I wanted to just briefly mention the recent paper that [quantifies] ocean heat uptake from changes in atmospheric O2 and CO2 composition, by Resplandy et al. The interesting thing about this paper is that it uses proxies to infer the change in ocean heat content. What it finds is that the change in ocean heat content is probably at the high end of earlier estimates, which are based mostly on direct measurements of ocean temperature.
What this implies is that observationally-based estimates of equilibrium climate sensitivity (ECS) are probably also on the low side. Fortunately, BlueSkiesResearch has already repeated this calculation. Essentially, it increases the lower limit by about 0.1K, the upper limit by about 0.5K, and the median by about 0.2K. This slightly resolves the issue I had with the slightly high TCR-to-ECS ratios that come out of these type of analyses.
However, the paper suggests that this increase in the change in ocean heat content implies a reduction in the carbon budget for staying below 2oC, and I don’t think that is correct. The carbon budget depends mostly on the transient climate response (or the transient response to cumulative emissions). This doesn’t really depend on how much the ocean heat content has changed; it primarily depends on how much we we’ve warmed for a given change in external forcing.
So, I don’t think that this new ocean heat content estimate really implies anything with respect to carbon budgets; I think the main significance is that it is suggests that some observationally-based estimates of equilibrium climate sensitivity are probably too low and that the likely range for equilibrium climate sensivity is still something like 2oC to 4.5oC.
There is now a guest post on Realclimate by one of the authors in which they discuss errors in the uncertainties and in the trend estimate.