Using remote sensing to quantify the additional climate benefits of California forest carbon offset projects

Nature-based climate solutions are a vital component of many climate
mitigation strategies, including California’s, which aims to achieve
carbon neutrality by 2045. Most carbon offsets in California’s
cap-and-trade program come from improved forest management projects
(IFMs). Since 2012, various landowners have set up IFMs following the
California Air Resources Board’s IFM protocol. As many of these projects
approach their tenth year, we now have the opportunity to assess their
effectiveness, identify best practices, and suggest improvements toward
future protocol revisions. In this study, we used remote sensing-based
datasets to evaluate the carbon trends and harvest histories of 37 IFMs in
California. Despite some current limitations and biases, these datasets
can be used to quantify carbon accumulation and harvest rates in offset
project lands relative to nearby similar “control” lands before and after
the projects began. Five lines of evidence suggest that the carbon
accumulated in offset projects to date has generally not been additional
to what might have otherwise occurred: (1) most forests in northwestern
California have been accumulating carbon since at least the mid-1980s and
continue to accumulate carbon, whether enrolled in offset projects or not;
(2) harvest rates were high in large timber company project lands before
IFM initiation, suggesting they are earning carbon credits for forests in
recovery; (3) projects are often located on lands with higher densities of
low-timber-value species; (4) carbon accumulation rates have not yet
increased on lands that enroll as offset projects, relative to their
pre-enrollment levels; and (5) harvest rates have not decreased on most
project lands since offset project initiation. These patterns suggest that
the current protocol should be improved to robustly measure and reward
additionality. In general, our framework of geospatial analyses offers an
important and independent means to evaluate the effectiveness of the
carbon offsets program, especially as these data products continue
improving and as offsets receive attention as a climate mitigation
strategy.