Beyond the single-basket mindset: a multi-gas approach to better constrain overshoot in near term warming

• Published in: Environmental research letters Vol. 19; no. 9; pp. 94011 - 94023
• Main Authors: Miller, Julie S, Dreyfus, Gabrielle B, Daniel, John S, Willis, Stephen, Xu, Yangyang
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01-09-2024
• Subjects: Baskets; Carbon dioxide; Climate change; climate mitigation; Datasets; Emission; Emissions; Emissions control; Global warming; Intergovernmental Panel on Climate Change; Methane; near-term warming; overshoot; Paris Agreement; Pollutants; Regression analysis; Regression models; Synthetic data; Tracks (paths)
• ISSN: 1748-9326; 1748-9326
• DOI: 10.1088/1748-9326/ad6461

Abstract The remaining carbon budget framework tracks progress towards the Paris Agreement’s goal to limit longer-term warming to well below 2 °C, but no analogous framework exists for constraining mid-century warming. Established single-basket methods of combining gases into CO 2 -equivalents using Global Warming Potentials (GWPs) lead to ambiguity over what combination of short- and long-lived emissions reductions are needed because they obscure the distinct warming impacts of each. We investigate to what extent a multi-basket approach that separates short-lived and long-lived pollutants can better estimate the likelihood for emission pathways to meet a near-term warming goal. We develop logistic regression models to categorize IPCC emission pathways (AR6) based on whether they exceed a mid-century temperature threshold. We focus on two baskets, using CO 2 for long-lived and methane (CH 4 ) for short-lived gases. For comparison, we consider several single-basket approaches (e.g. GWP100, GWP20, GWP*). We further apply our framework to a synthetic dataset covering a broader emissions space. Across both datasets, the two-basket outperforms all single-baskets. Using an illustrative near-term goal (1.7 °C), the two-basket approach reduces the magnitude of overshoot by a factor of 7 compared with the traditional single-basket. The two-basket’s advantage is smaller with the AR6 pathways, which we attribute to the high correlation between CO 2 and CH 4 emissions and confounding effects from other pollutants. Our results indicate that the two-basket approach better constrains overshoot magnitude, particularly if future emissions deviate from the AR6 assumption of correlated CO 2 and CH 4 reductions. Our approach allows the determination of a metric value and reduction target in the context of a chosen set of scenarios and temperature threshold; the outcome is a near-term methane-specific emissions budget that can be adopted by decisionmakers in a way that is analogous and complementary to the carbon budget. Future work could consider a third basket for very short-lived pollutants.