UC Irvine

Current standard methods result in significant discrepancies in carbon offset accounting compared to approaches based on representative community based subsamples, which provide more realistic assessments at reasonable cost. Perhaps more critically, neither of the currently approved methods incorporates uncertainties inherent in estimates of emission factors or non-renewable fuel usage (fNRB). Since emission factors and fNRB contribute 25% and 47%, respectively, to the overall uncertainty in offset estimates for Purépecha communities in Mexico, exclusion of this uncertainty is a critical omission. When the recommended uncertainty for default emission factors and the uncertainty associated with regional estimates of fNRB are included the lower 95% confidence intervals of both Clean Development Mechanism and Gold Standard methods exceed the total amount of carbon saved, which would result in zero marketable carbon savings if approaches recommended by the IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories, or Land use, Land-Use Change and Forestry (LULUCF) are to be followed. In contrast, for the same communities, methods using representative subsamples of emission factors and fuel consumption, combined with community-level fNRB estimates, result in significant carbon offsets with a lower 95% confidence interval of 2.3 tCO2e home − 1 year − 1. Given the misleading estimates, revision of the currently approved methodologies to provide robust estimates of carbon offsets is strongly recommended.