Towards robust regional estimates of carbon sources and sinks using atmospheric transport models

Abstract

Characterizing the sources and sinks of atmospheric CO2 has emerged as one of the most important problems facing the scientific and policy aspects of future atmospheric CO2 levels. Distinct from measurement of carbon stocks and flows within the biosphere and oceans, information about regional carbon sources and sinks can be inferred from variations in observed atmospheric CO2 concentrations through tracer transport inversion. However, inverse estimates of regional carbon fluxes over the last decade differ due to a number of factors including the transport model used. The impact of the transport model differences on inversion estimates has been difficult to assess since studies to date have employed one or few models. The TransCom 3 international experiment was constructed to explore the reasons why inverse estimates have differed with particular emphasis on the atmospheric transport aspect of the inverse problem. With nearly every active CO2 inverse modeling group in the world, inverse estimates were generated for long-term annual mean, seasonal, and interannual carbon exchange for sub-continental sized regions in the oceans and land. In the course of characterizing the estimation errors due to the many sensitivities of the carbon inverse problem, the model mean central estimates have emerged as surprisingly robust. At every opportunity, interpretation of the control results are made, connecting them to their biogeochemical implications.