Impact of climate change and ocean acidification on the marine nitrogen cycle

The marine nitrogen cycle is responsible for two climate feedbacks in the Earth System. Firstly, it modulates the fixed nitrogen pool available for phytoplankton growth and hence it modulates in part the strength of the biological pump, one of the mechanisms contributing to the oceanic uptake of anthropogenic CO₂. Secondly, the nitrogen cycle produces a powerful greenhouse gas and ozone (O₃) depletion agent called nitrous oxide (N₂O). Future changes of the nitrogen cycle in response to global warming, ocean deoxygenation and ocean acidification are largely unknown. Processes such as N₂-fixation, nitrification, denitrification and N₂O production will experience changes under the simultaneous effect of these three stressors. Global ocean biogeochemical models allow us to study such interactions. Using NEMO-PISCES and the CMIP5 model ensemble we project changes in year 2100 under the business-as-usual high CO₂ emissions scenario in global scale N₂-fixation rates, nitrification rates, N₂O production and N₂O sea-to-air fluxes adding CO₂ sensitive functions into the model parameterizations. Second order effects due to the combination of global warming in tandem with ocean acidification on the fixed nitrogen pool, primary productivity and N₂O radiative forcing feedbacks are also evaluated in this thesis. (author)