Hidden Concentration in Voluntary Carbon Markets: Registry Dependence, Vintage Exposure, and Financial Due Diligence

Voluntary carbon markets are often described as diversified because they contain thousands of projects, technologies, and jurisdictions. This study examines whether that visible variety corresponds to effective diversification once credit volumes and vintage exposure are considered. Using the Voluntary Registry Offsets Database, version 2026-06, we analyze 11,468 projects and 44,106 nonzero project-year records covering 2.655 billion tonnes of carbon dioxide equivalent (tCO2e) issued across six registries. The database includes 10,785 voluntary-only projects and 683 projects linked to regulatory programs. The empirical strategy combines market shares, raw and normalized Herfindahl–Hirschman indices, effective numbers of categories, vintage-weighted inventory measures, non-parametric tests, and fractional logit models with heteroskedasticity-consistent (HC3) robust inference. Registry concentration was high for issuance and positive remaining inventory, whereas detailed project-type concentration was low. The three largest registries accounted for 88.6% of issuance, and 46.8% of positive remaining inventory was more than five years old. Project age since first issuance was positively associated with the accumulated retirement share, and a quadratic specification suggested a diminishing association, with limited evidence at advanced ages. The findings identify institutional and temporal dependencies that can inform accounting controls and financial due diligence. They do not independently establish liquidity, fair value, registry performance, or environmental integrity.