A Modeling Study Of Interactive Feedbacks Between Carbon Dioxide,Global Warming,Ocean Acidification,and The Ocean Carbon Cycle

Since the preindustrial time,atmospheric CO2 concentration has increased from?280 ppm to-405 ppm.Increasing atmospheric CO2 causes global warming through the greenhouse effect.Meanwhile,the absorption of anthropogenic CO2 by the ocean decreases seawater pH,causing ocean acidification.In this study,we use a coupled climate-carbon cycle model(UVic ESCM,University of Victoria Earth System Climate Model)to conduct a series of simulations to investigate the interactions between atmospheric CO2,global warming,ocean acidification,and the ocean carbon cycle,as well as the underlying mechanisms.With the increase of atmospheric CO2,rising ocean temperature would affect the ocean carbon cycle and ocean's ability to absorb atmospheric CO2.Warming-induced decrease in CO2 solubility would reduce ocean's uptake of atmospheric CO2.Also,a warmer ocean inhibits vertical mixing between the surface and deep ocean,and slows down North Atlantic meridional overturning circulation,further suppressing the oceanic CO2 uptake.In addition,temperature change,via its impacts on ocean biological rates(including the rates of phytoplankton growth,phytoplankton mortality and detritus remineralization),also have important effects on oceanic CO2 uptake.Under RCP8.5 CO2 concentration scenario,between year 1800 and 2100,in the simulation that includes the effects from increasing CO2 alone,simulated cumulative oceanic CO2 uptake amounts to 586 PgC.Warming-induced decrease in CO2 solubility reduces cumulative oceanic CO2 uptake by 34 PgC,while warming-induced inhibition in ocean mixing and thermohaline circulation reduces the uptake by 13 PgC.Meanwhile,warming-induced changes in ocean biological rates increase the cumulative ocean CO2 uptake by 4 PgC.Our results show that,in addition to increase in atmospheric CO2 alone,CO2-induced global warming would have important effects on oceanic CO2 uptake through different physical and biogeochemical mechanisms.Increase in atmospheric CO2 would cause ocean acidification.In this study,we perform simulations under an idealized atmospheric CO2 concentration pathway in which CO2 increases at a rate of 1%from its preindustrial level of 280 ppm until it reaches four times of the preindustrial level,and then decreases at the same rate to the preindustrial level.Our simulations show that,when atmospheric CO2 increases to four times of its preindustrial level,ocean surface pH decreases from its preindustrial level of 8.2 to 7.7(i.e.,220%increase in[H+]),and global mean aragonite saturation horizon(ASH,the depth below which seawater becomes undersaturated with aragonite,a soluble form of CaCO3)shoals from 1288 m to 143 m.When CO2 concentration decreases to its initial level,ocean surface pH recovers to 8.1,but ASH recovers only to about 630 m.These results indicate that the change in surface ocean carbonate chemistry largely follows the change in atmospheric CO2,but the response of deep ocean chemistry substantially lags behind the change in atmospheric CO2.Our analysis indicates that,even if we could lower atmospheric CO2 concentration artificially(i.e.,via Carbon Dioxide Removal geoengineering),deep ocean acidification would continue for a long time,threatening deep-sea ecosystesms.Ocean acidification would in turn affect some key processes in the ocean carbon cycle and alter ocean's ability to take up atmospheric CO2.For example,ocean acidification could suppress calcification of some marine organisms,increasing sea surface alkalinity and promoting oceanic CO2 uptake(calcification feedback).Meanwhile,the suppression of calcification decreases the export flux of CaCO3 to the ocean depth,potentially reducing the export flux of organic carbon that is associated with CaCO3,inhibiting marine biological carbon pump and oceanic CO2 uptake(ballast feedback).In this study,we incorporate a set of parameterization schemes linking ocean acidification,calcification,and organic carbon flux into the UVic model.Under SRES A2 CO2 emission scenario,by the end of this century,in the simulation without calcification and ballast feedbacks,cumulative ocean CO2 uptake is 530 PgC.Calcification feedback increases cumulative ocean CO2 uptake by about 4%,and ballast feedback decreases cumulative ocean CO2 uptake by about 3%.On timescales of several centuries,calcification and ballast feedbacks become more important.Meanwhile,due to ballast feedback,more organic carbon would accumulate and decompose in the upper ocean,reducing oxygen concentration in the upper ocean,affecting marine biological processes.In this study,we investigate the responses of the ocean carbon cycle and ocean acidification to changes in atmospheric CO2 and CO2-induced global warming.We also examine the feedbacks of ocean acidification on the ocean's uptake of atmospheric CO2.This study helps us to gain a better understanding of interactive feedbacks between atmospheric CO2,global warming,the ocan carbon cycle,and ocean acidification.