Thread by @Knutti_ETH

1/ Two interesting comments in @Nature_Geo on carbon budgets.
Carbon budgets may be of limited value to trigger policy action, but they have been crucial in changing how people think about the CO2 problem. Here’s why…
@Peters_Glen @Oliver_Geden nature.com/articles/s4156…

2/ Many (inconveniently) simple facts that everyone needs to remember follow from carbon budgets (and related work). Summarized here: iac.ethz.ch/content/dam/et…. Namely…

3/ CO2 is the largest contributor to the total forcing and surface warming both in the past and future.

4/ A large fraction of the CO2 emitted stays in the atmosphere for centuries and longer. Much of the climate change is irreversible for millennia even if CO2 emissions are stopped (so going back requires CCS). pnas.org/content/106/6/…

5/ There is little warming commitment from past emissions beyond the warming observed so far. Additional future warming (above current levels) is largely determined by future emissions.
science.sciencemag.org/content/340/61…

6/ The real commitment and inertia is in infrastructure and society that is currently emitting CO2
science.sciencemag.org/content/329/59…

7/ Every ton of CO2 adds about the same amount of warming, no matter when and where it is emitted. Any temperature target
implies a limited budget of CO2 that we are allowed to emit to stay
below the target, largely irrespective of the scenario that leads to those emissions.

8/ Higher emissions early on imply stronger reductions later on for the same temperature target. We are using the emissions of the next generations and let them figure out a way to live without.

9/ Countries and generations approximately contribute(d) to past and future climate change in proportion to their total cumulative emissions. Past contributions have been very uneven.

10/ Global CO2 emissions need to decrease quickly and strongly to likely keep warming below 2 °C, because a large fraction of the CO2 budget has already been emitted.

11/ A higher temperature target would provide more time but simply postpones the decarbonization required. It needs to happen anyway.

12/ Fossil fuel reserves are much bigger than the budget allowed for limiting warming to below 2°C. As a consequence, much of the carbon will need to remain in the ground, or be sequestered again. nature.com/articles/natur…

13/ We do not know exactly how big the carbon budget is; we can only
say that it is smaller than X for a given likelihood that we can pick. A higher
certainty of achieving the desired goal implies a smaller budget.

14/ Uncertainty thus is not on our side. To prevent dangerous interference with the climate system, a larger uncertainty implies *stronger* emission reduction targets to be on the safe side.

15/ The global carbon budget can be framed as a global commons problem. I benefit more from burning CO2 by producing cheap energy than I suffer now from the problems this causes. But it may not be solved in that way: sciencedirect.com/science/articl…

16/ All of these points were crucial in UNFCCC leading to the Paris agreement. They don't prescribe policy. Now we may need other tools to establish the policies that make 2°C happen.

. @afreedma

Follow ups: 17/ Are scientists/carbon budgets to blame for the lack of action? No in my view. The reasons are mostly politics, focus on near term profit, economic growth, freedom, differences in cultures, values, priorities, etc., not science.
qz.com/1278776/what-i…

18/ Some scientists have argued for carbon budgets as a policy tool, but most have not. We saw it as the simplest way of calculating/explaining how close we are to two degrees, and that emissions have to approach zero quickly.
iac.ethz.ch/content/dam/et…

19/ One reason for IPCC AR5 WG1 highlighting the carbon budget (or TCRE) is that it is only a function of physical (climate sensitivity/TCR) and carbon cycle feedbacks (airborne fraction). (Almost) no assumptions on technology, cost, time preference…

20/ The budget does depend a bit on non-CO2, on policies taken, on pre-industrial baseline, on model agreement with obs,… but the basic conclusions in this thread are valid no matter how large the budget is.

21/ Time of zero emission may be more policy relevant but requires assumptions on technology, cost, negative emissions, discount rates, so is inevitably more policy prescriptive. Even worse on country levels where it requires assumptions on burden sharing.

22/ And it's very uncertain: Even if the budget were known exactly, zero emission could still be anywhere from ~2050 to 2100 depending on what we do before zero emission (and after with CCS). Many time series have the same integral.

23/ IPCC WG1 could never have defended a year for zero emission. Carbon budget is to first order a property of the climate, distribution over time a societal choice.

24/ My take: carbon budget is a simple concept for understanding climate and useful in communication: the more I use now the less somebody else can (now or later). It is policy relevant but to trigger policy action may need more.

25/ And honestly some of these debates on the size of the budget and 1.5/2°C are moot: we know emissions need to drop rapidly no matter what. As we wrote before Paris: “We need to agree how to start, not where to end mitigation”
nature.com/articles/ngeo2…