To follow up on my post about Understanding Lewis (2013), I thought I might highlight a recent paper called The dependence of transient climate sensitivity and radiative feedbacks on the spatial pattern of ocean heat uptake, by Rose et al. One of my main issues with Lewis (2013) was that adding 6 years of data (1995 – 2001) changed the ECS estimate from 2.0 – 3.6oC, to 1.2 – 2.2oC. This seems quite an alarming change, given such a small increase in the data being used.
What’s interesting about this new paper (which I discovered through a tweet from Gavin Schmidt) is that it attempts to understand how climate sensitivity depends on regional changes in the deep ocean heat uptake. I’m not sure I’ve quite understood precisely what they’ve done (I’m trying to finish off a paper of my own, so am trying to avoid reading too many climate science papers at the moment), but the key figure is – I think – below. Essentially they seem to have run 3 sets of simulations. One of which is a standard run in which they double CO2, and the other 2 are runs in which the deep ocean heat uptake is either mainly in the tropics, or mainly in the polar regions.
The main result in the paper seems to be that varying the regional ocean heat uptake can significantly change the sea surface temperatures and also changes the radiative forcing from feedbacks. The key figure is the top left one above which shows the SST results from the 3 sets of simulations (each set uses 4 different GCMs). My understanding is that this means that if one wants to estimate climate sensitivity (transient or equilibrium) using transient observations, it will be strongly influenced by the regional distribution of the deep ocean heat uptake. One would expect the regional variation in deep ocean heat uptake to be cyclical so, overall, these variations shouldn’t influence the actual climate sensitivities, but it does mean using transient observations to constrain them can be very difficult, and likely unreliable.
The paper itself actually says,
Transient climate response is governed both by an evolving pattern of sea surface warming activating different local feedbacks and by changes in the local feedbacks themselves as the pattern of OHU slowly evolves. This casts doubt on the possibility of estimating the feedbacks governing transient climate change from equilibrium mixed layer models (as noted by Shell ), and more importantly, of estimating equilibrium climate sensitivity from inherently transient climate observations.
So, the paper would seem to be suggesting that the equilibrium climate sensitivity cannot be reliably estimated from transient observations. That Lewis (2013)’s ECS estimate appears to have changed dramatically with the addition of only 6 years worth of data would seem to be consistent with this new paper. Alternatively, I guess, Lewis (2013) could just have made some kind of error. It’s possible, of course, that the Lewis (2013) result is somehow correct, but that would seem quite remarkable if true, even if the results in this paper do not turn out to be robust.