Modelling the inorganic ocean carbon cycle under past and future climate change

The increase in atmospheric CO2 concentration over the last 150 years is unprecedented during the past 420,000 years of Earth's history. The oceans are the largest sink for CO2 and it is unknown how the ocean carbon cycle will respond to increasing anthropogenic CO2 concentrations in the future. It is possible that climate feedbacks may act to reduce further uptake of carbon by the ocean. This thesis examines the inorganic ocean carbon cycle and how climate feedbacks influence future uptake using a coupled ocean-atmosphere-sea ice model with an inorganic carbon component. Past climate transitions are also studied, including both abrupt and glacial-interglacial climate transitions.; The sensitivity of the inorganic carbon cycle to increased atmospheric CO2 is examined and atmospheric carbon dioxide levels are projected under global warming scenarios. A transient weakening of the North Atlantic overturning is found in most simulations and increased SSTs are found in all simulations. Although these positive feedbacks act on the carbon system to reduce uptake, the ocean has the capacity to take up 65--75% of the anthropogenic CO2 increase once the forcing is stopped. The effects of climate feedbacks on future carbon uptake are analyzed and it is found that the ocean stores 7% more carbon when there are no climate feedbacks acting on the system. Sensitivity experiments are conducted with respect to the representation of ocean mixing and sea ice dynamics. The inclusion of the Gent McWilliams parameterization for mixing associated with mesoscale eddies leads to a further 6% increase in oceanic uptake, whereas the inclusion of sea ice dynamics leads to a 2% global difference in uptake.; Past climates have been marked by abrupt transitions from cold to warm states and mechanisms which led to these transitions most likely include variability in the thermohaline circulation (THC). Changes in atmospheric CO2 concentration that arise during abrupt climate change events are investigated. This is accomplished through the use of meltwater pulse scenarios applied to the coupled model. Transient simulations are carried out under a glacial equilibrium climate with increased freshwater discharge to high latitude regions in both hemispheres in order to simulate meltwater episodes. Changes in ocean circulation and carbon solubility are found to lead to significant increases in atmospheric CO2 concentrations when meltwater episodes are simulated in both hemispheres. The magnitude of increase in atmospheric CO2 is between 10--40 ppmv, which accounts for some of the changes in CO 2 as recorded in the ice core records.; The response of the carbon cycle during glacial-interglacial transition is investigated by applying 8 kyr BP boundary conditions to an LGM equilibrium climate. An LGM equilibrium with orbital parameters set to 21 kyr BP and CO 2 radiative forcing to 200 ppmv is used and 8 kyr BP boundary conditions, namely orbital parameters for 8 kyr BP and CO2 radiative forcing of 280 ppmv, are then imposed on this equilibrium. Atmospheric CO2 is then allowed to evolve and we find an increase of only ∼5 ppmv. Changes due to CO2 radiative forcing alone account for almost all of the change in atmospheric CO2 with effects of changing the orbital parameters almost negligible on global carbon uptake. It is suggested that the inclusion of both biological and carbonate pumps may be important components to unlocking the mystery surrounding the glacial-interglacial cycles.