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A detailed look at future warming and remaining carbon budgets in the IPCC WG1 AR6 reportMalte Meinshausen & Zebedee Nicholls, Australian-German Climate & Energy College

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SIXTH ASSESSMENT REPORTWorking Group 1 - The Physical Science Basis

24th August 2021

CARBON BUDGETS AND TEMPERATURE PROJECTIONS IN THE NEW IPCC WG1 AR6 REPORT

Malte Meinshausen, Zebedee NichollsClimate & Energy College, University of Melbourne

With thanks to the many IPCC authors and contributors, particularly those of the CCB7.1 and

carbon budget deliberations, namely Piers Forster, Jan Fuglestvedt, Joeri Rogelj, Jared Lewis, Chris

Smith, Chris Jones, Sebastian Milinski, Matt Palmer, Bill Collins, Sönke Zaehle, Keywan Riahi,

Jarmo Kikstra, Ed Byers

Outline1. Revisit key messages

a. Every tonne of CO2 we avoid

emitting puts us in a better place

than we would have been otherwise

2. Projected warming

3. 1.5C crossing time

4. Remaining carbon budgets

5. The role of emulators: Bringing

knowledge together across WG1 and

passing it on to WG3.

6. Conclusion

Projected warming

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SIXTH ASSESSMENT REPORTWorking Group I – The Physical Science Basis

Human influence has warmed the climate at a rate that is unprecedented in at least the last 2000 years

Figure SPM.1Comparison to the classical “Hockey stick”

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Warming projectionsTable SPM.1. See more in Table 4.5 and Fig 4.11


Figure 1.25, Chapter 1

AR6 Recipe: 1. Take CMIP6 model ensemble2. Constrain with historical

observations (basically, very warm models get down-weighted).

3. Use independent ECS and TCR assessment (via simple emulator)

4. Blend Constrained CMIP6 with ECS/TCR 50:50

… and then use other (more complex) emulators to calibrate towards the assessed ranges to inform WG3 and other findings


How to still use the very good high warming scenarios?

→ Global warming Level patterns.

Figure SPM.5


With every increment of global warming, changes get larger in regional mean temperature, precipitation and soil moisture

Figure SPM.5

Time of reaching 1.5°C

https://www.realclimate.org/index.php/archives/2021/08/we-are-not-reaching-1-5oc-earlier-than-previously-thought/

What is the 1.5°C goal, and is it

lost?

2°C

1.5°C

...hold increase to well below 2C…

...and pursuing efforts to limit warming to below 1.5C…

2°C

1.5°C



2°C

1.5°C



SR1.5 pathways “1.5C with no or limited overshoot”

The main scientific input to the Paris Agreement at the time, as well as the SR1.5 report, uses the definition of 1.5°C scenarios with temperatures BY 2100 being below 1.5°C, allowing for limited, temporary overshoot.

Of course, any overshoot comes with, potentially irreversible, climate impacts.

Carbon Budgets and how they changed

Figure 5.31 in IPCC AR6 Chapter 6 Canadell et al. (2021)

Schematic: The components of the carbon budget

Figure 5.31 in IPCC AR6 Chapter 6 Canadell et al. (2021)

The data on the components:

- Historical warming at around 1.07°C (2010-2019) + ~0.1°C (2020)

- non-CO2: ~0.1°C-----------

0.23°C left → 500 GtCO2*

*best estimate TCRE is 1.65C / 1000 GtC or 0.45C/1000 GtCO2

Remaining carbon budgetsTable SPM.2


Remaining carbon budgetsExceeding more than a given budget reduces, but does not immediately eliminate,

our chance of staying below 1.5C

Table SPM.2


650 GtCO2 (net zero by 2052) 500 GtCO2 (net zero by 2045) 400 GtCO2 (net zero by 2040)

<1.5C>1.5C <1.5C >1.5C <1.5C

>1.5C



Carbon budget for an 83% chance of 2C is the same as the carbon budget for a 17% chance of 1.5C (aiming for well below 2C might even get us to 1.5C if we’re lucky)

Remaining carbon budgets

Key implication of carbon budgets:

● To limit global warming to 1.5C requires emissions to reach zero● To limit global warming to 2.0C requires emissions to reach zero● To limit global warming to 3.0C requires emissions to reach zero● To limit global warming to 4.0C requires emissions to reach zero● To limit global warming to 5.0C requires emissions to reach zero

If we want to stabilise the climate, then the goal is always for emissions to reach zero. That applies whether we exceed a budget or not.

Adapted from

https://twitter.com/airscottdenning/status/1423771338527805445?s=20




480

What is our remaining carbon budget in GtCO

2

from now to keep warming with 50% chance below 1.5°C relative to pre-industrial?

580

0.43 ~400

300

SR1.5 budget looks slightly higher

SR1.5 just the same as AR6 when adjusted for recent emissions

… but actually, Earth System feedbacks had to be subtracted from SR1.5 budget (and they are already included in AR6 one).

… and both SR1.5 and AR6 refer to 1850-1900 rather than to pre-industrial. Best estimate for pre-industrial is around 0.05°C to 0.1°C cooler, which shaves off another ~180 GtCO2.

What’s changed since SR1.5

a. Historical warming down slightly (SR15: 0.97°C, AR6: 0.94°C for same period) ⇒ larger budgets

(~65 GtCO2)

b. TCRE: AR6 slightly narrower than SR1.5 but same median ⇒ same central budget, larger budget

for 67% chance, smaller budget for 33% chance

c. ZEC: no change

d. non-CO2: no change (pure luck given we updated almost everything in MAGICC)

e. Earth system feedbacks: SR15 assumed 100 GtCO2 flat rate, AR6 uses feedback of 26 +/- 97

GtCO2 per C ⇒ larger budgets

AR6 remaining carbon budget still tiny, but ~100 GtCO2 larger for like-with-like comparison.

