Defining national net zero goals is critical for food and land use policy

Many countries have pledged net zero targets, but terms such as net zero, climate neutral, carbon neutral, and GHG neutral are used inconsistently. The IPCC indicates NZ CO2 around 2050 and NZ GHG around 2070 to limit warming to 1.5 °C. Most national targets aim for NZ GHG, often aggregating gases using GWP100, but scopes and implementations vary. The land sector is pivotal, particularly for CO2 removals, yet land-based removals compete with existing agricultural uses and have social and environmental implications, including food security and potential leakage if production shifts to less efficient regions. Methane from livestock is a major contributor; alternative accounting approaches (e.g., GWP*) that reflect CH4’s short-lived nature could change land-use mixes compatible with national NZ and may emphasize separate CH4 targets. However, downscaling such approaches raises fairness concerns in burden sharing across nations. Ireland is a major exporter of milk and beef, with AFOLU accounting for over 40% of national emissions. Recent trends show rising AFOLU emissions due to increasing milk production and low afforestation rates, despite a national NZ-by-2050 goal (climate neutrality). This study asks: how do different NZ definitions affect feasible AFOLU configurations for Ireland by 2050, and what are the implications for milk and beef production with and without ambitious abatement?

The paper situates its work within literature on NZ target-setting and metrics for aggregating GHGs. It highlights that GWP100 is the internationally accepted metric for UNFCCC reporting and Paris Agreement NDCs, but alternative approaches (e.g., GWP*, separate methane targets) have been proposed to better align with temperature outcomes and the distinct behavior of short- vs long-lived climate forcers. Prior studies emphasize the land sector’s role for CO2 removals and the trade-offs with current land uses, including risks of leakage and impacts on global food security when livestock production is displaced. The fairness of national allocations for CH4 reductions has been debated, with approaches including grand-parenting (equal percentage reductions), equal per-capita, and allocations based on animal protein production. Literature also notes potential for mitigation in agriculture through technical abatement and the importance of afforestation and peatland rewetting for land-based CO2 removals. The authors build on these strands by systematically comparing ten NZ definitions, including fairness-adjusted CH4 targets and warming-equivalent metrics, in a national AFOLU modeling framework.

The study uses GOBLIN, a national biophysical AFOLU model, to generate 3000 randomized scenarios of Ireland’s agricultural production and land-use configurations under biophysical constraints, targeting 2050 (and extending emissions accounting to 2100 with fixed post-2050 land use). Inputs randomized via Latin hypercube sampling include dairy, beef, and sheep numbers (capped at 2021 levels), animal productivity, fertilizer rates, and land allocation rules for grassland spared from livestock. Spared grassland on organic soils could be rewetted; on mineral soils it could be afforested or maintained as ungrazed grassland; cropland area remained constant. Emissions and removals were computed using IPCC Tier 1/2 methods consistent with Ireland’s UNFCCC reporting. A parallel set of 3000 ‘abated’ scenarios applied an ambitious uniform 30% reduction to agricultural CH4 and N2O sources (enteric, manure; direct and indirect N2O). Ten NZ definitions were applied to classify scenarios as successful (S-NZ) or failed (F-NZ), with and without abatement (S-NZ-A, F-NZ-A): (1) GWP100 NZ in 2050 (net GHG balance based on AR5 values CO2=1, CH4=28, N2O=265); (2) GWP* NZ (CO2-warming-equivalent method aligning CH4 rate-of-change with CO2-equivalent warming to achieve no additional warming at 2050); (3–5) Separate national biogenic CH4 targets for 2050 compatible with 1.5 °C using fairness allocations from Prudhomme et al.: grand-parenting (equal percentage reductions), population (equal per-capita), and protein (proportional to animal protein production), combined with a GWP100 net balance for CO2+N2O; (6–7) eGWP* variants replacing the CH4 reference with fairness-based reference levels (population and protein) in the GWP* calculation to address preferential treatment concerns; (8) carbon neutrality (CO2-only net balance in 2050); (9–10) long-term (LT) definitions requiring cumulative GHG balance over 2051–2100 with constant post-2050 land use, assessed with both GWP100 LT and GWP* LT. Scenario outputs included land areas (new forest, rewetted organic soils), livestock populations, and milk and beef production, analyzed against NZ success for each definition. Sensitivity analysis varied technical abatement from 0% to 100% to assess effects on NZ compliance rates.

