1/ Yesterday (19th), @KevinClimate & @Peters_Glen, in a good & long multi-forked thread, aired their differing approaches to how to call for the rapid decarbonization the world so desperately needs.

I hope they won't mind me summarizing that here and offering a 3rd philosophy.

2/ Their discussion stemmed from this tweet by Glen: https://twitter.com/Peters_Glen/status/1351446872796389382?s=20.

I welcome their further thoughts here in my thread, if they wish.

3/ Their discussion was whether to call for cutting emissions "as fast as possible" regardless of carbon budget scenarios, or to do so using "a well-defined budget range with transparent reasoning & assumptions." It also centered broadly on understandings of policy "feasibility".

4/ Glen feels using a "budget has limited policy utility because of uncertainty" in modeling (etc.), so he "would mitigate as fast as feasible, regardless of the budget."

Kevin points out that "'fast as feasible' goals [may] sum to emissions nearer 'pursue 4oC'".

5/ Glen points out that "it is hard to avoid some level of feasibility in the discussion" and that, in contrast, calling for (e.g.) a 10%/yr emission cut is meaningless unless what that means in practice (a feasibility consideration) is part of policy discourse.

6/ Kevin's response is that "I don't want 'targets', rather duties, commitments, etc." and that, since the measure of a policy is the action stemming from it, "if the obligation is a minimum 10% cut each year, completely new policies emerge" from that.

7/ Kevin said earlier what's needed is "a far wider landscape of what's feasible & who defines it" instead of "reverse engineer[ing] our analysis to fit with 'feasibility' as defined by short-term politics." Glen says feasibility is not what's comfy for politicians & hi-emitters.

8/ ... and that it means "as fast as we can get emissions down." Kevin replied, "few mitigation scenarios take your or my view (which I think we may share) of what's feasible" because those favor tech solutions over what's deep & immediate and moves toward a GND & fair rationing.

9/ Here's a 3rd philosophical approach to what the two debated.

C budgets, being probabilistic, put a sliding scale of choices into public discussion & policymaking, making a slippery slope leading to delay. Temperature targets underlying the budgets poorly represent 'danger'.

10/ So I disagree with relying on C budgets, which Kevin uses to determine how fast emissions must be cut (e.g. his estimates of 10-12% per year in several northern high-emitting nations).

However, I think those same rates of cut can be roughly arrived at by another method.

11/ As both agreed, the matter of 'feasibility' is crucial. I think it is fine to discuss it initially as a broad concept (as they did); but, the concepts of 'feasibility' and 'as fast as possible' must be merged through consideration of a specific policy or policy framework.

12/ That is the only way those concepts make sense, other than as abstract notions. Moreover, feasibility rests upon the physical, social and political realms, all together. Beyond physical, economic & tech limits, feasibility is foremost a creative exercise.

13/ The world is in the utmost climate emergency; we can't even imagine what is in store absent a very literal 'fast as possible' cut in emissions. We - scientists and everyone else - were unable to image the severity of impacts of recent years and how soon they beset us.

14/ With those considerations, 'as fast as possible' means - if we have the creativity to persuasively argue its feasibility - exactly the rate of cuts Kevin has been proposing (10-12%/yr - if not more) for rich nations, with the others to do a fair share too. Without a C budget!

15/ But the task of cutting emissions at such a high rate, year-on-year, demands specifics if the call for it is to be more than just a notion.

Questions arising are: (1) How to ensure that performance, down to zero; & 2) How to make adopted policy popular enough to survive.

16/ I have been working on a policy framework to resolve those questions into action (if feasibility can be successfully argued by the climate movement) which I think can deliver the rate of emissions cuts Kevin seeks, at least for fossil fuels, cement and steel. /Fin

My apology to both if I have misconstrued anything in their original thread or summarized it inaccurately. (Did my best to understand it.)