The changing ocean carbon sink in the earth system

Eunice Foote, who was the first to measure the solar heating of CO2 in her early experiments already in the 1850s noted: “An atmosphere of that gas would give to our Earth a high temperature“ (Foote, 1856). Indeed, our planet is warming unprecedently fast due to rising anthropogenic CO2 emissions (Masson-Delmotte et al., 2021). Next to catastrophic floodings, wildfires and droughts on land, with tragic consequences for people, the ocean silently suffers from the ongoing heating, acidification, and deoxygenation with tragic impacts for marine systems.

The ocean plays an essential role in regulating Earth’s climate; it is also essential for regulating the Earth’s carbon cycle. The ocean contains around 38,000 Gt of carbon. This is 16 times more than the terrestrial biosphere (plant and the underlying soils), and about 60 times more than the pre-industrial atmosphere (Canadell et al., 2021). Therefore, even a small perturbation to the ocean carbon content by changing its capacity to store carbon would impact atmospheric CO2 concentrations (Fig.1.1), making the ocean carbon sink a major regulator of the Earth’s climate on a time scale of hundreds to thousands of years. As the ocean currently continuously absorbs anthropogenic carbon from the atmosphere, it thereby has a key role in moderating ongoing climate change.

Based on the Global Carbon Budget (GCB) estimates (Friedlingstein et al., 2020), the global ocean has already taken up about one third of the cumulative anthropogenic CO2 emissions (Fig.1.2). The strength of the ocean carbon sink is determined by chemical reactions in seawater (carbonate system), biological processes (photosynthesis, export flux, and remineralization by aerobic and anaerobic respiration), and physical processes (including ocean circulation and vertical mixing). But even though these key mechanisms are identified (Landschutzer et al., 2021), there are considerable uncertainties regarding their interannual and decadal variations, as well as their susceptibility to ongoing climate change. Here, a major uncertainty arises from the lack of knowledge regarding the contribution of the natural variability of the climate system (Ilyina, 2016).

In this essay, I present my research contributions based on my papers explicitly mentioned in the text. My research was guided by the following questions:

  1. How do ocean biogeochemical cycles accommodate perturbations brought about by anthropogenic activities or natural forcings?
  2. What are the predictability horizons of variations in the ocean carbon sink?
  3. What is the potential of the ocean carbon sink, artificially enhanced by ocean alkalinity additions, to mitigate climate change?

Ilyina T., 2022. The changing ocean carbon sink in the earth system. PhD Thesis, MPI for Meteorology, Max Planck Society. 66 p. Thesis. Thesis.

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