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

budget (rGCB) equitably aligned with meeting the Paris Agreement (PA) temperature goal. PA-aligned pathways in integrated assessment modelling typically rely on large scale carbon dioxide removal (CDR), even though such reliance is highly uncertain and likely inequitable. Moreover, radical reductions in fossil fuel combustion (FFC) cut CO2 emissions but result in a parallel reduction in cooling aerosols, severely reducing the available rGCB. Consequently, substantial land use change (LUC) CO2 and agricultural non-CO2 mitigation is now crucial to achieve global carbon budget outcomes aligned with meeting the stringent Paris temperature goal. However, most negative emissions literature undervalues or ignores the importance of early, sustained, and permanent methane (CH4) in equitable mitigation limiting CDR dependence. Therefore, to meet a national PA “fair share” temperature goal, we use the recently developed GWP* CO2 warming equivalence method in illustrative scenarios to assess trade-offs between: mitigation of CO2 and N2O; and temperature impact reduction (TIR), via CH4 mitigation and limited CDR. Ireland provides an informative, developed nation case study due to: carbon budgeting law and policy; rising agricultural CH4 and N2O emissions; and net emissions of LUC CO2, due to drained organic soils, ongoing peat extraction, and declining forestry CO2 removals. Practically, Ireland’s CDR potential may be only 200 MtCO2. Based on the IPCC database scenario sets for 1.5C low overshoot and Lower 2C, GWP* is used to benchmark a “prudent” multi-gas rGCB* of 640 GtCO2we for CO2+N2O+CH4 from 2015. Ireland’s population-based share of this is a multi-gas national carbon quota (NCQ*) of 410 MtCO2we, depleting from 2015 onward. Using a spreadsheet tool, illustrative scenarios are assessed for different CO2+N2O+CH4 reduction pathways over time. Only radical scenarios, achieving net zero CO2 before 2050 while permanently cutting CH4 by 50%, limit overshoot and enable a return to the Paris-aligned national quota by 2050 without exceeding the practical CDR limit to 2100. Key findings include guidance on use of GWP* in mitigation analysis, the certainty-importance of early and deep CH4 mitigation in addition to rapid CO2 reduction to limit CDR reliance. Given mitigation delay, the need for immediate and deep CO2 and CH4 mitigation is confirmed, requiring regulatory limits on GHG-intensive agriculture and land use activities as well as radical reductions in fossil carbon combustion for energy and cement use