Monitoring and interpreting the ocean uptake of atmospheric CO ₂

Abstract

The oceans are an important sink for anthropogenically produced CO 2 , and on time scales longer than a century they will be the main repository for the CO 2 that humans are emitting. Our knowledge of how ocean uptake varies (regionally and temporally) and the processes that control it is currently observation-limited. Traditionally, and based on sparse observations and models at coarse resolution, ocean uptake has been thought to be relatively invariant. However, in the few places where we have enough observations to define the uptake over periods of many years or decades, it has been found to change substantially at basin scales, responding to indices of climate variability. We illustrate this for three well-studied regions: the equatorial Pacific, the Indian Ocean sector of the Southern Ocean, and the North Atlantic. A lesson to take from this is that ocean uptake is sensitive to climate (regionally, but presumably also globally). This reinforces the expectation that, as global climate changes in the future owing to human influences, ocean uptake of CO 2 will respond. To evaluate and give early warning of such carbonclimate feedbacks, it is important to track trends in both ocean and land sinks for CO 2 . Recent coordinated observational programmes have shown that, by organization of an observing network, the atmosphere-ocean flux of CO 2 can, in principle, be accurately tracked at seasonal or better resolution, over at least the Northern Hemisphere oceans. This would provide a valuable constraint on both the ocean and (by difference) land vegetation sinks for atmospheric CO 2 .

Detecting the anthropogenic influences on recent changes in ocean carbon uptake

“…However, as the ocean carbon sink is sensitive to climate as well as atmospheric CO 2 , models predict a future weakening of the CO 2 uptake rate in response to ongoing climate change, resulting in a positive carbon-climate feedback [Friedlingstein et al, 2006]. The effort to identify this climate-carbon feedback promoted the investigation of trends in ocean carbon uptake [Le Quéré et al, 2007;Schuster and Watson, 2007;Thomas et al, 2008;Schuster et al, 2009;Metzl et al, 2010;Watson et al, 2011] in order to detect substantial deviations from the expected response to rising atmospheric CO 2 . However, the causes of the recent variations in the ocean carbon uptake over the last decades are still poorly understood [Le Quéré et al, 2007;Lovenduski et al, 2008;McKinley et al, 2011;Bates, 2012].…”

Dynamical and biogeochemical control on the decadal variability of ocean carbon fluxes

“…Among these regions potentially impacted by anthropogenic forced climate change, the North Atlantic has been extensively studied over the three last decades (Watson et al, 2011;Schuster et al, 2009;Brown et al, 2010;Metzl et al, 2010;Corbière et al, 2007;McKinley and Follows, 2004;McKinley et al, 2011). Debate prominently arises from disagreement between causes of trends in the North Atlantic carbon sink estimated from data or numerical model output.…”

“…On the basis of in situ observations, several authors (i.e. Corbière et al, 2007;Watson et al, 2011;Brown et al, 2010;Metzl et al, 2010) have reported a more rapid growth in surface ocean pCO 2 than in atmospheric pCO 2 , which translates into a decline of the net ocean carbon uptake over the last decade. Possible mechanisms driving this decline are associated to climate-induced modifications of both oceanic and atmospheric dynamics, e.g., rising sea-surface temperature (Corbière et al, 2007), increasing ocean stratification (Schuster et al, 2009) or changes in horizontal oceanic currents owing to a shift in the North Atlantic Oscillation (Thomas et al, 2007;Schuster et al, 2009).…”

Effect of gas-transfer velocity parameterization choice on air–sea CO<sub>2</sub> fluxes in the North Atlantic Ocean and the European Arctic

“…In this work we have analysed solely the effects of the choice between various published empirical wind-driven gas-transfer parameterizations. The North Atlantic is one of the regions of the world ocean best covered by CO 2 fugacity measurements (Watson et al, 2011), the coverage of the Arctic seas is much poorer, especially in winter .…”