An emission pathway classification reflecting the Paris Agreement climate objectives

The paper analyses the impacts of the IPCC AR6 WGIII report’s use of scenario categories and suggests an alternative approach to determining what scenario pathways align with or are compatible with the Paris Agreement. 

IPCC’s scenarios in AR6 include those based on whether scenarios have an at least 50% probability of keeping warming below 1.5°C in 2100 and then on the likelihood of keeping warming below 2°C. Additional categories around limiting warming to 2.5 °C, 3°C, and 4°C and exceeding 4°C with a greater than 50% chance are also included. The paper suggests the categorisation of emissions pathways between either keeping warming below 1.5°C or 2°C does not reflect Article 2.1 of the Paris Agreement, and that the IPCC’s dichotomy between the two temperature targets mischaracterises the opportunities for policies to achieve Paris. It also suggests overlap between 1.5°C or 2°C is considerable, which is not reflected in the IPCC’s pathway categories. Furthermore, the pathways based around 2100 are not rooted in the Paris Agreement or the UNFCCC.  

The paper sets out to provide guidance on what pathways should be considered Paris Agreement compatible. Three criteria are proposed: 

1. Criterion I - pursuing efforts to limit warming to 1.5°C; this is interpreted as including pathways that never have a greater than 66% probability to overshoot 1.5 °C. 
   
2. Criterion II – hold warming to well below 2°C; this is proposed as a very likely chance (>90%) of never exceeding 2 °C, which is the proposal of how to translate the “well below 2 °C”. 
   
3. Criterion III - Achieving net zero greenhouse gases; net zero greenhouse gases need to be achieved in the second half of the 21st century as per Article 4. 

The paper notes that its proposed criteria, that are better aligned with the Paris Agreement, can provide insights on Paris compliant mitigation benchmarks. It suggests these pathway categories can advance the relevance of pathway analysis as we consider how to implement Paris. 

This paper can be used to provide evidence on what a Paris compliant scenario should be, narrowing the types of pathways that can be used. This is important given the large number of scenarios that often feature, and are claimed as Paris compliant.