Thread by @JoeriRogelj

Carbon budgets tell us how much CO2 we can still emit while keeping warming below specific limits.

The latest @IPCC_CH report provides updated estimates of these budgets.

Here’s an insider's view with a deep dive looking at how they have changed since previous reports. (1/n)

I have been involved in the estimation of carbon budgets since the IPCC Fifth Assessment Report in early 2010s.

And since the first IPCC estimates published in 2013, we have learned a lot and have gotten much better at estimating remaining carbon budgets. (2/n)

The scientific basis underlying a carbon budget is our robust scientific understanding that global warming is near-linearly proportional to the total amount of CO2 we ever emit as a society.

This is shown in Fig. SPM10, both for the past and future projections. (3/n)

The remaining carbon budgets also made it into the Summary for Policymakers - the most prominent place that can be given for any finding of the report.

Table SPM.2 gives an overview of the latest estimates, for different temperature limits and different probability levels. (4/n)

How have these changes since previous reports?
Since the first time @IPCC_CH reported carbon budgets in 2013, important advances have been made in how we estimate budgets.
Five puzzle pieces combine to give carbon budget estimates, and allow us now to understand the updates (5/n)

Starting with the key message:
carbon budget estimates in AR6 are very similar to those published in the 1.5C Special Report in 2018, but they represent a significant update since AR5 in 2013.
(6/n)

When adjusting for the emissions since AR5 and SR15, AR6 remaining carbon budget for limiting warming to 1.5C with 50% chance is about 300 GtCO2 larger than in AR5, but virtually the same as in SR15.
(7/n)

For 66% probability, the AR6 budget is about 60 GtCO2 larger than in SR15 (8/n)

The budget is so much larger than in AR5, with more accurate methods published since which ensure that model uncertainties over the historical period are not accumulated into the future.
This is best illustrated by this technical figure from SR15.
(9/n)

Between SR1.5 and AR6 every piece of the carbon budget was reassessed:
- warming to date
- how much warming we get per tonne of CO2
- how much warming would occur once we reach net zero CO2
- how much non-CO2 warming we expect
- Earth system feedback otherwise not covered
(10/n)

Let's dive in each piece of the puzzle to understand what has changed.
(11/n)

SR1.5 used a 0.97°C warming estimate between 1850-1900 and 2006-2015. The AR6 uses 0.94°C for the same period. (12/n)

In isolation, this update results in central estimates being about 65 GtCO2 larger in AR6 than in SR15.

For the 33% and 67% estimates that's about 110 and 50 GtCO2 higher, respectively. (13/n)

The next piece of the puzzle is the warming we project per tonne of CO2. SR1.5 used an estimate of 0.8-2.5°C per 1000 GtC (=3664 GtCO2).

AR6 assessed this quantity, aka Transient Climate Response to Cumulative Emissions of CO2 (or TCRE), to fall in the 1.0-2.3°C range. (14/n)

Having the same central estimate, the update in TCRE causes no shift in 50% estimates, but the higher and lower percentiles are narrowed.

For a 67% chance, AR6 estimates are about 50 and 100 GtCO2 larger compared to SR1.5 for 1.5°C and 2°C of global warming, respectively. (15/n)

The third piece of the puzzle is the how much warming is expected to still occur once global CO2 emissions reach (and remain at) net zero.

This is known as the Zero Emissions Commitment to emissions of CO2 (or ZEC). (16/n)

The AR6 estimate confirms the SR1.5 estimate of no further CO2-induced warming or cooling once global CO2 emissions reach and stay at next zero. The uncertainty surrounding this value are reported separately.

ZEC therefore causes no changes between SR1.5 and AR6. (17/n)

The fourth puzzle piece is the projected warming from non-CO2 emissions. As SR1.5, AR6 uses deep mitigation pathways assessed by SR1.5, but with climate projections updated entirely with dedicated climate emulators that integrate the scientific information across chapter. (18/n)

By coincidence (and it is really coincidence), the updates in radiative forcing from tens of different gases, climate sensitivity, and carbon-cycle uncertainties result in no shift in the estimate of non-CO2 warming for remaining carbon budget. (19/n)

Pure luck, given the many updated pieces of scientific knowledge that were integrated in AR6, but convenient for explaining differences in carbon budget estimates.

Updated non-CO2 warming estimates lead to no change in remaining carbon budget estimates compares to SR1.5. (20/n)

The last piece is to account for Earth system feedbacks that would otherwise not be covered.

SR1.5 assumed an additional blanket reduction of 100 GtCO2 for this century for these feedbacks. AR6 updates this assessment entirely and includes it in its main estimates. (21/n)

Taking into account not only permafrost thaw, but also a host of other biogeochemical and atmospheric feedbacks, the AR6 estimates to appropriately account for these feedbacks, remaining carbon budgets have to be reduced by 26 ± 97 GtCO2 per °C of additional warming. (22/n)

Altogether these updates mean AR6 remaining carbon budget estimates are very similar compared to SR1.5.

Bottom line of this long thread: these budgets are small, our current ~40 GtCO2/yr emissions are reducing them rapidly, and all require CO2 to decline to net zero. (23/n)

If you reached this point and you felt the above thread was just missing that tiny bit of depth that you are yearning for, have a look at Chapter 5 of the new IPCC AR6 WG1 report. (24/n)

Section 5.5 tells you everything you need to know about the AR6 assessment of remaining carbon budgets

Box 5.2 gives an even more detailed comparison with earlier reports.

Check it out!
(end)
ipcc.ch/report/ar6/wg1/