suggests. However, I still think that a key point is that at any time our committed warming is closer to the transient response to the forcing at that time, than to the equilibrium response, with some caveats related to forcing due to aerosols and short-lived greenhouse gases. This essentially means that any amount of emission reductions can have an impact, even on relatively short timescales.