North America's net terrestrial carbon exchange with the atmosphere 1990–2009

Abstract

Scientific understanding of the global carbon cycle is required for developing national and international policy to mitigate fossil-fuel CO 2 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 CO 2 exchange for North America over the period (1990–2009). 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 America land surface was a sink for atmospheric CO 2, with a net transfer from atmosphere to land. Estimates ranged from −890 to −280 Tg C yr −1, where the atmospheric inversion estimate forms the lower bound of that range (a larger land-sink) and the inventory-based estimate the upper (a smaller land sink). 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 CO 2 in the atmosphere in the late 20th and early 21st century. The continent's CO 2 source to sink ratio for this time period was likely in the range of 4 : 1 to 3 : 1.