The role of marine biota in the climate system - an Earth system model approach

Abstract

Marine phytoplankton are unicellular algae that are the first link of the marine food chain. They can influence the climate system via biogeochemical and biogeophysical mechanisms, especially during large blooms. Phytoplankton can absorb light at the surface of the ocean, modifying the distribution of radiative heat along the water column. These changes in heat budget alter the oceanic properties, the atmospheric properties and finally the overall climate system. In this thesis, I investigate the role of the marine biota in the climate system by using an Earth system model of intermediate complexity called EcoGENIE. I modified the oceanic and ecosystem model components to consider phytoplankton light absorption. Over the past years, the number of plankton functional types in models has increased but the relative importance of biological processes such as phytoplankton light absorption is still unclear. As a logical extension, I compared the relative importance of phytoplankton light absorption with an increase in marine ecosystem complexity. I show that phytoplankton light absorption increases the atmospheric CO2 concentration and the overall heat budget of the planet. In contrast, increasing ecosystem complexity only slightly affects the carbon cycle and thus the heat budget. In conclusion, phytoplankton light absorption has a higher impact on the climate system than an increase in marine ecosystem complexity. After demonstrating that phytoplankton light absorption has an impact on the climate system, I focus on the climate pathways behind the atmospheric warming due to this biogeophysical mechanism. Phytoplankton light absorption increases the oceanic temperature with consequences on the air-sea heat and CO2 fluxes. I evidence that changes in air-sea CO2 exchange due to phytoplankton light absorption have a larger contribution to the atmospheric heating than phytoplankton-induced changes in air-sea heat flux. After demonstrating that phytoplankton light absorption increases the atmospheric temperature via an increase in atmospheric CO2 concentration, I explore the effects of this biogeophysical mechanism would have in a warmer climate. To shed light on this question, I conduct simulations under RCPs and pre-industrial conditions. First, I evidence that the overall warming due to phytoplankton light absorption is smaller than the overall warming due to climate change. Secondly, chlorophyll biomass is expected to decrease under global warming and my results indicate that phytoplankton light absorption enhances the reduction of the chlorophyll biomass. As a consequence, less heat is trapped by chlorophyll and the effect of phytoplankton light absorption on the climate system is reduced. Thirdly I demonstrate that prescribing atmospheric CO2 concentration in model simulations blur the real effect of phytoplankton light absorption on the climate system. This thesis supports the idea that phytoplankton light absorption should be considered in climate studies as an internal constituent of the climate system for long-term climate adjustment.