There is a new post on Watts Up With That (WUWT) by Nic Lewis discussing a new paper that claims that the transient climate response (TCS) is less than 2oC. The paper is called Energy budget constraints on climate response and Nic Lewis is one of the authors.
The basic conclusion of the paper is that the Equilibrium Climate Sensitivity (ECS) has a 5-95% range of 1.2-3.9oC while the Transient Climate Response (TCR) has a 5-95% range of 0.9-2.0oC. Maybe I should try and define these two terms before carrying on (if I get anything wrong, someone who knows more about this than me is welcome to correct me). The ECS is the required increase in global surface temperature so that a new equilibrium is reached if CO2 concentrations are doubled. It is normally determined via models or via past history as one has to also include all the other effects that accompany a doubling of CO2. However, the ECS is not reached instantaneously and so there is a lag between the CO2 concentrations doubling and the ECS being reached. The TCR is what the change in global surface temperature will be at the point in time at which the CO2 concentration has doubled, assuming that CO2 concentrations rise at 1% per year.
The WUWT post starts by saying
Headline best estimates of 2.0°C for equilibrium climate sensitivity (ECS) and 1.3°C for the – arguably more policy-relevant – transient climate response (TCR) are obtained, based on changes to the decade 2000–09, which provide the best constrained, and probably most reliable, estimates.
As far as I can tell the IPCC’s most likely range for the ECS is 1.5-4.5oC, which seems consistent with the results presented in this paper. Nic Lewis is making the claim that the lower TCR (1.3oC rather than 1.8oC) is significant and relevant. My understanding is that by using the time period 2000 – 2009, they’ve used the period during which global surface temperatures have risen more slowly than expected. One of the explanations for this is that more energy is going into the oceans than anticipated and hence less is available to heat the surface. This presumably explains why the TCR from this study is lower than previous estimates.
Now, if the oceans continue to absorb more energy than previous studies had expected, the TCR may well end up lower than the 1.8oC estimated by earlier work. The ECS is, however, largely unchanged. This means that the ultimate effect of doubling CO2 will be no different, it will just take longer to reach this new equilibrium. However, one of Bob Tisdale’s constant claims is that ENSO cycles and PDOs bring heat from the oceans and act to heat the surface. Every 10 – 20 years one sees sudden jumps global surface temperatures. It therefore seems that one cannot assume that this excess energy will remain in the oceans over long timescales. The TCR may, therefore, appear low for a decade or so, but sudden jumps in surface temperatures will see it increase. Therefore, one might expect the net change in global surface temperature at the instant CO2 has doubled to be bigger than the 1.3oC suggested by this study.
Essentially, this study seems fine in the sense that it doesn’t appear to have done anything particularly strange. The ECS is within the range predicted by other work and so nothing seems to have really changed there. The TCR is quite a bit lower than previously predicted but they appear to have based this on a time period (2000 – 2009) during which global surface temperatures rose more slowly than expected, possibly due to more energy going into the oceans than expected. In fact, in this paper the estimate based on the period 1970-2009 seems to have a 5-95% range of 0.75-2.5oC. All I can really say is that I hope the TCR is lower than we expect as that might gives us more time sort out the mess we’re in. That may, however, not be obviously true since if the ECS is unchanged, putting the CO2 into the atmosphere is enough to guarantee that we’ll eventually get there.