Converging science and policy-based greenhouse gas budgets for North America for the Second Regional Carbon Cycle and Processes Study (RECCAP-2)

Abstract:

Sustained increases in atmospheric carbon dioxide, methane, and nitrous oxide concentrations from human activities are contributing to climate change and driving changes in the Earth system. Comprehensive greenhouse budgets that account for sources and sinks across a range of anthropogenic and natural sectors and their changes over time have focused mainly on individual gases, i.e., the US State of the Carbon Cycle Report, and the Global Carbon Project Regional Carbon Cycle and Processes study (RECCAP-1). In 2017, a second RECCAP activity, RECCAP-2, was initiated to provide a basis for a multi-greenhouse gas accounting activity for 10 key land regions and 5 ocean regions, and covering the most recent decade. Here we discuss the strategy and motivating science questions that are being used to develop the budgets for North America, which includes Canada, the United States, Mexico, Central America and the Caribbean. Three independent approaches are used, referred to as ‘top-down’, ‘bottom-up’, and ‘data-driven’, with the differences between the three approaches used to assess confidence in the budget and to identify sources of uncertainty. Reconciling top-down and bottom up estimates, in part, relies on how lateral fluxes are treated for trade and aquatic flows of carbon, which affects the interpretation of the carbon dioxide and methane budgets. In addition, the reporting of land use and land cover change fluxes from dynamic global vegetation models and for country-level reporting have important differences in definitions that contribute to discrepancies. Acknowledging these uncertainties via Tier 1 to Tier 3 styled approaches will contribute to a better understanding of how science-based accounting can inform policy-based requirements.