Journal Article

The Impact of Recent Climate Change on the Global Ocean Carbon Sink

Nissen, Cara, Hauck, Judith, Bunsen, Frauke

2024

Overview

In recent decades, the ocean CO2 uptake has increased in response to rising atmospheric CO2. Yet, physical climate change also affects the ocean CO2 uptake, but magnitude and driving processes are poorly understood. Using a global ocean biogeochemistry model, we find that without climate change, the mean carbon uptake 2000–2019 would have been 13% higher and the trend 1958–2019 would have been 27% higher. Changes in wind are the dominant driver of the climate effect on CO2 uptake as they affect advective carbon transport and mixing, but the effect of warming increases over time. Roughly half of the globally integrated wind‐driven trend stems from the subpolar Southern Ocean and polar oceans in both hemispheres. Warming reduces the solubility of CO2 and acts rather homogeneously over the world oceans. However, the warming effect on pCO2 is dampened by limited exchange of surface and deep waters. Plain Language Summary At the ocean surface, the greenhouse gas CO2 is exchanged between atmosphere and ocean. Because the concentration of CO2 in the atmosphere has increased through man‐made CO2 emissions, the ocean has taken up an increasing amount of CO2 (about 25% of the emissions). Beside the atmospheric CO2 concentration, other climate variables affect the oceanic CO2 uptake: Firstly, winds set the ocean in motion, drive ocean currents and thus control the transport of dissolved forms of CO2 with ocean circulation. In particular, winds drive the exchange between the surface ocean and the deep ocean, where the bigger part of the ocean's carbon is stored. Secondly, global warming affects the oceanic CO2 uptake because the solubility of CO2 in water is temperature‐dependent. In recent decades, changes in winds and global warming have reduced the capacity of the ocean to remove CO2 from the atmosphere. Yet, this climate effect is not well understood. Here, we use computer simulations from 1958 to 2019 to quantify the climate effect and find that climate change reduced the oceanic CO2 uptake of the last two decades by 13%, with winds having more of an effect than sea surface warming. The effect of warming increases over time. Key Points Climate change reduced ocean CO2 uptake by 13% (2000–2019) primarily induced by wind‐driven changes in dissolved inorganic carbon transport A feedback between the surface dissolved inorganic carbon concentration and air‐sea flux dampens warming‐driven outgassing of natural carbon The effect of wind changes stems primarily from high latitudes, whereas the effect of warming is globally relatively uniform

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: air‐sea CO2 flux / Atmosphere / Biogeochemistry / Carbon / Carbon dioxide / Carbon dioxide atmospheric concentrations / Carbon dioxide concentration / Carbon dioxide emissions / Carbon dioxide exchange / Carbon sinks / Carbon uptake / Climate change / Climate effects / Climate models / Deep water / Emissions / Exchanging / global carbon cycle / Global warming / Global warming effects / Greenhouse effect / Greenhouse gases / Hemispheres / Mathematical models / ocean biogeochemical modeling / ocean carbon uptake / Ocean circulation / Ocean currents / Ocean surface / Oceans / pCO2 / Sea surface / Sea surface warming / Solubility / Surface temperature / Temperature dependence / Water circulation / Winds