Global Combustion CO2 Accounting Neglects the Origin of Traded Fossil Fuels.

Combustion CO2 emissions are commonly estimated by multiplying fuel consumption by the corresponding emission factors. However, national inventories may apply domestic factors to imported fuels, overlooking differences in fuel properties across extraction origins and creating discrepancies in emission estimates. Here, we trace fossil fuels to their countries of extraction and assign emission factors specific to the extraction country and fuel type. Using 2020 data for 150 countries and regions, we compare this origin-traced estimate with a nonorigin-traced control using the same energy-consumption data but combustion-country emission factors. Globally, the control estimate is 339.2 Mt of CO2 lower than the origin-traced estimate, equivalent to 1.0% of the origin-traced total, with coal accounting for 89.9% of the discrepancy. Although crude oil had the largest energy-based trade volume among primary fuels, oil-related fuels contributed less because their emission factors varied less across countries, particularly for refined petroleum products. Such discrepancies can be particularly consequential for highly import-dependent countries. For example, the national-level discrepancy exceeds 4% in India, which is a major fossil-fuel importer. Our origin-tracing approach can improve the accuracy of combustion CO2 estimates by assigning emission factors that better reflect the fuels actually consumed. This provides more robust emission information for assessing progress toward Nationally Determined Contributions.