Climate mitigation potential of natural climate solutions and clean energy on The Nature Conservancy properties in California, USA

best management practices. The Nature Conservancy, California chapter decided to restore a small area of wetland (176 ha) and convert corn to rice (1,671 ha) at Staten Island. This blend of two NCS activities across a portion of the property will account for 11,180 MTCO2e year-1 once fully implemented, which represents 25% of the potential reduction from cultivated to managed wetland (45,315 MTCO2e year-1) only on the property. Across all properties, seven had overlapping wetland restoration and other NCS activities. If only wetland restoration or agricultural NCS activities were implemented, this non-overlapping total represented 96% of the total including overlapping activities, indicating the overestimate had a small impact on the overall CO2e reduction potential. Discussion The CO2e reduction potential of restoration, agricultural management, and clean energy tran- sitions on all fee-owned properties (63,663 MTCO2e year –1) would not reach the goal of 72,000 MTCO2e year –1 set for 10% of The Nature Conservancy’s fee-owned properties in Cal- ifornia. One explanation is that the 2030 goal had a high sequestration rate (10 MTCO2e ha –1 year –1) compared to the mean values reported in the literature for cover cropping (1.50–3.78 MTCO2e ha –1 year –1) which represented almost half of the total area identified for NCS on fee properties. Another explanation is that new studies on sequestration rates were initiated in 2015 when the Cap-and-Trade market began funding NCS in California, and these studies had

lower sequestration rates for some NCS activities. In this study, tidal wetland restoration sequestration was less than in an earlier study in California published before the Cap-and- Trade market started (mean 14 and 21 MTCO2e ha –1 year –1, respectively) [3]. Additionally, the main land cover type is annual grassland on fee properties where only one feasible NCS activity applied, wetland restoration of former peatlands. If compost application became feasible for The Nature Conservancy or there were sequestration rates available for woodland restoration, both of which would apply to annual grasslands, it is likely that total CO2e reduction potential would be greater and closer to the goal. The elimination of compost application because of potential biodiversity impacts limited the CO2e reduction potential. Fig 4. Mean and range of annual CO2e reduction from potential NCS activities and ongoing projects. The dashed green line shows The Nature Conservancy’s annual reduction goal of 72,000 MTCO2e per year by 2030. Note the log-scale on the x-axis. https://doi.org/10.1371/journal.pone.0311195.g004 PLOS ONE Climate mitigation potential of natural climate solutions and clean energy: The nature conservancy, California PLOS ONE | https://doi.org/10.1371/journal.pone.0311195 October 21, 2024 9 / 14