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Did 1.5°C suddenly get easier?
The document discusses the complexities of carbon budgets and their implications for climate targets, specifically focusing on the temperature thresholds of 1.5°C and 2°C. It highlights the significant impact of cumulative emissions and non-CO2 factors on carbon budgets, noting that small temperature changes can drastically alter budget estimates. The analysis reveals that different modeling approaches and definitions can lead to diverging estimates, underscoring the importance of considering nuanced pathways in climate policy.
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Did 1.5°C suddenly get easier?
- 1. Did 1.5°C suddenlyget easier? Glen Peters (CICERO) Carbon Debate, 22/01/2018
- 2. 0.1°C is ~220GtCO2 (~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. 0.1°C is ~220GtCO2 (~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. • Two keyexplanations 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. Bringing out thenuances
- 6. • Why isthere 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. 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. “Complex” models assess1-4 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. Two main definitionsare 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. Non-CO2 pathways areimportant, 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. Add about 150GtCO2for 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. Summing the 2°Cemission 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. • 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.