North American terrestrial CO uptake largely offset by CH and NO emissions: toward a full accounting of the greenhouse gas budget.

The terrestrial ecosystems of North America have been identified as a sink of atmospheric CO though there is no consensus on the magnitude. However, the emissions of non-CO greenhouse gases (CH and NO) may offset or even overturn the climate cooling effect induced by the CO sink. Using a coupled biogeochemical model, in this study, we have estimated the combined global warming potentials (GWP) of CO, CH and NO fluxes in North American terrestrial ecosystems and quantified the relative contributions of environmental factors to the GWP changes during 1979-2010. The uncertainty range for contemporary global warming potential has been quantified by synthesizing the existing estimates from inventory, forward modeling, and inverse modeling approaches. Our "best estimate" of net GWP for CO, CH and NO fluxes was -0.50 ± 0.27 Pg CO eq/year (1 Pg = 10 g) in North American terrestrial ecosystems during 2001-2010. The emissions of CH and NO from terrestrial ecosystems had offset about two thirds (73 %±14 %) of the land CO sink in the North American continent, showing large differences across the three countries, with offset ratios of 57 % ± 8 % in US, 83 % ± 17 % in Canada and 329 % ± 119 % in Mexico. Climate change and elevated tropospheric ozone concentration have contributed the most to GWP increase, while elevated atmospheric CO concentration have contributed the most to GWP reduction. Extreme drought events over certain periods could result in a positive GWP. By integrating the existing estimates, we have found a wide range of uncertainty for the combined GWP. From both climate change science and policy perspectives, it is necessary to integrate ground and satellite observations with models for a more accurate accounting of these three greenhouse gases in North America.