I had been intending to write a post about aerosols, given what’s been said about them recently. In particular, Nic Lewis at the Parliamentary Select Committee said a couple of things that I wanted to check. Essentially he was arguing that the aerosol estimates had changed from AR4 to AR5 and that climate models were using an aerosol forcing that was too high (negative) and hence over-estimating climate sensitivity.
Fortunately, someone else has done the job for me. Will Morgan has written what seems to be a very informative and clear post about what did the IPCC say about aerosols. I encourage you to read it, but I’ll my summarise my understanding below. This isn’t so that you can learn from me (read Will Morgan’s post), it’s so that those more informed than me can correct my understanding, so that I might learn something.
- Anthropogenic aerosols can influence the radiation budget in a number of ways : they can scatter incoming sunlight and produce cooling, they can seed clouds also producing cooling, and dark aerosols can absorb incoming radiation and produce warming.
- Overall it is thought to be that aerosols produce a net cooling. The exact amount (i.e., the radiative forcing) is, however, very uncertain.
- Determining the net radiative forcing due to aerosols is a crucial part of estimating climate sensitivity using recent observations (energy budget constraints). The large uncertainty, however, means that such estimates are likely to be very uncertain.
- There are two ways in which the aerosol forcing is estimated. Satellite based measurements (giving a value of -0.85 Wm-2 with a range from -0.93 Wm-2 to -0.45 Wm-2) and using a subset of climate models (giving -1.38 Wm-2 with a range from -1.68 Wm-2 to -0.81 Wm-2).
- The value presented in AR5 was based on expert judgement – using the two methods described above – and was -0.9 Wm-2 with a range from -1.90 Wm-2 to -0.1 Wm-2. The aerosol forcing in AR5 was reduced slightly compared to AR4 (-1.2 Wm-2 with a range from -2.30 Wm-2 to -0.2 Wm-2).
- Here’s the kicker (I think) – the claim, by Nic Lewis and others, that climate models are using aerosol forcings that are higher (more negative) than the IPCC best estimate appears to be based only on the climate models used to estimate the aerosol forcing. Therefore, I don’t believe that the claim that climate models – overall – have too high an aerosol forcing is necessarily correct.
The table below gives the various estimates and the change from AR4 to AR5
Aerosol radiative forcing estimates (credit : Will Morgan)
So, my basic summary is that the anthropogenic aerosol radiative forcing is uncertain and hence means that energy budget estimates of the climate sensitivity will also be very uncertain (possibly to the extent that they’re not particularly useful). The range has changed from AR4 to AR5, but not by much. The claim that climate models are using aerosols forcings that are too high is confusing the subset of models used to estimate the aerosol forcing with all climate models, and hence isn’t – strictly speaking – correct. Corrections and clarifications through the comments are welcome though.