Promising climate progress from net-zero ambitions to the Paris Agreement goal: keeping below 2C [my foot]
John Nissen
Michael MacCracken
Hi John--Hesitantly, a few thoughts after only a cursory look at the paper trying to address your questioning about optimism, I sensed a few things that have tended to get ignored for quite some time in similar studies and may apply in this study:
1. If you look at the emissions file, I don't see any listing of changes in the emissions of SO2 and so I wonder if they have accounted for the loss of the sulfate aerosol cooling effect by having SO2 emissions go to zero.
2. The emissions of black carbon (mostly from coal fired plants and two-stroke engines, both of which are being phased out) are also not listed. Of course, as the snow and ice melt back, their darkening effect will be reduced.
3. It is not at all clear that the MAGICC model allows for changes in cloud cover, such as reductions in marine stratus from warming of for Jim Hansen's views on the higher longer-term climate sensitivity.
4. I don't think the studies using MAGICC really provide full treatment of spatial variations in the response, so the Arctic amplification that you care about so much is not seen given their focus on the global average. There has in the past been to associate the spatial distributions of warming in global model runs with the changes by calibrating using changes in global average temperature which seems plausible for small changes but likely not plausible for large changes in global average temperature.
Finally, a thought on implications for climate intervention, while at least some of the omissions mentioned might suggest that warming might well be greater, the potential for mitigation to be effective that the article suggests could well be interpreted as undercutting the Termination Shock argument against SAI/SRM, for it shows that the amount of SAI needed to get warming below 1 C (or even 0.5 C) warming does not have to be very much if mitigation works this well and so much restoration of Arctic sea ice and slowing of ice sheet melting could likely be accomplished by polar-SAI that you (and I) are pushing for, at least as a useful starting point (our viewpoint being that warming greater than 0.5 C leads to unacceptable risks and that the 1.5 -2 C warming viewed as endurable in the Paris Accord was a political decision not supported by the science). And if, worst case, polar SAI were suddenly eliminated as critics like to focus on in objecting, the paper's results suggest that the Termination Shock warming would only be large if the article's optimism about mitigation were not achievable--and, if that is likely, then we should be planning to move faster toward global-SAI (and other SRM approaches like planetary sunshades, etc.) to keep the planet livable.
Best, Mike MacCracken
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