Methane emissions from rice paddies globally: A quantitative statistical review of controlling variables and modelling of emission factors

Greenhouse gas (GHG) modelling tools or the Intergovernmental Panel on Climate Change (IPCC) inventory methods are often used to identify suitable mitigation strategies for GHG emissions from rice, since measuring them in field is challenging and costly. Here we report an up-to-date quantitative review on methane (CH4) emission from rice paddies using information obtained from peer-review articles. Statistical analysis was conducted on the factors controlling CH4 emissions and a generalised additive model (GAM) was developed to estimate emission factors (EFs). Results showed that emissions were strongly linked to water regime, soil texture and organic amendment practices. Fields that were rainfed during the dry season or saturated emitted 70% and 56% that of continuously flooded fields, while applying straw off-season instead of within-season could decrease emissions by 48%. An independent dataset was used to evaluate the new model performance against existing models with the new model showing R2 values of 0.47 (n: 169), compared to 0.01-0.09 (n: 169) for the existing models. New baseline EFs was estimated at global, regional, and Country scale with result showing that using different pre-season water management when calculating baseline EFs at country level is vital in order to reflect the variation between tropical and temperate rice regions accurately. Our findings shows that the new model is more sensitive in capturing differences in management practices between tropical and temperate rice, and their impacts on CH4 emissions with baseline EF calculations accounting for these differences providing sound mitigation strategies.