Comparing process-based models with the inventory approach to predict CH4 emission of livestock enteric fermentation

Livestock production is the largest anthropogenic methane (CH4) source globally over the decades. Enteric fermentation of ruminants is responsible for the majority of global livestock CH4 emissions. Both inventory-based models (IvtMs) and process-based models (PcMs) are extensively used to assess the livestock CH4 emission dynamics. However, the model performance and the associated uncertainty have not been well quantified and understood, which greatly hamper our credibility of the regional and global CH4 emission predictions. In this study, we compared the CH4 emissions of livestock enteric fermentation (CH4,ef) predicted by multiple IvtMs and PcMs across Inner Mongolia, a region dominated by typical temperate grasslands that are widely used for animal husbandry. Twenty predictions from five IvtMs, and ten predations from five PcMs were explicitly calculated and compared for the reference year of 2006. The CH4,ef predicted from PcMs is lower than IvtMs and the variation between PcMs is substantially higher, i.e. 0.34 +/- 0.36 g CH4/m(2)yr and 0.78 +/- 0.14 g CH4/m(2)yr for PcMs and IvtMs, respectively. Different model strategies undertaken, i.e. the demand-oriented strategy for IvtMs and the resource-demand co-determined one for PcMs, cause the different predictions of CH4,ef between the two model groups. Using the results from IvtMs as the baseline scalar, we identified and benchmarked the performance of individual PcMs in the study region. The quantitative information provided can facilitate the understanding of key principles and processes of CH4,ef estimations, which will contribute to the future model development of global CH4 emission.