Abstract
Carbon sequestration in grasslands has been proposed as an important means to offset greenhouse gas emissions from ruminant systems. To understand the potential and limitations of this strategy, we need to acknowledge that soil carbon sequestration is a time-limited benefit, and there are intrinsic differences between short- and long-lived greenhouse gases. Here, our analysis shows that one tonne of carbon sequestrated can offset radiative forcing of a continuous emission of 0.99 kg methane or 0.1 kg nitrous oxide per year over 100 years. About 135 gigatonnes of carbon is required to offset the continuous methane and nitrous oxide emissions from ruminant sector worldwide, nearly twice the current global carbon stock in managed grasslands. For various regions, grassland carbon stocks would need to increase by approximately 25% – 2,000%, indicating that solely relying on carbon sequestration in grasslands to offset warming effect of emissions from current ruminant systems is not feasible.
Generated Summary
This research investigates the potential of soil carbon sequestration in grasslands to offset greenhouse gas (GHG) emissions from ruminant systems. The study employs a climate model to assess the impact of soil carbon sequestration on the climate, considering the differences between short- and long-lived GHGs. The primary goal is to determine the feasibility of relying on soil carbon sequestration as a strategy to mitigate the warming effect of emissions from current ruminant systems. The study acknowledges that soil carbon sequestration provides time-limited benefits and the intrinsic differences between short and long-lived greenhouse gasses. The study assessed the climate impacts of different GHGs by examining pulse emissions and continuous emissions over time. The cumulative climate impacts were assessed by analyzing the required soil C-sequestration to cancel the CH4 and N2O emissions from ruminant systems globally. The research also incorporates conversion ratios to quantify how much CO2 sequestration is needed to offset the climate impact of continuous CH4 or N2O emissions across time.
Key Findings & Statistics
- One tonne of carbon sequestered can offset radiative forcing of 0.99 kg methane or 0.1 kg nitrous oxide per year over 100 years.
- Approximately 135 gigatonnes of carbon is required to offset the continuous methane and nitrous oxide emissions from the ruminant sector worldwide.
- Grassland carbon stocks would need to increase by approximately 25% – 2,000% in various regions.
- The RF of a pulse emission of one t of CO2, for example, decreases from 0.0017 in year one to 0.0007 nW m⁻² in year 100, and 0.0005 nWm⁻² in year 500.
- CH4 has a much higher impact on RF than CO2.
- The RF of one t of CH4, as a result of its relative short perturbation lifetime, drops drastically within a few decades (i.e., from 0.2 nW m⁻² in year one to 0.0003 nW m⁻² in year 100 and 0.00005 nW m⁻² in year 500).
- N₂O has a relative longer perturbation lifetime (109 years) and is also more potent compared to CO2 and CH4.
- The RF associated with a pulse emission of one t of N2O gradually reduces over time and lasts for more than half a millennium (i.e., 0.36 nW m⁻² in year one and 0.005 nW m⁻² in year 500).
- In a 100-year timeframe, the amount of CO2 that needs to be stored to compensate the climate impact of a continuous flow of CH4 (one t per year) equals about 3.7 kt for RF and 3.5 kt for global surface temperature change.
- A one-off sequestration of one t of C would offset the RF of a continuous emission of 0.99 kg CH4 per year or the global temperature change of 1.05 kg CH4 per year over 100 years.
- A one-off sequestration of one t of C would offset the RF of a continuous emission of 0.1 kg N₂O per year over 100 years or 0.04 kg N2O per year over 500 years.
- The (maximum) soil C-sequestration potential among grasslands (0-50 t ha⁻¹) was estimated.
- One hectare of grassland potentially sequestering an additional 50 t SOC can compensate enteric CH4 emissions of about 1.25 heads of cattle.
- The global ruminant sector released approximately 110 megatons of CH4 and 2.4 megatons of N₂O annually.
- To offset these emissions, we need to increase SOC stock in global grassland by 135 gigatonnes (Gt) of C.
- The required SOC stock increase to offset CH4 and N2O emissions from ruminant systems varies across global regions.
Other Important Findings
- The study found that the climate impact of a continuous flow of CH4 stabilizes while that of N₂O shows a trend to stabilize.
- The research found that the current SOC stock in global grasslands is nearly double the current global carbon stock in managed grasslands.
- It was found that soil C-sequestration in grasslands can only possibly cancel out a continuous flow of enteric CH4 emissions in rather extensive systems.
- The study highlights the importance of reducing emissions and increasing SOC stocks to mitigate climate change.
- The conversion ratios of the climate impact of a one-off pulse of CO2 and a continuous emission of CH4 or N₂O over 500 years were displayed.
Limitations Noted in the Document
- The study uses a 100-year timeframe, which favors the positive impact of C-sequestration over the negative impact of GHG emissions.
- The study assumes constant yearly GHG emissions from ruminants, while global animal numbers are projected to increase.
- Uncertainties exist regarding the current SOC stock and sequestration, as well as the definition and extent of grasslands.
- The analysis uses a simple linearized model, which may not accurately reflect global aggregated values.
Conclusion
The study’s findings underscore the limitations of solely relying on soil carbon sequestration in grasslands to offset the climate impacts of ruminant systems. The analysis reveals that the amount of carbon sequestration needed to counteract methane and nitrous oxide emissions from the ruminant sector is substantial, often requiring increases in soil carbon stocks that are unrealistic given current conditions. The research suggests that strategies to reduce emissions are crucial. This includes reducing the sources of GHGs and increasing soil carbon stocks. The study also highlights that reducing emissions from ruminant farming requires the implementation of several strategies, including decreasing livestock numbers, improving feed quality, and enhancing animal health. The study supports the claim that ruminant systems can have a negative annual GHG balance via soil C-sequestration is overly optimistic and could be misleading, particularly at global or regional levels. The analysis of this study was built on a simple linearized model, which was initially developed for small perturbations and may not necessarily reflect global aggregated values accurately as in our results. The study’s findings underscore the need for a balanced approach that combines emission reduction strategies with efforts to enhance soil carbon sequestration, while acknowledging that the latter alone is insufficient. Therefore, the study recommends an integrated approach to manage grassland, reduce emissions, and explore the benefits of soil carbon sequestration. It emphasizes that the current animal densities may need to be reduced to fully compensate for climate impacts by means of soil C-sequestration in grasslands.