Commercial forest carbon protocol over-credit bias delimited by zero-threshold carbon accounting

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The validity of forest carbon offsets is increasingly called into question. However, despite the use of commercial forest carbon protocols (CFCPs) for more than two decades, claiming ∼566 MMtCO2e and a market value of ∼USD $15.7 billion, comparative analysis and quality assurance of CFCP methodology and offset results are limited. In this study, five widely used biometric-based CFCPs are characterized and compared with results of directly measured CO2 by eddy covariance, a meteorological method integrating vertical fluxes of forest and soil carbon and the only alternative non-biometric source of net forest carbon sequestration data available. We show that CFCPs share a structural feature delimiting forest carbon values by zero-threshold carbon accounting (gC m−2 y−1 or d−1 ≤ 0), confirming reported forest carbon uncertainty, elucidating the data gap across CFCPs, and emphasizing the need for urgent improvement. The CFCP pattern of values is in contrast to known natural emissions of global forest CO2 exchange that is based on direct measurement, obviating a fundamental biological constraint on net forest carbon storage (i.e., soil efflux, ecosystem respiration). Exclusion of forest CO2 sources to the atmosphere precludes net carbon accounting, resulting in unavoidable over-credit of CFCP project offsets. CFCP carbon results are significantly different from global forest CO2 net ecosystem exchange population results (FLUXNET2015 gC m−2) at the 95 to 99.99% confidence levels, inferring an annual median error of ∼247% (gC m−2), consistent with over-crediting. Given the urgency of reliably reducing CO2 emissions and achievement of net-zero and carbon neutral goals, direct CO2 measurement provides a quality-assured alternative method for commercial forest carbon products with the potential to harmonize global markets, catalyzing the role of forests in managing climate change through nature-based solutions.