Multi‐Century Changes in the Ocean Carbon Cycle Controlled by the Tropical Oceans and the Southern Ocean

Abstract

The oceanic absorption of anthropogenic carbon dioxide (CO2) is expected to continue in the following centuries, but the processes driving these changes remain uncertain. We studied these processes in a simulation of future changes in global climate and the carbon cycle following the RCP8.5 high emission scenario. The simulation shows increasing oceanic uptake of anthropogenic CO2 peaking towards the year 2080 and then slowing down but remaining significant in the period up to the year 2300. These multi‐century changes in uptake are dominated by changes in sea‐air CO2 fluxes in the tropical and southern oceans. In the tropics, reductions in upwelling and vertical gradients of dissolved carbon will reduce the vertical advection of carbon‐rich thermocline waters, suppressing natural outgassing of CO2. In the Southern Ocean, the upwelling of waters with relatively low dissolved carbon keeps the surface carbon relatively low, enhancing the uptake of CO2 in the next centuries. The slowdown in CO2 uptake in the subsequent centuries is caused by the decrease in CO2 solubility and storage capacity in the ocean due to ocean warming and changes in carbon chemistry. A collapse of the Atlantic Meridional Overturning Circulation (AMOC) predicted for the next century causes a substantial reduction in the uptake of anthropogenic CO2. In sum, predicting multi‐century changes in the global carbon cycle depends on future changes in carbon chemistry along with changes in oceanic and atmospheric circulations in the Southern and tropical oceans, together with a potential collapse of the AMOC.

Long‐Term Slowdown of Ocean Carbon Uptake by Alkalinity Dynamics

“…As a result, the rate at which carbon is stored as DIC may no longer be the primary factor controlling the air‐sea difference in CO 2 partial pressure driving CO 2 uptake. Other factors, such as warming or changes in alkalinity, could become equally important drivers of CO 2 uptake on these longer timescales (Chikamoto & DiNezio, 2021; Egleston et al., 2010; Matsumoto et al., 2010; Riebesell et al., 2009). However, little is known about the relative importance of these processes and how they interact with a changing climate despite recent advances simulating these processes and the global ocean carbon cycle using numerical models.…”

Emergence of the ocean CO2 uptake hole under global warming

“…1d). This is mainly caused by decreasing in the buffer capacity of the carbonate system-the efficiency of ocean CO2 absorption-due to change of multiple physical and chemical processes as anthropogenic CO2 dissolves into the ocean 12,20,21 .…”

“…1c). In the Southern Ocean, turbulent exchange is expected to accelerate as the open ocean increases and westerly wind strengthens 12 . At the same time, a weakening of the equatorial trade wind is followed by a weakening of the upwelling.…”

“…In future climate scenarios, global ocean carbon uptake shows a linear behavior: continuously increasing in response to rising atmospheric CO2 levels 8,9 . The Tropical Ocean, as well as the Southern Ocean, are expected to be a more favorable environment for CO2 uptake due to changes in upwelling and sea ice [10][11][12] . In the case of the NAO, several studies suggest that the weakening of the AMOC leads to surface warming in the NAO, followed by a slowdown in CO2 uptake and a decline in CO2 storage [13][14][15][16] .…”

A shift in the mechanism of CO2 uptake in the Southern Ocean under high emission-scenario

“…The intergovernmental panel on climate (IPCC) high emission scenario projects over 1000 ppm atmospheric CO2 (~2.5-factor increase) and associated warming of up to 4.5 °C by 2100 in the global surface oceans relative to the beginning of the 21 st century 10,11 . Simulated changes include, but are not limited to, sea-ice loss, ocean acidification, deoxygenation, and a decrease in CO2 buffer capacity [12][13][14][15][16][17] . The decrease of the CO2 buffering capacity is projected to enhance biological-induced CO2 uptake in summer, and amplify the seasonality of air-sea CO2 fluxes (FCO2) as well as hydrogen ion concentration ([H + ]), both of which will likely have a significant implications for the efficiency of the ocean CO2 uptake and marine calcifying organisms in the Southern Ocean 18 .…”