Thread by @damon_matthews

Our Perspective is out in @NatureGeosci! “Opportunities and challenges in using remaining carbon budget to guide climate policy” -- everything you ever wanted to know about carbon budgets in only 10 pages! Or keep reading for the 10-tweet version ... 1/10
nature.com/articles/s4156…

2/ First off. What is a remaining carbon budget (RCB)? Total future CO2 emissions (from fossil fuels and/or land-use) that is consistent with limiting warming to a particular target, from now until the time emissions reach net-zero. Only CO2 mind … none of them other GHGs

3/ Why is the RCB finite? Each CO2 emission increases global temperature, as per the TCRE (Transient Climate Response to cumulative CO2 Emissions -- a remarkably robust linear relationship, despite many uncertainties). So, stable global temp requires (net)-zero CO2 emissions

4/ So we can use the TCRE to estimate the RCB, given some estimate of available future CO2-only warming … or we can define an “effective TCRE” as the total anthropogenic warming (caused by all types of emissions) per unit CO2 at the time the temperature target is reached

5/ TCRE uncertainties propagate to the RCB. But the effective TCRE is *more* uncertain b/c it includes non-CO2 geophysical uncertainties re: forcing and response *and* socio-economic uncertainties related to future emission pathways … these also propagate to RCB uncertainty

6/ Clarity re: methodological choices and assumptions in calculating RCBs is essential! There are enough real uncertainties complicating things, that we don’t need to add artificial ones by using inconsistent definitions and approaches. See very long Table for a complete list ...

7/ So: net-zero CO2 targets are essential to stabilize climate. For 1.5°C, an RCB of 400 b tonnes CO2 means net-zero by 2040; for well-below 2°C (800 bT), net-zero by 2060. Imagine discrete 5-year budgets, each smaller than the last. Current emission pledges will not achieve this

8/ What does this mean for countries? We all like to claim our national targets are “fair and ambitious” but “fair” means many different things, and so far no country’s target is fair&ambitious enough to meet the Paris goal. Needs stronger international cooperation and oversight

9/ So where does this leave us? Clearly need to step up on all fronts to get this done. COVID lockdowns caused emissions to drop, and smart investments could drive emissions down further in coming years. Net-zero CO2 emissions w/i decades is possible if we decide it is a priority

10/ Thanks to all co-authors or our paper! @kasia_tokarska @NichollsZeb @JoeriRogelj @pepcanadell @PFriedling @froeltho @piersforster #NathanGillett @ilitat #RobJackson #ChrisJones @cdkoven @Knutti_ETH @AndrewHMacDoug1 @meinshausen @NadineMengis #RolandSéférian @KirstenZickfeld