Some more about Hansen et al.

I thought I would expand a bit on my previous post about the recent Hansen et al. paper. Something I did like is that the paper highlighted that there is no known paleoclimate analogues for the current anthropogenic forcing pathway. Humans are producing a substantial perturbation to a complex, non-linear system. Even though we have a good understanding of how the system might respond to such a perturbation, we shouldn’t rule out the possibility of surprises.

Additionally, the paper seems to be making a few key points. Firstly, it’s arguing that equilibrium climate sensitivity (ECS) is probably ~4^oC, rather than the more standard best estimate of ~3^oC. It’s also highlighting that the greenhouse gas (GHG) forcing is already 4 Wm^-2, equivalent to a doubling of atmospheric CO₂. So, the fast-feedback equilibrium warming (ECS) for today’s GHG forcing is already 4^oC. Furthermore, it’s arguing that this implies a relatively large aerosol forcing that is currently masking about ~1^oC of warming, implying a large amount of warming in the pipeline if aerosol emissions are substantially reduced.

Although the above arguments could well be correct, there are some other factors to consider. Even though todays’ GHG forcing is indeed probably around 4 Wm^-2, it’s a combination of a number of different GHG species. The dominant one is CO₂, but methane and some other species, contribute about one-third of this forcing. These species have relatively short atmospheric lifetimes. Hence, if human emissions of these species were also to substantially reduce, this would counteract some of the warming that would result if we were to substantially reduce aerosol emissions, although the timescales wouldn’t be quite the same.

Also, even though CO₂ has a long atmospheric lifetime, this doesn’t mean that all the CO₂ that has been emitted will remain in the atmosphere for a very long time, it just means that a significant fraction of what has been emitted will remain in the atmosphere for a very long time. So, if CO₂ emission go to zero, the CO₂ forcing will also decrease slightly. This is why the best estimate for the zero emission committment (how much further warming there will be if emissions go to zero) is very close to zero. In other words, the warming that has been locked in due to emissions to date is very close to the amount of warming that has occured to date.

Essentially, what this implies is that the warming in the pipeline depends mostly on future emissions and isn’t locked in due to past emissions. There’s, of course, no problem with estimating constant forcing/concentration warming committments, but maintaining a constant forcing/concentration does require continued emissions, so I do think it’s worth making clear that future warming depends mostly on future emissions, rather than being locked in due to past emissions. It is still possible to influence future warming, even if it will be very challenging to do so.