Observation-constrained estimates of the global ocean carbon sink from Earth system models

Abstract

Abstract. The ocean slows global warming by currently taking up around one-quarter of all human-made CO2 emissions. However, estimates of the ocean anthropogenic carbon uptake vary across various observation-based and model-based approaches. Here, we show that the global ocean anthropogenic carbon sink simulated by Earth system models can be constrained by two physical parameters, the present-day sea surface salinity in the subtropical–polar frontal zone in the Southern Ocean and the strength of the Atlantic Meridional Overturning Circulation, and one biogeochemical parameter, the Revelle factor of the global surface ocean. The Revelle factor quantifies the chemical capacity of seawater to take up carbon for a given increase in atmospheric CO2. By exploiting this three-dimensional emergent constraint with observations, we provide a new model- and observation-based estimate of the past, present, and future global ocean anthropogenic carbon sink and show that the ocean carbon sink is 9 %–11 % larger than previously estimated. Furthermore, the constraint reduces uncertainties of the past and present global ocean anthropogenic carbon sink by 42 %–59 % and the future sink by 32 %–62 % depending on the scenario, allowing for a better understanding of the global carbon cycle and better-targeted climate and ocean policies. Our constrained results are in good agreement with the anthropogenic carbon air–sea flux estimates over the last three decades based on observations of the CO2 partial pressure at the ocean surface in the Global Carbon Budget 2021, and they suggest that existing hindcast ocean-only model simulations underestimate the global ocean anthropogenic carbon sink. The key parameters identified here for the ocean anthropogenic carbon sink should be quantified when presenting simulated ocean anthropogenic carbon uptake as in the Global Carbon Budget and be used to adjust these simulated estimates if necessary. The larger ocean carbon sink results in enhanced ocean acidification over the 21st century, which further threatens marine ecosystems by reducing the water volume that is projected to be undersaturated towards aragonite by around 3.7×106–7.4×106 km3 more than originally projected.

Ocean acidification in emission-driven temperature stabilization scenarios: the role of TCRE and non-CO2 greenhouse gases

“…Similarly, the range in volumes changes with Ω A between 1 and 2, 2 and 3, and above 3 vary strongly across the different model setups. Under prescribed atmospheric CO 2 in SSP1-2.6, the simulated decrease in volume that is projected to remain saturated towards aragonite (Ω A > 1) in 2100 is 89 ± 6 × 10 6 km 3 [56]. Thus, the uncertainty of future interior ocean acidification rates under a prescribed temperature target when using the AERA is around twice as large as under prescribed atmospheric CO 2 .…”

“…The wide range of simulations with the AERA demonstrates that the main uncertainty of ocean acidification projection for a given warming stems mainly from the uncertainty in the knowledge of TCRE and the choice in the reductions in non-CO 2 emissions. Moreover, the AERA simulations with Bern3D-LPX underline the importance of emission-driven temperature stabilization simulations in the CMIP framework for projections and uncertainty assessments of the Earth system, its carbon cycle [56], extreme events [43,57], and ecosystem stressors [19] in a stabilized climate.…”

“…By 2150, possible atmospheric CO 2 in a 1.5 • C world under the 'baseline' scenario range from 320 ppm to 368 ppm after having peaked at 433-439 ppm between 2030 and 2035, and the cumulative ocean carbon sink from 1765 to 2150 varies from 190 Pg C to 271 Pg C. In comparison, the SSPs that are used by the IPCC have by definition no uncertainty in the future atmospheric CO 2 as atmospheric CO 2 is prescribed as a boundary condition. While the uncertainty of the atmospheric CO 2 in 2100 is by definition non-existent, the range of the cumulative carbon sink is also smaller when atmospheric CO 2 is prescribed, for example 274-332 Pg C for SSP1-2.6 across the CMIP6 ensemble [56]. Thus, when CO 2 emissions evolve freely to converge to a temperature target, the range of the future sink is ∼40% larger than under fixed atmospheric CO 2 .…”

“…As opposed to surface ocean acidification, longterm ocean acidification below the surface depends not only on the increasing surface ocean dissolved inorganic carbon but also on the quantity of additional dissolved inorganic carbon at the ocean surface that is transported below the ocean surface [31,37,39,40,56,88] and distributed within the ocean [63]. The shading indicates the range of all model setups with ECSs from 2.3 • C to 4.7 • C (5%-95% ECS likelihood range [63]).…”

“…Thus, the uncertainty of future interior ocean acidification rates under a prescribed temperature target when using the AERA is around twice as large as under prescribed atmospheric CO 2 . The difference in uncertainty under prescribed temperature targets and prescribed atmospheric CO 2 is smaller in the ocean interior (factor 2) than at the ocean surface (factor 10), because most water masses are not in direct contact with the atmosphere so that the differences in simulated ocean circulation and deep-water formation affects both kinds of simulations [37,39,40,56,90,91].…”