The Missing "Missing Sink"

Abstract

To assess CO2 fluxes of the terrestrial biosphere, ground-based inventories, flux measurements and bottom-up modeling approaches are used. In most cases inventory-based approaches are not able to produce a full carbon account (FCA). The FCA refers to a carbon budget that is complete, encompasses all components, and is applied continuously in time. Atmospheric inversion modeling implicitly measures the sum of all fluxes, meaning a FCA. Eddy-covariance measurements have huge variations and are difficult to scale up to regional and decadal levels. Bookkeeping up to more complex process-based models rely on land-use change estimates over time, which have large uncertainties. To overcome the accounting gap between top-down and bottom-up measurements, the IPCC introduced the terrestrial "missing sink" concept by taking long-term land-use changes into account to further break down the global carbon budget. IIASA has developed a bottom-up FCA approach that breaks down the terrestrial carbon balance of Russia for 1990 (1988–1992) resulting in a sink. This was then combined with the terrestrial sink strength of the extra-tropical Northern Hemisphere (approximately > 30°N) determined via top-down atmospheric inversion. Using this approach, the remainder, the terrestrial sink strength of the extra-tropical Northern Hemisphere without Russia, could then be determined with a relative uncertainty that is smaller (i.e., < 100%) than the uncertainties exhibited by inverse models. From the analysis it can be concluded that the "missing sink" issue can be reduced to an issue of relevant accounting due to the fact that the combined top-down/bottom-up approach does not identify any missing sink. Key words: carbon balance, flux, missing sink, inverse modeling, inventory approaches, full carbon accounting, top-down/bottom-up approaches, terrestrial ecosystems, Russia