Evaluation of California’s forest carbon offset projects: stocks, trends, and vulnerabilities

Abstract:

California’s Cap-and-Trade Program was launched in 2013 to help reduce carbon emissions and achieve carbon neutrality by 2045. A major sub-component of Cap-and-Trade is the Compliance Offset Program, where credits are issued to different categories of activities, such as improved forest management projects, for voluntarily increasing carbon sequestration in un-capped sectors. The forest carbon offset protocol relies on periodic, verified, allometric inventories of carbon stocks to measure performance. Remotely sensed spectral indices could provide a compelling, lower-cost alternative to quantify the benefits of forest offset projects, compared to their regional common practice. Here we systematically compare 40 offset project aboveground carbon inventories to remotely sensed records of forest carbon and vegetation properties, quantifying agreement and change over time. We also quantify the potential vulnerability of the project locations, based on historical disturbance patterns and future climate change risks.

We find some disagreement between ground-based inventories and eMapR biomass, a dataset derived from satellite imagery, field observations, and LiDAR. More than half (58%) of the project inventories reported steeper increases in carbon than the eMapR record. We also report a background increase in carbon in the projects’ overall regions, which can account for approximately 21% of the projects’ total increases throughout 2013-2017. However, 30% of these projects are not keeping up with this background trend. Further, these project areas have historically had fewer fire disturbances than their regional averages, but may be more vulnerable to future climate change based on a space-for-time-substitution random forest model. These background trends and vulnerabilities are not accounted for in the current protocol. This work highlights the utility of different geospatial datastreams for improved carbon offset monitoring to contribute towards achieving the state’s greenhouse gas emission reduction goals.