The global push towards net-zero (NZ) greenhouse gas (GHG) emissions, with 151 countries representing 92% of the world economy making pledges, highlights the crucial role of consistent definitions. Terms like "net zero," "climate neutral," and "carbon neutral" are often used interchangeably, despite variations in scope and implementation. The Intergovernmental Panel on Climate Change (IPCC) emphasizes the need to achieve NZ carbon dioxide (CO2) emissions by 2050 and NZ GHG emissions by 2070 to limit warming to 1.5 °C. However, national targets frequently lack clarity and deviate from the 100-year global warming potential (GWP100) aggregation used in UN Framework Convention on Climate Change (UNFCCC) reporting. The land sector, particularly agriculture, is central to meeting NZ targets, offering potential CO2 removal. However, land-based CO2 removal competes with existing land uses, primarily livestock grazing. Shifting land use has significant societal and environmental implications, impacting global food security and potentially causing GHG "leakage" if food production shifts to less efficient regions. Livestock methane (CH4) emissions significantly contribute to global GHG emissions. Alternative GHG accounting methods, such as GWP*, which account for the short-lived nature of CH4, could result in different land use mixes compatible with NZ. Introducing separate CH4 targets could reduce the demand for CO2 removals while aligning with IPCC emission trajectories. However, downscaling these methods to the national level presents challenges regarding fairness in global CH4 emission burden-sharing. Differing principles for allocation, such as using historical emissions as a baseline or requiring equal percentage reductions, could be perceived as unfair. This lack of consensus on CH4 emission accounting contributes to inconsistent NZ target definitions. Ireland, a significant exporter of milk and beef with the AFOLU sector contributing over 40% of national GHG emissions, faces a rising emissions challenge despite an NZ target by 2050. This study uses detailed AFOLU GHG flux scenarios for Ireland to explore the land use combinations needed to achieve NZ under various definitions and examines the effects on milk and beef production with and without ambitious GHG abatement measures.

Existing literature highlights the vagueness and inconsistencies surrounding net-zero targets, emphasizing the need for clear definitions to guide effective policymaking (Rogelj et al., 2021). Studies have explored the agricultural sector's contribution to climate change and its potential for mitigation, emphasizing the distinct role of agriculture compared to fossil fuel-emitting sectors (Lynch et al., 2021). The efficiency of land use changes in mitigating climate change has also been a subject of research, highlighting the need to consider trade-offs and potential consequences (Searchinger et al., 2018). The debate surrounding the appropriate accounting of short-lived climate pollutants like methane (CH4) has been extensively discussed, with various methodologies proposed to better reflect their impact on warming (Allen et al., 2018, Allen et al., 2022). The fairness and equity implications of different accounting approaches at the national level have also been addressed, raising concerns about potential biases against countries with low baseline emissions or those providing global food security (Kartha et al., 2018; Rogelj & Schleussner, 2019; Richards et al., 2018). Previous research has also examined the challenges and opportunities associated with transforming the agricultural sector to achieve climate neutrality, recognizing the need for integrated social, economic, and environmental actions (Harrison et al., 2021).

This study utilized the GOBLIN (General Overview for a Back-casting approach of Livestock INtensification) model, a national biophysical AFOLU model. GOBLIN generates randomized scenarios of agricultural activities and land use combinations within biophysical constraints for the year 2050, calculating annual GHG emissions to 2100. Maximum animal numbers were constrained at 2021 levels, and land spared from livestock was allocated to carbon-neutral or carbon-positive uses (wetland restoration, afforestation, ungrazed grassland). An ambitious 30% reduction in agricultural CH4 and N2O emissions (representing optimistic technical abatement by 2050) was applied post hoc. Ten NZ definitions were applied to filter scenarios (Figure 1b): NZ CO2 (only), NZ GHG based on GWP100, no net warming based on GWP*, and separate CH4 targets alongside GWP100 balance for N2O and CO2. Variations incorporated international fairness and longer-term (LT) horizons (to 2100). Scenarios were classified as successful (S-NZ, S-NZ-A) or failed (F-NZ, F-NZ-A) to reach NZ with and without abatement. The GWP100 metric, using AR5 emission values for CO2, CH4, and N2O, was employed to calculate GHG neutrality. The GWP* metric, reflecting the rate of change in short-lived climate pollutants, was calculated using the latest GWP equation, considering CO2-warming-equivalent emissions. Methane targets were based on allocation methods from Prudhomme et al. (2021): grand-parenting, population, and animal protein security. The eGWP* method adjusted national reference emission levels for CH4 for improved international fairness. Carbon neutrality considered only CO2 emissions and removals. Long-term NZ scenarios assessed cumulative emissions between 2050 and 2100. Abatement analysis explored the sensitivity of NZ compliance to varying levels of agricultural emission reductions (0–100%). 3000 scenarios were generated using Latin hypercube sampling, varying input parameters within defined ranges (Table S1).

