I was wondering if a anyone had any insight into this new paper by Bjorn Stevens called Rethinking the lower bound on aerosol forcing. The basic goal seems to be to try and more tightly constrain the forcing due to anthropogenic aerosols. The IPCC AR5 5-95% range was -0.1 to -1.9 Wm-2. Stevens (2015) suggest – via various calculations – that the lower limit should really be -1.0 Wm-2, with an upper limit of around -0.3 Wm-2.
Of course, constraining the range is a good thing, but this result has the “skeptico-sphere” all excited, because it has implication for climate sensitivity. Nic Lewis has a post on Climate Audit and on Climate Etc.. It’s also mentioned on Bishop-Hill and, I presume (although I haven’t looked) on WUWT. As an aside, Stoat has an amusing post on how best to describe Willard Anthony Watts and his blog.
The reason the skeptico-sphere is all a flutter over this, is that if you redo Nic Lewis’s energy balance calculation, the reduced aerosol forcing means you get an ECS range (5-95%) of 1.05 – 2.2K and a best estimate of 1.45K. The TCR range drops to 0.9-1.65K. So, nice and low. Fantastic, if true. However, not only does this seem really implausibly low, it also means that Nic Lewis’ method is now diverging from the IPCC range (1.5 – 4.5K for ECS). In such a scenario, someone presumably has to be wrong (unless reality happens to just lie in the overlap region). Who is it? Well, I guess we don’t know, but we could hazards some guesses. For example:
- Paleo-climate is largely inconsistent with an ECS that is lower than 1.5K. It’s probably difficult to see how we could have moved out of a snowball Earth or between glacial and inter-glacials if the ECS was as low as 1.45K.
- The greenhouse effect itself is inconsistent with such a low ECS, unless – for some reason – the ECS has a very strong temperature dependence.
- The net feedback response (water vapour, lapse rate, clouds) is thought to be around 2 Wm-2K-1. An ECS as low – or lower then – 1.45K would suggest a feedback response of around 0.6Wm-2K-1, much lower then we would expect. I also have a feeling that there is something logically inconsistent about an ECS range that encompasses the possibility of no feedbacks, but maybe not.
Now, I don’t know the answer and am not claiming that such a low ECS must be wrong: it just seems clear that an awful lot of our current understanding must be wrong in order for it to be this low.
There is also an interesting irony about this paper. One of the arguments made in the paper is that the CMIP5 climate models typically underestimate the warming between 1920 and 1950. One way to resolve this would be if these models were over-estimating the aerosol forcing during this period (more negative than it actually is). However, the CMIP models have an ECS range from 2.1K to 4.6K. Therefore it seems a little inconsistent to use the result in Stevens (2015) to argue for an ECS with a range from 1.05K to 2.2K when the result in the paper is partly based on models with an ECS range from 2.1K to 4.6K.
Anyway, those are just my thoughts and the whole aerosol thing is quite a complex topic about which I know little. Hence, I wondered if any of my much more knowledgable readers might have some other insights.