Sensitivities of marine carbon fluxes to ocean change

Abstract

Throughout Earth's history, the oceans have played a dominant role in the climate system through the storage and transport of heat and the exchange of water and climate-relevant gases with the atmosphere. The ocean's heat capacity is Ϸ1,000 times larger than that of the atmosphere, its content of reactive carbon more than 60 times larger. Through a variety of physical, chemical, and biological processes, the ocean acts as a driver of climate variability on time scales ranging from seasonal to interannual to decadal to glacial-interglacial. The same processes will also be involved in future responses of the ocean to global change. Here we assess the responses of the seawater carbonate system and of the ocean's physical and biological carbon pumps to (i) ocean warming and the associated changes in vertical mixing and overturning circulation, and (ii) ocean acidification and carbonation. Our analysis underscores that many of these responses have the potential for significant feedback to the climate system. Because several of the underlying processes are interlinked and nonlinear, the sign and magnitude of the ocean's carbon cycle feedback to climate change is yet unknown. Understanding these processes and their sensitivities to global change will be crucial to our ability to project future climate change.climate change ͉ marine carbon cycle ͉ ocean acidification ͉ ocean warming T he ocean is presently undergoing major changes. Over the past 50 years, the ocean has stored 20 times more heat than the atmosphere (1). The Arctic Ocean surface layers have recently experienced a pronounced freshening (2), and although there is still considerable uncertainty in current observational estimates (3), climate models run under increasing atmospheric CO 2 levels essentially all predict a slowdown of the Atlantic meridional overturning circulation (MOC), which is part of the global thermohaline circulation (4). Since preindustrial times, the ocean has also taken up Ϸ50% of fossil fuel CO 2 , which has already led to substantial changes in its chemical properties (5).Carbon fluxes from the sea surface into the ocean interior are often described in terms of a solubility pump and a biological pump (6). The abiotic solubility pump is caused by the solubility of CO 2 increasing with decreasing temperature. In present climate conditions, deep water forms at high latitudes. As a result, volume-averaged ocean temperatures are lower than average sea-surface temperatures. The solubility pump then ensures that, associated with the mean vertical temperature gradient, there is a vertical gradient of dissolved inorganic carbon (DIC). This solubility-driven gradient explains Ϸ30-40% of today's ocean surfaceto-depth DIC gradient (7).A key process responsible for the remaining two thirds of the surface-to-depth DIC gradient is the biological carbon pump. It transports photosynthetically fixed organic carbon from the sunlit surface layer to the deep ocean. Integrated over the global ocean, the biotically mediated oceanic surface-to-depth DIC grad...

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Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans

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