CO2-CarbonCycle-Climate-Interactions (C4I)

Oceans represent 70% of Earth's surface, supporting vast biodiversity and providing major food resources for humankind. Since the industrial revolution, the oceans have restricted the extent of global warming by taking up approximately 50% of the CO2 from fossil fuel burning and cement manufacture. CO2 forms carbonic acid when dissolved in seawater and lowers ambient pH in a phenomenon known as 'ocean acidification'. An important facet of ocean acidification is a decline in the concentration of carbonate ions in the ocean, a form of dissolved carbon that is depleted in the acidification reaction when CO2 is added to seawater. This is critical, because the shells and skeletons of many marine organisms are made of calcium carbonate (CaCO3) which dissolves at low carbonate ion concentrations (known as 'under-saturated' conditions). However, ecological thresholds of disruption may be crossed long before conditions of under-saturation actually occur and marine organisms' shells start dissolving around them, because calcium carbonate shells and skeletons will require more metabolic energy to maintain their thickness as carbonate ion concentrations fall. Experiments in the laboratory and field have already demonstrated this effect and find that calcifying algae called coccolithophorids generally produce less CaCO3 shell material in more acidic conditions. Slowing the rate of production of CaCO3 by algae living in the ocean surface may have a 'beneficial' impact by helping neutralize fossil fuel CO2, but a detrimental impact as ecosystems are disrupted. There may also be serious implications for the supply of organic detritus to organisms on the seafloor, as it is suspected that this food supply depends heavily on particles of CaCO3 to weigh down the fluffy organic matter and help it sink. Other impacts of ocean acidification may include changes in the amount and nutritional content of organic matter produced in the ocean, and loss of nutrients: converting nitrate to the powerful greenhouse gas nitrous oxide. How the ocean carbon cycle 'works' and whether CaCO3 particles are really important to weighting down fluffy organic matter as well as how exactly algae like coccolithophorids will respond to changing ocean chemistry are subject to significant uncertainties. This means that we would have no way of knowing whether a single computer model prediction for the future is correct or not. In this project we will tackle this question of uncertainty head-on - running out computer models of ocean carbon cycling and climate hundreds and hundreds of times to see what future impacts are possible and what are not. We will be greatly helped in this by using vast datasets describing what the modern ocean 'looks' like (in terms of the distributions of nutrients and patterns recorded in the sediments) to constrain the swarm of models so that they all agree on what the modern ocean looks like to begin with. The outcome of our work will firstly be a better understanding of the modern ocean carbon cycle, which is essential to get right before worrying about the future. We will also make predictions about the range of changes in ocean carbon and nutrient cycles we can expect in the future and how the ocean may affect the degree of future warming by emitting more or less greenhouse gases such as carbon dioxide and nitrous oxide.