Climate change, driven by increasing atmospheric greenhouse gas (GHG) concentrations, is a significant global challenge. Food systems contribute approximately one-third of human-induced GHG emissions, with the ruminant sector being a major source of methane (CH4) and nitrous oxide (N2O). Various mitigation strategies have been suggested, including carbon (C) sequestration in soils used for animal feed. Grasslands, with their higher belowground biomass allocation, tend to have higher soil organic carbon (SOC) stocks than croplands. However, soil C sequestration is typically temporary, with sequestration rates declining to zero as SOC reaches a new equilibrium. The cooling effect of soil C sequestration is not well-integrated into current GHG inventories. Existing studies often use a single value, the sum of annual GHG emissions minus CO2 removal via soil C sequestration, which oversimplifies the complex dynamic.

Many studies express climate impact in CO2-equivalents (CO2-eq) using global warming potentials (GWPs), implying equal integrated radiative forcing for different GHGs. However, GWPs mask the endpoint impact of emissions and are considered inappropriate for the goals of the Paris Agreement as they don't account for temporal differences between short and long-lived GHGs. For instance, CH4 has a much higher initial impact on radiative forcing than CO2 but a much shorter lifetime. GWP*, which relates the climate impact of a one-off CO2 release to a change in CH4 emission rates, has also been criticized for its reliance on arbitrary baseline emissions. A climate model provides a more accurate method to assess the cumulative climate impacts of GHG fluxes over time, accounting for differences between short-lived GHG emissions and (theoretically) long-lived but finite soil C sequestration.

This study uses a simple climate model to assess the cumulative climate impacts of GHG fluxes over time. The model was used to estimate the required soil C sequestration to offset CH4 and N2O emissions from ruminant systems globally. The climate impact of different GHGs was analyzed, considering both pulse emissions and continuous flows. The conversion ratios between the climate impact of a one-off pulse of CO2 and continuous emissions of CH4 or N2O were calculated. The study also considered the number of cattle that could be supported on a given area of grassland while offsetting enteric CH4 emissions via soil C sequestration. Data on GHG emissions from the global ruminant sector were obtained from the Global Livestock Environmental Assessment Model (GLEAM 3.0). Data on SOC stocks in managed grasslands were derived from the Global Soil Sequestration Potential Map (GSOCseq V1.1). Three alternative scenarios were tested to represent the long-term process of soil C sequestration, comparing the climate benefits of soil C sequestration with continuous CH4 emissions.

The analysis reveals significant differences in the climate impact of CO2, CH4, and N2O, both for pulse emissions and continuous flows. A one-off sequestration of one tonne of C offsets the radiative forcing of a continuous emission of 0.99 kg CH4 per year or 0.1 kg N2O per year over 100 years. Globally, a 135-gigatonne increase in SOC stock in grasslands would be needed to offset current CH4 and N2O emissions from ruminants, almost triple the current global SOC stock. Regional variations were also found; SOC stocks would need increases ranging from 25% to 2000% to offset regional emissions. The study also showed that soil C sequestration can only possibly cancel out a continuous flow of enteric CH4 emissions in rather extensive systems (mostly with a cattle density lower than one head per hectare).

The findings highlight the limitations of relying solely on soil C sequestration in grasslands to offset climate warming from ruminant systems. Reducing GHG emissions is imperative, focusing on both CO2 reduction and mitigation of CH4 and N2O emissions from ruminant farming (reducing livestock numbers, improving feed, animal health, inhibiting methanogenesis, and managing manure). Simultaneously, efforts are needed to restore degraded grasslands, preserve current C stocks, and increase stocks where possible. The authors emphasize the need to consider trade-offs between increasing SOC stocks and other soil functions (e.g., yield, N2O emissions).

This study demonstrates that current reliance on soil C sequestration in grasslands to offset climate warming caused by ruminant systems is overly optimistic. While preserving and enhancing SOC stocks in grasslands remains crucial, emission reductions must be prioritized. Future research should focus on refining models to better capture the dynamic nature of soil C sequestration and its interactions with other soil functions. Furthermore, exploring and implementing sustainable practices that effectively reduce GHG emissions from the ruminant sector are essential for climate neutrality.

The study utilizes a 100-year timeframe, favoring the positive impact of C sequestration. Yearly GHG emissions from ruminants were assumed constant, while global animal numbers are projected to increase. Uncertainties in SOC stock and sequestration data exist due to methodological choices. The translation of soil C sequestration into a one-off pulse of CO2 at year one neglects the long-term nature of the process. The analysis was based on a simple linearized model, which might not accurately reflect global aggregated values. However, testing with a reduced-complexity climate model supports the main conclusion.