Effective forest-based climate change mitigation requires our best science

California's Cap-and-Trade Program, established in January 2013, sets a limit on the major sources of greenhouse gas emissions and allows a portion of excess emissions to be offset through purchase of credits for climate benefits accrued elsewhere. Reflecting the critical role of forests in climate regulation, the majority of these offsets are related to Improved Forest Management (IFM) following the U.S. Forest Project Protocol- the first legally enforceable offset program for existing forests. Soberingly, however, Badgley et al. (2021) show that this program has resulted in widespread over-crediting, the issuance of credits in excess of actual climate benefits, to the tune of 30 million t CO2 -eq, or 29.4% of the credits analyzed, valued at ~US$410 million. This stemmed from the use of coarse, regional averages of forest carbon (C) stocks in the calculation of project baselines (Fig. 1). Reliance on regional average C stocks is a core element of several commonly used forest C offsetting methodologies, implying that the type of problem identified by Badgley et al. could be more widespread. This should serve as a call to action for scientists and policymakers to create a robust framework for forest C offset accounting, based on the most current scientific information, that delivers real, verifiable climate benefit, and which is flexible enough to adapt as scientific data improves.