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# Did 1.5°C suddenly get easier?

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A paper in 2017 argued for a considerable revision on the carbon budget for 1.5°C (https://www.nature.com/articles/ngeo3031), & I had some concerns (http://www.cicero.uio.no/no/posts/nyheter/commentary-did-15c-suddenly-get-easier). This is an extended presentation from a debate with the authors (https://www.youtube.com/watch?v=L7JS6x5fBNk)

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### Did 1.5°C suddenly get easier?

1. 1. Did 1.5°C suddenly get easier? Glen Peters (CICERO) Carbon Debate, 22/01/2018
2. 2. 0.1°C is ~220 GtCO2 (~5 years) 0.2°C is ~440 GtCO2 (~10 years) 0.3°C is ~660 GtCO2 (~15 years) … 0.5°C is ~1100 GtCO2 (to shift from 2°C to 2.5°C is a lot of carbon) The power of cumulative emissions 1 GtCO2 is 1 billion tonnes CO2, numbers are for 50% chance (median values), assumes non-CO2 constant Source: 0.5°C makes a big difference for mitigation
3. 3. 0.1°C is ~220 GtCO2 (~5 years) 0.2°C is ~440 GtCO2 (~10 years) 0.3°C is ~660 GtCO2 (~15 years) Millar et al ~660 GtCO2 (~15 years) larger The power of cumulative emissions 1 GtCO2 is 1 billion tonnes CO2, numbers are for 50% chance (median values), assumes non-CO2 constant Did 1.5°C suddenly get easier?
4. 4. • Two key explanations for difference with other estimates: – 0.9°C versus 1.1°C: Explains 400GtCO2 (10 years) – Non-CO2: 0.1°C (of 0.3-0.7°C) explains 200GtCO2 (5 years) • Millar et al – Consistent with the existing literature (at extreme high end) – Made valid and defendable choices – Completely misleading (did not put in context) Bridging the differences
5. 5. Bringing out the nuances
6. 6. • Why is there no “magic” carbon budget number? – Alternative modelling approaches (fixable) – Alternative definitions (fixable) – Alternative temperature datasets (fixable) – Alternative non-CO2 emissions (unfixable) – Ambiguity with negative emissions (unfixable) • In reality, the pathway actually does matter! Carbon budgets: Oversimplification
7. 7. Earth System Models (ESMs) consider limited scenarios (hence limited variations in non-CO2), have large model spread (below), and have internal variability. Not appropriate for carbon budgets (in my view). Source: IPCC (AR5), via Millar et al (2017) but using my own calculations Carbon budgets with “complex” models Even though a single scenario is used (RCP8.5), there is large model spread due to 20 different Earth System Models In RCP8.5, 1.5°C could be crossed already or in 2050
8. 8. “Complex” models assess 1-4 scenarios (RCPs), “simple” models assess hundreds of scenarios (dots below) Complex models are Earth System Models, simple models are Integrated Assessment Models Source: Rogelj et al (2016); Peters (2017), Avoid/Exceed Blog Model type has a big impact Sample result from a complex model
9. 9. Two main definitions are to exceed or avoid a given climate target (e.g., 2°C) Exceed budgets are generally higher, but need not be higher Source: Rogelj et al (2016); Peters (2017), Avoid/Exceed Blog Definitions have big impact Exceed target Avoid target
10. 10. Non-CO2 pathways are important, and can lead to a large range in cumulative emissions A 0.1°C change in non-CO2 temperature contribution, changes budget about 200GtCO2 Source: Rogelj et al (2016); Peters (2017), Avoid/Exceed Blog Non-CO2 emissions have big impact
11. 11. Add about 150GtCO2 for 2016-2020. Need to deduct cement, what to use? Source: Riahi et al. 2016; IIASA SSP Database; Global Carbon Budget 2017 Carbon budget for 2°C of warming (66%) Remainingcarbonbudget(GtCO2) (from2020) 300 GtCO2 1300 GtCO2
12. 12. Summing the 2°C emission scenarios gives the carbon budget (66% chance), with large uncertainty ranges Carbon Capture and Storage (CCS) and “Negative Emissions” allows the budget to be exceeded Note: Totals are not always consistent because medians are not additive, and some columns have different numbers of scenarios Source: Peters (2016) Some perspectives Non-CO2 emissions No CCS Millar et al 66% 2°C
13. 13. • Could (almost) equivalently use – Carbon budget – Year of net-zero emissions – Specify the pathway • All figures use the same carbon budget (1.5°C), but the policy implications are all very different Why use a carbon budget?
14. 14. Peters_Glen cicero.oslo.no cicerosenterforklimaforskning glen.peters@cicero.oslo.no Glen Peters