The release of fossil fuel CO2 to the atmosphere by human activity has been implicated as a major cause of global climate change. The marine carbon uptake plays a crucial role in mitigating the effects of this disturbance to the climate system, as the oceans are currently holding roughly a third of the cumulative anthropogenic CO2 (Cant) emissions from the atmosphere. However, there are indications that the oceanic carbon sink may have changed during the past few decades. As there are remaining uncertainties to this question, identifying the oceanic carbon inventory and its variability is vital for understanding the global carbon cycle and how it might change over time. Robust estimates of the magnitude and variability of the storage and distribution of Cant in the ocean are therefore important for understanding the human impact on climate. In the CarboOcean project, a synthesis was conducted reviewing observational and modelling estimates of the storage and transport of Cant in the ocean. The CarboChange project is now taking these results further.
Estimating the storage of Cant in the ocean is a difficult task for a variety of reasons. First, the quantity of Cant is not directly measurable; it has to be inferred via indirect means. Second, the Cant signal in the ocean is small (of order a few percent at the most) compared to the (unknown) natural or preindustrial background distribution of carbon. A further complication is that carbon in the ocean participates in complex biogeochemistry reactions. Lastly, the Cant distribution in the ocean is rather heterogeneous. As a consequence, unlike the atmosphere, which is relatively well mixed and where observations extend back many thousands of years, the ocean is much more challenging in this regard.
In the on-going project, there has been a review of the methods for estimating the air-sea flux and interior transport of Cant quantities. However, the results are found to be highly dependent on modelled circulation. A compilation of records of Cant could provide a “best” estimate of the global ocean inventory of anthropogenic carbon, but includes a broad range of values suggesting that a combination of approaches is necessary in order to achieve a robust quantification of the ocean sink of anthropogenic CO2.
S. Khatiwala, T. Tanhua, S. Mikaloff Fletcher, M. Gerber, S. C. Doney, H. D. Graven, N. Gruber, G. A. McKinley, A. Murata, A. F. R´ıos, C. L. Sabine, and J. L. Sarmiento.
Global ocean storage of anthropogenic carbon.
Biogeosciences Discuss., 9, 8931–8988, 2012