Risk to rely on soil carbon sequestration to offset global ruminant emissions

The study addresses whether increasing soil organic carbon (SOC) in grasslands can offset greenhouse gas emissions from global ruminant systems. Ruminants are a major source of anthropogenic methane (CH4) and nitrous oxide (N2O). While grasslands can accumulate SOC, sequestration is generally finite and slows as soils approach a new equilibrium. Conventional accounting often uses CO2-equivalents based on global warming potentials (GWPs), which mask temporal differences among gases and the time-limited nature of sequestration. The authors aim to develop an approach that captures the distinct climate dynamics of short-lived (CH4) versus long-lived (CO2) gases and finite SOC sequestration, in order to quantify realistically the potential for grassland SOC to mitigate ruminant emissions and inform pathways toward climate neutrality.

Prior work often calculates single CO2-equivalent values using GWPs, sometimes suggesting that soil carbon sequestration in grasslands can offset grazing system emissions based on short-term data. However, sequestration is finite and depends on the soil’s distance from equilibrium. GWPs equate integrated radiative forcing for pulse emissions over a chosen time horizon but obscure the time profile of warming and differences between short- and long-lived gases. GWP* was proposed to better reflect the climate impact of changing CH4 emission rates, but it depends on baseline choices that can yield perceived unfairness. Existing studies rarely incorporate soil C-sequestration into ruminant GHG accounting using climate models that resolve temporal dynamics and historical warming. Grasslands typically have higher SOC than croplands due to belowground allocation, yet sequestration rates decline to zero as new equilibria are reached, and long-term SOC increases are uncertain and regionally variable.

- Climate model: The authors used a simple linearized climate model (Persson & Johansson Version 2.0), parameterized consistently with IPCC AR6 radiative efficiencies and perturbation lifetimes, to simulate radiative forcing (RF) and global mean surface temperature change from both pulse and continuous emissions of CO2, CH4, and N2O. The model captures indirect effects and updated carbon-cycle responses for CH4 and N2O. - Equivalency approach: They quantified conversion ratios that equate the climate impact (RF or temperature change) of a continuous emission flow of CH4 or N2O to a one-off pulse of CO2 removal (representing cumulative SOC increase). Results were generated over up to 500 years; for application, a 100-year horizon was used, consistent with soil equilibration and common practice. - Representation of soil C sequestration: Long-term SOC sequestration was represented as a one-off CO2 removal at year one to facilitate application. Sensitivity analyses tested alternative sequestration timing scenarios, showing broadly consistent results. - Application to ruminant systems: The approach was applied first to offset continuous CH4 emissions; only residual SOC capacity, if any, was then compared to N2O emissions. - Data sources: Global ruminant CH4 and N2O emissions by species and region were drawn from FAO’s GLEAM 3.0 (2015 baseline). Current SOC stocks in managed grasslands and their spatial extent were taken from FAO’s GSOCseq v1.1 (0–30 cm topsoil, 2020 reference). For cattle density illustrations, enteric CH4 emission factors were from IPCC 2019 Tier 1 defaults (rounded display range 40–160 kg head−1 yr−1), and grassland soil C sequestration potentials (5–50 t C ha−1) were derived from IPCC stock change factors across selected climate zones and soil types. - Cross-check: A reduced-complexity climate model (MAGICC7) was used in a supplementary comparison to assess uncertainties from model linearization. - Time horizons: 500-year simulations illustrated gas-specific dynamics; 100-year horizon was adopted for offset calculations given soil equilibration and near-term policy relevance.