- NZ feasibility was highly sensitive to the definition used. Across aggregation methods for CO2, CH4, and N2O, 1–85% of scenarios met NZ criteria (excluding the very permissive CO2-only case). Including carbon neutrality, up to 99% succeeded. - Success rates among 3000 scenarios (non-abated vs abated): carbon neutrality 2969 (99%) both; GWP* 2464 (82%) and 2547 (85%); CH4 Target Grand-parenting 1744 (58%) and 2560 (85%); eGWP* Protein 1816 (61%) and 2511 (84%). Lowest success: CH4 Target Population 35 (1%) and 92 (3%); GWP100 LT 551 (18%) and 805 (27%); eGWP* Population 770 (26%) and 1172 (39%). GWP100 37% and 50%. - Common features of successful NZ scenarios across definitions: substantially larger areas of new forestry and rewetted organic soils, with lower milk and beef outputs relative to 2021. - New forest area: median among S-NZ ranged 1096–2267 thousand ha across definitions versus 313–1078 thousand ha among failures. Minimum new forest required to succeed was lowest for carbon neutrality (median ~+142% of 2021 forest cover) and highest for CH4 Target Population and GWP100 LT (median +294% and +255% vs 2021 forest cover in non-abated runs). - Rewetted organic soils: median new wetland area for S-NZ and S-NZ-A ranged 209–339 thousand ha; maximum rewetting (339 thousand ha) corresponds to ~28% increase and was frequently needed for stricter definitions (e.g., GWP100 LT, eGWP* Population, CH4 Target Population). - Production impacts: Across successful scenarios, aggregate milk and beef outputs declined vs 2021. With abatement, the 95th percentile milk output achievable under NZ varied from 11% to 91% of 2021 output, typically requiring large reductions in suckler-beef output (up to 98%) and major forest expansion (+47% to +387% forest cover, i.e., from 11% of land in 2021 to 16–53% in 2050 for high-milk S-NZ-A cases). Sheep populations could decline by up to 97% in high-milk NZ cases. - Livestock herd trade-offs: Except under carbon neutrality, no abated NZ scenario maintained 2021 levels of both dairy and suckler cow populations. Scenarios maintaining 2021 dairy numbers required at least a two-thirds reduction in suckler cows. Stricter definitions (CH4 Target Population, Protein; eGWP* Population; GWP100 LT) required significant reductions in dairy cow numbers even with abatement. - Land-use and milk output linkage: Higher milk outputs among S-NZ-A scenarios were associated with smaller spared areas for new forests, but minimum afforestation needs formed a rising lower boundary at higher milk outputs (especially under GWP100), indicating necessary offsets for remaining emissions. - Technical abatement sensitivity: Increasing abatement raised NZ compliance shares across definitions. For GWP100, success rose from 45% at 20% abatement to 85% at 80% abatement. For CH4 Target Population, success jumped from 18% at 70% abatement to 88% at 90%. Rankings of definitions by success changed with abatement level. Even at 100% abatement, long-term definitions achieved only 91–93% success (GWP100 LT and GWP* LT), reflecting challenges maintaining long-term CO2 flux balance in soils and biomass. - Policy-relevant synthesis: Regardless of definition, achieving NZ in Ireland’s AFOLU sector requires transformative land-use change: high afforestation rates, extensive rewetting of organic soils, and cattle destocking. Ambitious abatement moderates but does not replace these actions.

The study demonstrates that the AFOLU configurations compatible with national net zero targets depend strongly on how NZ is defined and how gases are aggregated. This directly addresses the research question by quantifying the sensitivity of feasible land-use and production mixes to ten alternative NZ definitions, including fairness-adjusted CH4 approaches and warming-equivalent metrics. While divergent NZ definitions create a broad and potentially confusing range of futures, the results reveal robust commonalities across all NZ-compliant scenarios: substantial expansion of forest cover, large-scale rewetting of organic soils, and reductions in cattle numbers. These actions are therefore high-priority regardless of the ultimate NZ definition adopted. Ambitious technical abatement of agricultural CH4 and N2O can significantly increase the feasibility space and reduce the scale of land-use change required, but cannot eliminate the need for structural shifts in land use and livestock production. The findings highlight difficult trade-offs—particularly between sustaining milk output and reducing suckler-beef production—that will shape rural livelihoods and export-based agrifood sectors. They also underscore fairness considerations in CH4 accounting and the risk of international leakage if production shifts to less efficient systems. For policy, this implies an urgent need to clarify NZ end-goals and metrics, prioritize no-regrets actions with long lead times (e.g., afforestation and peatland rewetting), and develop a just transition strategy and bioeconomy pathways to compensate farmers for foregone livestock production while supporting climate goals.

This work provides new evidence on how differing NZ definitions reshape the set of viable AFOLU futures, highlighting both the diversity of potential configurations and the consistent need for transformative land-use change in Ireland. Key contributions include: (i) systematic comparison of ten NZ definitions (including fairness-based CH4 targets, eGWP*, and long-term balances) applied to 3000 randomized national scenarios; (ii) quantification of success rates and associated land/production outcomes; and (iii) identification of common actions across NZ definitions (high afforestation, extensive peatland rewetting, and cattle destocking) that remain necessary even with ambitious technical abatement. The study emphasizes the urgency of implementing land-use changes with long response times, the importance of clear internationally aligned NZ definitions to guide policy and stakeholder action, and the opportunity to support a just transition through development of a circular bioeconomy (e.g., biomass for materials and energy with cascading use). Future research should further assess economic feasibility, social impacts, leakage risks, and interactions between land-based sinks and downstream bioeconomy innovations (including carbon storage longevity), as well as refine long-term flux dynamics and uncertainties beyond 2050.

Model-based limitations include: (i) scenario space constrained to positive land-use transformations on spared grassland (rewetting on organic soils, afforestation on mineral soils, or ungrazed grassland), excluding potentially adverse or alternative land-use changes; (ii) linear interpolation of trends to 2050 may influence metrics sensitive to emission trajectories (e.g., GWP*); (iii) post-2050 equilibrium assumption for AFOLU with static flux factors (except modeled forest growth-harvest cycles to 2100) increases uncertainty for long-term NZ definitions; (iv) animal numbers capped at 2021 levels and parameter ranges biased to concentrate scenarios near NZ boundaries; (v) abatement assumptions are speculative—30% uniform reduction is already ambitious, while upper-bound sensitivity (up to 100%) is illustrative rather than realistic; and (vi) broader economic feasibility, behavioral adoption, and supply-chain dynamics were not assessed. The study also notes uncertainty in long-term activity–flux relationships and interactions between land-based CO2 sinks and bioeconomy options (e.g., cascading wood use, BECCS) that could extend sink longevity.