Forests and land use represent a substantial portion of the world’s carbon budget; inventories of associated emissions and removals are integral to climate policy. However, the degree to which existing inventory guidelines reflect current scientific evidence has not been evaluated for key carbon stocks such as forest soils. Here, we compare forest-soil inventory guidelines to available scientific data, identifying key differences and informing refinements to inventory approaches.
Most countries report greenhouse gas emissions and removals using Intergovernmental Panel on Climate Change (IPCC) methods. These methods allow countries to measure their emissions (e.g., from the transportation sector) as well as sinks (for example, through forestry practices). Inventory guidelines provide default values for forest soil carbon stocks, which embody net changes in both emissions and removals. Forest carbon merits attention due to its potentially influential uncertainties, magnitude of stocks, and relevance to current policy. How do IPCC guidelines compare to available data? We use a two-pronged approach to assess forest carbon. First, we compare IPCC default inventory values with large-scale soil core data containing 100,000 profiles globally. Second, we incorporate a modelled dataset, developed by the International Soil Reference and Information Center using machine learning techniques, to IPCC guidelines for additional comparison.
Results/Conclusions
In consequential ways, current inventory guidelines for forest soils differ from the state of the science. Our first approach, based on crosscutting empirical soil-carbon data, reveals discrepancies between these data and policy guidelines, particularly the range of possible carbon stocks compared to singular IPCC values. Our second approach, using modelled data, provides a more comprehensive global comparison to the guidelines. We find many instances in which modelled predictions differ from IPCC guidelines, in the case of varying climate regions and soil types. Both approaches emphasize opportunities to refine greenhouse gas inventory methodologies.
Finally, our research reveals underlying methodological challenges for forest soil carbon reporting. IPCC guidelines include categories that may be mischaracterized, leading to potentially inaccurate carbon reporting. Beyond these methods, the default reference values themselves carry error uncertainties that might hinder the effectiveness of forest-based mitigation efforts. In light of these challenges, we provide recommendations for improving these guidelines. Our research emphasizes the importance of grounding policy guidelines in available evidence, particularly relating to the contribution of forests in the global carbon budget. Robust national inventory reports are crucial to the success of international climate actions, such as the Paris Agreement, as well as national and regional efforts.