- Gas dynamics: Continuous CO2 emissions drive ever-increasing RF and temperature, while continuous CH4 (lifetime ~11.8 years) and N2O (lifetime ~109 years) stabilize as atmospheric removal balances emissions after decades to centuries. - Conversion ratios (100-year horizon): A one-off sequestration of 1 tonne of carbon offsets the RF of a continuous emission of approximately 0.99 kg CH4 per year, or approximately 0.1 kg N2O per year over 100 years. For N2O, the required CO2 removal to offset continuous emissions grows with time (e.g., equivalence increases from ~35 kt CO2 at year 100 to ~87 kt at year 500 per 1 t yr−1 N2O), reflecting its longer lifetime. - Global requirement: Offsetting current global ruminant CH4 (~110 Mt yr−1) and N2O (~2.4 Mt yr−1) emissions requires an SOC stock increase of about 135 Gt C. This is nearly double the current SOC stock in managed grasslands (estimated at ~78 Gt C) and would require almost tripling managed grassland SOC stocks. - Regional gaps: All regions show large gaps; required SOC increases range from ~25% up to nearly 2,000% of current managed grassland SOC. Example: South Asia requires ~34 Gt additional SOC, equivalent to ~563 t C ha−1, far exceeding current stocks (~28 t C ha−1). - Cattle density illustration: Even optimistic sequestration (50 t C ha−1) offsets only low enteric CH4 intensities; 1 ha at 50 t C ha−1 can compensate ~1.25 cattle emitting 40 kg CH4 yr−1. Typical real-world cattle densities are often higher, and achieving 50 t C ha−1 additional SOC is itself challenging. - Implication: Sole reliance on grassland SOC sequestration to cancel the warming effects of present-day ruminant emissions is infeasible globally and in all examined regions.

By explicitly modeling the temporal climate response of short-lived versus long-lived gases and finite SOC sequestration, the study shows that the stabilizing impact of continuous CH4 and N2O emissions cannot be durably neutralized by one-off SOC increases at the scales required. The findings directly address the research question: while SOC sequestration contributes to mitigation, it cannot reasonably offset the ongoing warming impact of current ruminant emissions at global or regional scales. This underscores the need to prioritize emission reductions—especially phasing out fossil CO2 and reducing CH4 and N2O from livestock—alongside protecting and enhancing SOC where feasible. The regional analysis highlights where preserving existing high SOC stocks is crucial and where sequestration potentials are limited. Policy strategies aiming for climate neutrality in livestock must recognize the finite, time-limited nature of SOC sinks, integrate realistic sequestration estimates, and avoid over-reliance on soil carbon to balance persistent emission sources.

The paper introduces a climate-model-based approach to embed finite soil carbon sequestration into GHG accounting that differentiates short- and long-lived gas dynamics. Applying this framework shows that approximately 135 Gt C would be needed to offset global ruminant CH4 and N2O emissions—an amount far exceeding current managed grassland SOC stocks and realistic sequestration potentials. Therefore, grassland SOC alone cannot deliver climate neutrality for the ruminant sector. The main contributions are: (1) quantifying conversion ratios linking continuous CH4/N2O emissions to one-off CO2 removals; (2) providing global and regional estimates of required SOC increases; and (3) illustrating practical limits via cattle density scenarios. Future work should improve SOC datasets and MRV, refine sequestration dynamics and permanence assumptions, explore region-specific mitigation portfolios combining emission reductions with SOC preservation, and further test results with more complex climate models.

- Time horizon: Using a 100-year timeframe favors the apparent benefit of one-off CO2 removals relative to still-increasing CH4/N2O impacts; different horizons alter equivalences. - Emission trajectories: Assuming constant ruminant emissions likely underestimates future gaps if livestock numbers and emissions increase. - SOC data and definitions: Current SOC stock estimates and managed grassland extents are uncertain and sensitive to choices of land definitions, soil depth, and datasets. - Sequestration dynamics and permanence: SOC sequestration is dynamic and typically finite; crediting temporary storage and potential reversals is challenging. Representing long-term sequestration as a year-1 pulse simplifies timing; sensitivity tests suggest robustness but detailed dynamics remain uncertain. - Model structure: The primary model is a linearized simple climate model suited to small perturbations; although a MAGICC7 comparison supports the main conclusions, global aggregate results may still carry model-form uncertainty. - Practical potential: The assumed maximum SOC increases (e.g., up to 50 t C ha−1) are difficult to achieve broadly; management trade-offs (e.g., potential N2O increases, biodiversity impacts) can limit realizable sequestration.