Andrew Dessler’s paper (technically Dessler and Forster), which he mentioned in this comment, has now appeared as a pre-print. Essentially, they use an energy balance approach to estimate equilibrium climate sensitivity (ECS), but – as I mentioned in this post – they use the tropical average temperature at 500-hPa, rather than using global average surface temperatures.
Typically, one can estimate the ECS by using that the planetary energy imbalance is given by
where is the change in external forcing, is the change in surface temperature and relates to the ECS through
is the change in forcing due to a doubling of atmospheric CO2.
The problem, though, is that it seems that – due to internal variability – the planetary energy imbalance, , correlates poorly with changes in surface temperature, . It appears, however, that there is a better correlation with changes in 500-hPa temperatures in the tropics, . Hence it seems that it is better to use
where is the equivalent of . The ECS can then be estimated using
where the latter term is because the ECS refers to the surface, not to tropical temperatures at 500-hPa. I’m also slightly glossing over what we would estimate to be from actual observations, and what it would be after a doubling of atmospheric CO2
The bottom line, however, is that they estimate the ECS to likely be between 2.4oC and 4.5oC (17-83% confidence interval), with a mode of 2.9oC and a median of 3.3oC. It seems, therefore, to be another example of an analysis suggesting that the ECS is probably above 2oC. However, unlike the recent Cox et al. paper, it doesn’t rule out some of the higher values (around 4oC, or slightly higher). This is maybe interesting, given another recent paper that suggested greater future global warming.
However, I don’t think we should read too much into this. I think the key points are that it again seems to largely ruling out an ECS less than 2oC, finds a best estimate of around 3oC, and does not yet confidently rule out ECS values of around 4oC, or higher. Would be nice if we could do the latter, but it would be wrong to think we have, when we probably haven’t.