The study revealed significant variations in the percentage of scenarios meeting NZ criteria (1–85%) depending on the definition employed (Table 1). Definitions resulting in the highest percentage of successful scenarios included carbon neutrality (99%), GWP* (82–85%), CH4 Target Grand-parenting (58–85%), and eGWP* Protein (61–84%). Conversely, definitions resulting in the lowest success rates included CH4 Target Population (1–3%), GWP100 LT (18–27%), and eGWP* Population (26–39%). Large differences in land use changes were observed between successful and unsuccessful scenarios (Figure 2). Successful scenarios exhibited considerably larger areas of new forestry and wetlands but lower milk and beef outputs. Median new forest areas for successful scenarios ranged from 1096–2267 kha, compared to 313–1078 kha for unsuccessful scenarios. Similarly, median new wetland areas ranged from 209–339 kha for successful scenarios versus 39–207 kha for unsuccessful ones. Even with ambitious abatement, few scenarios could maintain 2021 levels of both dairy and suckler beef cow populations (Figure 3a), except under the carbon neutrality definition. Maintaining high milk production (Figure 4) required substantial land use changes: 26–90% reduction in grassland for grazing and a 47–387% increase in forest cover. Significant reductions in suckler beef and sheep populations were also necessary (Figure 5). Technical abatement increased the share of scenarios achieving NZ across all definitions (Figure 6). For instance, a 20–80% increase in abatement increased the success rate of the GWP100 definition from 45–85%. Even at 100% technical abatement, the LT definitions only achieved 91–93% success rates.

This study demonstrates that achieving national NZ targets necessitates significant transformations in Ireland's land sector and agricultural practices. The wide range of potential futures highlighted by varying NZ definitions underscores the critical need for international consensus on clear and consistent accounting methodologies, especially for methane emissions. While significant variations exist across definitions, common actions emerge: afforestation, organic soil rewetting, and reductions in cattle numbers. Even with optimistic abatement measures, substantial land use change is unavoidable. The study highlights trade-offs between prioritizing milk or beef production and achieving NZ goals, emphasizing the need for difficult policy decisions. Reducing agricultural outputs could lead to international GHG leakage. The choice of NZ definition impacts not just the technical aspects of mitigation but also the equity and fairness of national commitments. The study's findings are particularly relevant for countries with significant agricultural contributions to national emissions.

This research demonstrates the significant sensitivity of achievable net-zero agricultural and land use configurations to the definition of net zero itself. While a wide range of potential futures emerges, several commonalities exist across definitions, highlighting the importance of afforestation, wetland restoration, and cattle reductions. Even ambitious technical abatement cannot fully substitute for these land use changes. International consensus on consistent net-zero definitions, particularly regarding methane accounting, is crucial for effective policymaking and a just transition. Ireland's case study highlights the need for evidence-based engagement with stakeholders to achieve net-zero while addressing social and economic implications.

The GOBLIN model's limitations include its focus on positive land use transformations (excluding negative impacts) and the linear interpolation of scenario values. Uncertainty in long-term projections beyond 2050 is acknowledged, with the model assuming static flux factors after 2050 (excluding forest growth-harvest cycles). The ambitious 30% abatement assumption might overestimate achievable reductions in the short term. Interactions between land-based CO2 sinks and downstream bioeconomy innovations are not fully accounted for.