Award: OCE-1314209

Anthropogenic CO2 emissions have caused an unprecedented change in upper ocean carbonate chemistry, which is manifested by an overall observed increase of the partial pressure of CO2 in water and decreasing pH and carbonate ion concentrations. Future projected trends in these quantities can impact the functionality of marine life in the ocean. Exposing fish to very high CO2 concentrations can lead to hypercapnia and trigger harmful physiological responses. Low carbonate ion concentrations, on the other hand, reduce the ability of calcifying organisms to form their shells and protective layers. This effect will be most pronounced for organisms that form aragonite, such as pteropods and corals. Even though the atmospheric CO2 forcing is spatially uniform, the carbonate chemical response in the ocean is not. The main purpose of this project was to determine how physical, chemical and biological processes work together to generate spatio-temporal shifts in future marine carbonate chemistry. By analysing future earth system model simulations conducted as part of the Coupled Model Intercomparison Project 5, we found that the seasonal cycle and the amplitude of interannual fluctuations in upper ocean CO2 will intensify considerably in response to increasing future CO2 gas emissions. In an RCP8.5 greenhouse gas emission scenario, the subtropical and equatorial regions will experience a near-doubling of the seasonal cycle amplitude almost exclusively due to a larger background CO2 concentration and solubility. This mechanism is further reinforced by an overall increase in the seasonal cycle of temperature as a result of stronger ocean stratification and a projected shoaling of mean mixed layer depths. The Southern Ocean will experience an even stronger variability amplification of CO2 which can be attributed to changes in the overall available dissolved carbon in the ocean. Due to the upwelling of CO2-rich deep waters the Southern Ocean already operates very close to threshold conditions that can cause aragonite undersaturation. This could trigger a rapid drop in calcification rates of organisms such as pteropods, which contribute significantly to the pelagic foodweb and carbon export fluxes in this region. Due to its biological vulnerability, our analysis focused in particular on the spatio-temporal variability of aragonite undersaturation in the Southern Ocean.Using an ensemble of ten Earth system models, we show that starting around 2030, aragonite undersaturation events will spread rapidly, affecting ∼30% of Southern Ocean surface waters by 2060 and >70% by 2100, including the Patagonian Shelf. On their onset, the duration of these events will increase abruptly from 1 month to 6 months per year in less than 20 years in >75% of the area affected by end-of-century aragonite undersaturation. This is likely to decrease the ability of organisms to adapt to a quickly evolving environment. The spatial characteristics of projected twenty-first century aragonite undersaturation events are largely determined by the present-day mean meridional gradient of carbonte concentrations. The largest zonal inhomogeneity, driven by climate-induced changes to biology and ocean circulation, shortens and lengthens the duration of surface aragonite undersaturation events by only one month south. Our results demonstrate that unabated CO2 emissions will generate unprecedented biogeochemical conditions in our global oceans, both in terms of mean conditions, seasonal and interannual variability. Avoiding ocean hypercapnia in the longterm and slowing down ocean acidification trends and their impact on marine biota can only be achieved through drastic reductions in CO2 emissions. Last Modified: 02/06/2022 Submitted by: Axel Timmermann