Methane mitigation achievement, including agriculture, is crucial to limiting dependence on uncertain CDR in national carbon budgeting equitably meeting Paris goals

DOI: 10.5281/zenodo.7534820 This is a pdf version of the slides presented by Paul R Price to the 2nd International Conference on Negative CO2 Emissions, June 14-17, 2022, Göteborg, Sweden. A video version of the presentation is at the following link: https://t.co/Hijftooo9R This Carbon Budgeting Fellowship research at Dublin City University is funded by the Climate Change Advisory Council (Ireland). The Dublin City University fellowship supervisors for this research are Prof. Barry McMullin and Dr. Aideen O’Dochartaigh. Abstract The Paris Agreement, in 2015, committed nations to limit global heating to ‘well below’ 2°C above the pre-industrial level and to pursue efforts to limit the increase to 1.5°C through policies ‘implemented to reflect equity and the principle of common but differentiated responsibilities and respective capabilities, in the light of different national circumstances’ (CBDR-RC). Given a rise to 2015 of nearly 1.1ºC, by 2025 many developed nations are likely to overshoot their equal per capita share of the remaining global CO2 budget corresponding to the Paris goal, even on a “minimally equitable” basis ignoring historic responsibility. Paris-aligned national policy therefore needs to plan to limit and cancel any “carbon debt” from the resulting overshoot through warming reduction measures that are usually framed in terms of carbon dioxide removal (CDR). However, large-scale CDR achievement is highly uncertain and so likely intergenerationally inequitable, contrary to CBDR-RC. Although recognised in physical climate science and assessment models, negative emissions literature to date frequently undervalues or ignores the importance of early, sustained, and permanent methane (CH4) mitigation in limiting CDR policy dependence. Moreover, radical reductions in fossil fuel use before 2050 achieves necessary cuts in CO2 emissions, limiting CO2 heating, but non-CO2 heating occurs in parallel, as related cuts in fossil fuel combustion aerosols are not compensated for by cuts in fossil CH4 emissions. Therefore, substantial non-CO2 mitigation, particularly reduction in annual CH4 emission, from global agriculture, forestry and land use (AFOLU), is critical to enable global carbon budgets aligned with a demanding Paris temperature target (Mengis and Matthews, 2020). Usefully, if used with care, the recently developed GWP* emissions equivalence metric allows evaluation of “CO2 warming equivalent” (CO2we) trade-offs between CH4 mitigation and gross emissions and CDR to inform national mitigation policy. Ireland provides an informative case study of this issue due to relatively high agricultural CH4 and N2O emissions, primarily from beef and dairy production, and rising net land use CO2 emissions, due to drained organic soils and ongoing peat extraction with declining forestry CDR. Since 2010, expansionary agricultural policy, especially in dairy production, has increased CH4 and N2O emissions by 19% and 5% respectively. Previous work identified an upper bound, prudent, minimally equitable, CO2-only national carbon quota of ~400 MtCO2 from 2015 for Ireland that is likely to be exceeded by 2024, and by 2050 could well be in overshoot this by 600 MtCO2. Ireland’s technical negative emissions potential may be 600 MtCO2 to 2100, but practically may be only 200 MtCO2, and less than 150 MtCO2 to 2050 even with stringent land use policy. The research project Society-wide Scenarios for Effective Climate Change Mitigation in Ireland (McMullin and Price, 2020), funded by Ireland’s Environmental Protection Agency (EPA), included assessment of the non-CO2 emissions contribution to achieving Paris goal mitigation within defined CBDR-RC terms. Based on the IPCC database scenario sets meeting the 2°C Paris temperature goal, GWP* was used to estimate a “prudent”, multi-gas [CO2+N2O+CH4] global GHG budget from 2015 of 840 GtCO2we. Ireland’s population-based share of this is a GHG national carbon quota (NCQ*) of 540 MtCO2we, depleting from 2015. Using a spreadsheet tool developed for the project, six alternative illustrative scenarios were assessed for varying [CO2+N2O+CH4] reduction pathways over time. Only radical scenarios achieving net zero CO2 before 2050 and permanently cutting CH4 by 50% (a temperature reduction equivalent to negative CO2 emissions of -500 MtCO2 by 2050) limited overshoot and enabled a return to the Paris-aligned national quota by 2060 without exceeding the practical CDR limit to 2100. Essential recommendations are made, regarding the use of GWP* in mitigation analysis, illustrating the “certainty advantage” of early and deep CH4 mitigation in addition to rapid CO2 reduction, and identifying crucial trade-offs between different climate pollutants in any Paris-aligned mitigation policy that is reliant on negative emissions assumptions. The need for extremely urgent and deep mitigation to adequately address the declared climate emergency is confirmed, including necessary limits on GHG-intensive agriculture and land use activities. References McMullin, B., Price, P., 2020. Synthesis of Literature and Preliminary Modelling Relevant to Society-wide Scenarios for Effective Climate Change Mitigation in Ireland. 2016-CCRP-MS.36 (EPA Research Report No. 352). Environmental Protection Agency. https://www.epa.ie/publications/research/climate-change/Research_Report_352.pdf Mengis, N., Matthews, H.D., 2020. Non-CO2 forcing changes will likely decrease the remaining carbon budget for 1.5 °C. NPJ Clim. Atmospheric Sci. 3, 19. https://doi.org/10.1038/s41612-020-0123-