→ An “Earth-System feedback adjusted” SR1.5 budget against 1850-1900 would be approximately

equivalent to a AR6 budget that assumes a pre-industrial temperature adjustment of 0.05C.

Scenarios

Future warming

dominated by CO2.

And carbon budgets can

also be used to figure out

GHG budgets over the

short timeframe until 2050

(like Victoria does).

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Role of emulators(Cross-chapter Box 7.1)

How to model the link between human emissions and global-mean temperature

for arbitrary emissions scenarios?


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Emissions to climate change cause-effect chain● Multiple steps in the chain

● Multiple interactions between steps

in the chain

● Requires comprehensive models


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Figure TS.4



in the chain


If we stop at global-mean warming, our

job is slightly easier


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Figure TS.4



in the chain


If we stop at global-mean warming, our

job is slightly easier

But, WG3 needs to assess 1 000’s of

scenarios


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Figure TS.4

The challengeCreate a tool which:

● Includes every link in the emissions to climate change cause-effect chain

e.g. the link between○ Emissions and concentrations (CO2 and non-CO2)

○ Concentrations and effective radiative forcing (CO2 and non-CO2)

○ Effective radiative forcing and global-mean warming

○ Feedbacks (e.g. carbon cycle feedbacks)

● Reflects the assessment of WG1 as closely as possible

● Is computationally cheap enough to be run for 1 000’s of scenarios in

WG3


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The process● Internal team focussed on Cross-Chapter Box 7.1

● Met every week for months

● Four emulator teams (MAGICC7, FaIR, CICERO-SCM, OSCAR)

● Included experts from multiple chapters to ensure tools reflected the

assessment sufficiently well


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The process● The targets for various metrics


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5.00

Table 7.SM.4

The results● Emulator performance compared to the targets


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5.00

Table 7.SM.4

The results● Colour scheme: darker shades ⇒ larger disagreement


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CCB7.1 Table 2

The results● Colour scheme: darker shades ⇒ larger disagreement


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CCB7.1 Table 2

The resultsEach model had strengths and

weaknesses

CICERO-SCM:

+ Historical warming

- Aerosol ERF


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CCB7.1 Table 2


weaknesses

OSCAR:

+ Airborne fraction

- Projected warming


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CCB7.1 Table 2


weaknesses

FaIR and MAGICC7:

+ Forcing and projected warming

- TCRE


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CCB7.1 Table 2

The results- FaIR and MAGICC7 assessed to

represent WG1 assessment to within

small* differences

- CICERO-SCM and OSCAR provide

additional information for evaluating

sensitivity of scenario classification


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CCB7.1 Table 2

*defined here as within typical rounding precisions of ±5% for central estimates and ±10% for ranges across more than 80% of metric ranges

The resultsOverall, there is high confidence that

emulated historical and future ranges of

GSAT change can be calibrated to be

internally-consistent with the assessment

of key physical-climate indicators in this

Report: greenhouse gas ERFs, ECS and

TCR.


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CCB7.1 Table 2

The results


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CCB7.1 Figure 1a)

The results


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CCB7.1 Figure 1a)

Assessed ranges based on observations (what the emulators should ideally match)

The results


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CCB7.1 Figure 1a)

For context, the raw CMIP6 range (the difference from the assessment is clear)

The results


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CCB7.1 Figure 1a)

Comparison of emulators and assessed ranges

Assessed ranges based on observations (what the emulators should ideally match)

CCB7.1 Figure 1c), 1d)

CMIP6 models for context (difference from raw CMIP6 ensemble is clear)


Comparison of emulators and assessed ranges


Emissions-driven mode used in WG3 leads to slightly less warming and wider uncertainties (in line with emissions-driven CMIP6 runs in Fig. 4.3 and carbon cycle assessment in Ch. 5)


Outlook for WG3● Consistency between WG1 physical assessment and WG3 scenario

categorisation has increased

● Validation of tools used for WG3 scenario categorisation has increased

● Availability of multiple emulators gives a more sophisticated picture of

the scenario categorisation of 1 000’s of scenarios

WG3 will be released in early 2022 with insights from WG1 as conveyed by

the calibrated emissions-driven emulators

(As we understand it) WG3 pipeline will be available for users beyond IPCC


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Other emulatorsUsed in multiple places throughout the report

● Ch. 1 to estimate warming pre-1850

● Ch. 4 to translate Ch. 7’s ECS, TCR and forcing estimate into projected

warming

● Ch. 3 and Ch. 7 to attribute warming to individual forcers (e.g. CO2, CH4)

● Ch. 5 to estimate non-CO2 importance for remaining carbon budget

● Ch. 9 for sea-level rise projections

● Ch. 11 for regional land temperature projections

● And many more


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CCB7.1 Table 1

Key takeaways● A problem we have created

● What happens next is up to us

● Greater certainty that if we get to net zero CO2, CO2-induced

warming stops

● Since first IPCC report in 1990, we burnt through 2/3rds of our

remaining carbon budget for 1.5C.

● Decisions made this decade determine what our climate

future looks like


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BY THE NUMBERS

Author Team 234 authors from 65 countries

28% women, 72% men

30% new to the IPCC

Review Process 14,000 scientific publications assessed

78,000+ review comments

46 countries commented on Final Government Distribution

Upcoming seminars:

Tuesday 7 September, 4.30 PM (AEST)

Developments in physical understanding and how to use it to improve climate policies - Insights from IPCC AR6 presented by two of Europe’s leading climate scientistsPiers Forster (University of Leeds) & Joeri Rogelj (Imperial College, London)

For more details and to register, please visit: climatecollege.unimelb.edu.au/seminars

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