1Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA
2Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania, USA
3Canadian Forest Service, Natural Resources Canada, Victoria, British Columbia, Canada
4School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Arizona, USA
5El Colegio de la Frontera Sur, Unidad Campeche, Campeche, Mexico
6US Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska, Fairbanks, Alaska, USA
7Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware, USA
8Joint Global Change Research Institute, Pacific Northwest National Laboratory, College Park, Maryland, USA
9Northern Research Station, USDA Forest Service, Saint Paul, Minnesota, USA
Received: 29 May 2014 – Discussion started: 17 Jul 2014
Abstract. Scientific understanding of the global carbon cycle is required for developing national and international policy to mitigate fossil fuel CO2 emissions by managing terrestrial carbon uptake. Toward that understanding and as a contribution to the REgional Carbon Cycle Assessment and Processes (RECCAP) project, this paper provides a synthesis of net land–atmosphere CO2 exchange for North America (Canada, United States, and Mexico) over the period 1990–2009. Only CO2 is considered, not methane or other greenhouse gases. This synthesis is based on results from three different methods: atmospheric inversion, inventory-based methods and terrestrial biosphere modeling. All methods indicate that the North American land surface was a sink for atmospheric CO2, with a net transfer from atmosphere to land. Estimates ranged from −890 to −280 Tg C yr−1, where the mean of atmospheric inversion estimates forms the lower bound of that range (a larger land sink) and the inventory-based estimate using the production approach the upper (a smaller land sink). This relatively large range is due in part to differences in how the approaches represent trade, fire and other disturbances and which ecosystems they include. Integrating across estimates, "best" estimates (i.e., measures of central tendency) are −472 ± 281 Tg C yr−1 based on the mean and standard deviation of the distribution and −360 Tg C yr−1 (with an interquartile range of −496 to −337) based on the median. Considering both the fossil fuel emissions source and the land sink, our analysis shows that North America was, however, a net contributor to the growth of CO2 in the atmosphere in the late 20th and early 21st century. With North America's mean annual fossil fuel CO2 emissions for the period 1990–2009 equal to 1720 Tg C yr−1 and assuming the estimate of −472 Tg C yr−1 as an approximation of the true terrestrial CO2 sink, the continent's source : sink ratio for this time period was 1720:472, or nearly 4:1.
Revised: 25 Nov 2014 – Accepted: 09 Dec 2014 – Published: 21 Jan 2015
King, A. W., Andres, R. J., Davis, K. J., Hafer, M., Hayes, D. J., Huntzinger, D. N., de Jong, B., Kurz, W. A., McGuire, A. D., Vargas, R., Wei, Y., West, T. O., and Woodall, C. W.: North America's net terrestrial CO2 exchange with the atmosphere 1990–2009, Biogeosciences, 12, 399-414, doi:10.5194/bg-12-399-2015, 2015.