The need for “apples-to-apples” comparisons of carbon dioxide source and sink estimates



The global land-based carbon dioxide (CO2) sink can be derived from the difference between fossil fuel emissions and the sum of estimated increases of CO2 in the atmosphere and in the ocean [Houghton, 2010]. For the purposes of developing policy to limit CO2 emissions, it is necessary to refine scientific understanding of the land CO2 flux in terms of its spatial and temporal patterns, as well as the underlying drivers. Net ecosystem exchange (NEE) is the commonly used measure of the land flux and is defined as the net vertical exchange of CO2 between a specified horizontal surface and the atmosphere above it over a given period of time. NEE estimates are reported from the atmospheric perspective, such that a positive value represents emissions (a land source) and a negative value represents removals (a land sink). This term represents a seemingly simple concept, and it was given a clear definition as one of the key carbon cycle variables by Chapin et al. [2006]. However, considerable confusion still arises around its usage, and the ambiguity is traceable primarily to the suite of different approaches used to estimate it.