Unlocking Blue Carbon's Potential Requires Targeted Uncertainty Reductions

Shawn Doyle , Hoonshin Jung, Tim Carruthers

Coastal “blue” carbon ecosystems like swamps and marshes are among the most efficient natural carbon sinks on Earth. Consequently, restoring and conserving these wetlands is gaining interest from private and government stakeholders as a nature-based solution to offset greenhouse gas emissions and generate revenue through carbon credits. However, the financial viability of blue carbon crediting is currently challenged by major uncertainties in quantifying net greenhouse gas fluxes from these dynamic systems. To better understand the drivers of this uncertainty, we conducted a sensitivity analysis to identify the most influential parameters affecting net carbon flux estimates over 30 years. Our analysis examined parameters from sub-basin (~60,000 ha) to the coastal ecosystem (~3,000,000 ha) scale, considering parameters such as the fate of soil carbon from marsh erosion, methane and nitrous oxide emissions, and changes in plant productivity. We found that the relative importance of these parameters varied by wetland type as well as the spatial and temporal scale of analysis. This confirmed that applying broad assumptions or default values in carbon flux calculations result in highly uncertain estimates, which can often vary by over 100%. In fresh and intermediate salinity wetlands, carbon flux estimates were most sensitive to the high uncertainty in methane and nitrous oxide emission rates. In contrast, brackish and saline wetlands showed greater sensitivity to variability in annual plant growth. However, where wetlands were prone to erosion, the fate of eroded soil carbon stores became the dominant source of uncertainty in net carbon flux estimates, particularly over longer (20+ years) time scales. By identifying which parameters contribute most to uncertainty in coastal carbon flux estimates at different spatial and temporal scales, our study helps target research and monitoring efforts towards those that can provide the greatest reduction in uncertainty, moving blue carbon crediting projects towards financial viability.