Emergence of Anthropogenic Signals in the Ocean Carbon Cycle.

Abstract

Attribution of anthropogenically-forced trends in the climate system requires understanding when and how such signals will emerge from natural variability. We apply time-of-emergence diagnostics to a Large Ensemble of an Earth System Model, providing both a conceptual framework for interpreting the detectability of anthropogenic impacts in the ocean carbon cycle and observational sampling strategies required to achieve detection. We find emergence timescales ranging from under a decade to over a century, a consequence of the time-lag between chemical and radiative impacts of rising atmospheric CO on the ocean. Processes sensitive to carbonate-chemical changes emerge rapidly, such as impacts of acidification on the calcium-carbonate pump (10 years for the globally-integrated signal, 9-18 years regionally-integrated), and the invasion flux of anthropogenic CO into the ocean (14 globally, 13-26 regionally). Processes sensitive to the ocean's physical state, such as the soft-tissue pump, which depends on nutrients supplied through circulation, emerge decades later (23 globally, 27-85 regionally).