1/n To conclude from the π©πͺ CO2 budget analysis & programmes of political parties that "1.5 is lost" is not a correct conclusion. Globally, 1.5 is still feasible, the world would have to be neutral by about 2050. It just means that Germany is lagging in its contribution.
2/n That is: lagging IF one measures the German contribution by population share. Some specialists question whether this is helpful, whether high-emitting nations should be held to such a standard when what really counts is the global outcome, to be fought for internationally.
3/n So the renewed discussion on a π©πͺ CO2 budget ceiling & what it means has elicited various responses & I'd like to particularly thank @CarlSchleussner for very good discussions. Carl is one of Germany's best experts not just on CO2 and rad. forcing but also on climate policy.
4/n The criticism is (in my words) that a national CO2 budget disregards that an intl. CO2 reduction regime could meet the Paris global temp goal irrespective of CO2 budget share fairness by population size, which the atmosphere does not care about as long as it happens.
5/n The arguement goes (in my words) that such an approach requires 2 things: an intl. financing system funded by the rich high emitters, as foreseen in the Paris agreement (& central to upcoming climate negotiations). Fairness through finance, the global goal made achievable.
6/n And second that national reduction paths at minimum follow the global cost-optimal path (including multiple GHG) - because not even hard-core free market economists could question whether that is the most advantageous course of action, so economic buy-in.
7/n The suggestion then is that if this all is the critical objective of the Paris project, confronting national politicians with national CO2 budget numbers is not the required focus - the focus should be on an international reduction regime, not detrimentally on "equal shares".
8/n I agree with the basic argument: of course the world, particularly the rich & high-emitting world, has to take on decarbonizing the whole world & to establish systems for that. In turn, from a purely global persp. they *might* be able to follow somewhat milder reduction paths
9/n But I do think that such discussions about intl. financial regimes and contributions, oriented toward results rather than distribution and equity, requires are morally justifyable baseline from which to launch such a regime. And that would be justly distributed CO2 budgets.
10/n I believe that to launch into intl. regimes without a national baseline means being adrift on the high oceans without a map. Global stocktaking is not enough; nations need a yardstick as to what their minimal baseline ambition should be irrespective of intl. deals on top.
11/n Only this keeps the conversation honest & measures progress - to plan to go global with shares is credible IF there were such an agreement, which there isn't. One could say Paris is, and we're bootstrapping, but does that help the national discussion, to wait for the whole?
12/n Two corollaries are important here. The first is that the CO2 budgets I & others compute are not "the" national budget. Given large uncertainties and normative choices, they are the maximal national budgets, the ceilings that do NOT rely on an assumption of what others do.
13/n They treat uncertainties and normative choices by going middle of the road with possible choices, which often is already a generous choice, e.g. when neglecting historical responsibilities. So they are somewhat generous ceilings to independent national shares. A baseline.
14/n Second, one might question whether focussing on a national 1.5 degrees contribution that seems impossible to achieve is counterproductive, undermining commitment, engagement, when at the same time a global 1.5 degrees path is still feasible but requires international focus.
15/n However, the main reason for focussing on a national 1.5 degrees budget (which the SRU report never did, btw) is that it is the publicly declared goal not just of the political parties, but also of the government, of the EU, and of the G7. They don't admit to emitting above.
16/n I'm afraid that if natl. governments & parties are allowed to proclaim their commitment to such goals but the path is a currently still insufficient & shaky, evolving regime with huge financing needs, then it may too easily be used as a smoke screen for insufficient action.
17/n Therefore I still think that a national minimal baseline (a maximal national CO2 budget, computed in a transparent manner (suggestions welcome, we published our's) is a useful concept for establishing and maintaining the required pressure for sufficiently ambitious action.
18/n It ensures honesty in a hot political field rife with claims. Whether a budget is strategically beneficial or not is a matter on which different views are possible. I still think that a number is helpful - sometimes society needs a number to guide its ambition: a challenge.
19/n I think complexity, such as an intl. climate regime, comes in parallel. It must be the focus of the Glasgow COP. π CO2 budgets are NOT instruments: they are baselines, the numbers that need to add up, globally. How to get there is not the realm of climate physics.
20/n Comments, remarks, thoughts very welcome! These are important issues and the debate among scientists a good thing. Transparency in public climate policy is another thing. Thanks to all who are in touch! /end/
For German readers, worthwhile additional comments by @MGSchmelzer of @NeueOekonomie
https://twitter.com/MGSchmelzer/status/1433673599995588609
One more remark: the Green Party reflects upon the dimensions discussed in this thread: their CO2 budget is to meet 1.5 obligations in the end through "climate partnerships" with other countries. The idea appears in general terms elsewhere, too, but remains vague.
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