Towards better-constrained assessments of the carbon balance of North America in the 21st Century: a comparison of recent model and inventory-based estimates

The North American C sink is generally considered to account for a large, but highly uncertain, portion of the northern extra-tropical land based sink, with estimates ranging from 15% to 100%. This uncertainty is owing to a number of sources, including the limitations of the methodologies used to develop estimates of C stocks and flux, the lack of comprehensive and accurate data on key driving forces (particularly disturbance, land management and land-use change), and the incomplete knowledge of long-term ecosystem responses to these driving forces and their interactions. Here, we examine the ability of various modeling approaches to identify sources and sinks of carbon across the North American continent by comparing model estimates with those based on analysis of available national forest and agricultural inventories for Canada, the U.S. and Mexico. For North America, inventory-based estimates of C stocks and flux in the early 21st Century (2000 - 2006) are being collected by either political state units in the case of the United States and Mexico, or the Kyoto Protocol reporting units for Canada. Flux estimates from more than 20 forward- and inverse- based models have been collected for the Regional / Continental Interim Synthesis activity under the North American Carbon Program, and these estimates have been processed to allow comparison at the spatial and temporal scales of the inventories. Preliminary analysis of the inventory data suggest that Canada’s Managed Forest Area acted as a net sink of atmospheric CO2 on the order of 46 TgC yr-1 from 2000 to 2006. This estimate includes the release of 26 TgC yr-1 from forest fires, while an additional 50 TgC yr-1 was removed from the forest as harvested products over this time period. In the U.S., inventory data indicate net C stock gains of 167 TgC yr-1 in the forest sector and 17 TgC yr-1 in croplands from 2000 to 2005. Model estimates of net ecosystem exchange (NEE) for the continent range from -78 to -645 TgC yr-1 over the 2000 - 2006 time period, where negative NEE represents a net terrestrial sink for atmospheric CO2. In general, inverse models tend to suggest larger net sinks than do forward models. We will present detailed analyses comparing model and inventory estimates of the component fluxes and stock changes, the spatial and temporal patterns of C sources and sinks, and the contribution of the forest and agricultural sectors to the overall C balance. In addition, we will identify key data gaps and discuss sources of uncertainty that should be prioritized for efforts towards better-constrained assessments of the C balance of North America in the 21st Century.