The Impact of Internal Climate Variability on Marine Phytoplankton in a Warming Climate

Elsworth, Geneviève Wheeler

Graduate Thesis Or Dissertation

The Impact of Internal Climate Variability on Marine Phytoplankton in a Warming Climate Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/nz806090m

Abstract

Marine phytoplankton (algae) play a key role in the Earth system by influencing ocean biogeochemical cycling, the flux of carbon dioxide from the atmosphere to the ocean, and the productivity of fisheries. The growth of these microscopic, unicellular primary producers is strongly affected by the oceanic physical and biogeochemical environment. As such, the variable and changing climate system has a large influence on phytoplankton abundance, its spatial distribution, and its temporal variability. Internal variability naturally arises from interactions between components of the coupled climate system, for example, between the ocean and the atmosphere. Whereas, anthropogenic changes to the climate system are considered to be externally forced, as they arise from greenhouse gas emissions. Phytoplankton experience both internal climate variability and externally forced anthropogenic changes, and it can be difficult to discern the influence of internal and external processes in the marine biosphere. Recent research suggests that it may be possible to separate internal and external influences on the coupled Earth system using large ensembles of Earth system models (ESMs). However, ESMs may not skillfully predict observed spatial patterns and temporal dynamics in real-world marine phytoplankton. In this dissertation, I use observational records and ESM ensembles to investigate the role of internal climate variability in marine phytoplankton in a warming climate. I first use a novel statistical emulation technique to place the remotely sensed record of surface ocean chlorophyll concentrations into the large ensemble framework. Much like a large initial condition ensemble generated with an ESM, the resulting observationally constrained synthetic ensemble represents multiple possible spatiotemporal evolutions of observed ocean chlorophyll, each with a different phasing of internal climate variability. I use the observationally constrained synthetic ensemble to contextualize the interpretation of long-term trends in the presence of internal variability and identify a wider range of possible trends in chlorophyll due to the sampling of internal variability in subpolar regions than in subtropical regions. Next, I evaluate the statistical methodology of the observationally constrained synthetic ensemble in the context of a large ensemble of an ESM. When applying the statistical approach to the Community Earth System Model Large Ensemble (CESM1-LE) over the historical period, simulated variability in surface ocean chlorophyll concentration is able to be reproduced using the statistical method. Finally, I quantify the influence of anthropogenic climate change on variability in phytoplankton biomass using the CESM1-LE. I find a significant decrease in the interannual variance of phytoplankton biomass under a business-as-usual (RCP8.5) emission scenario, with heterogeneous regional trends. Statistical analysis of regional trends reveal zooplankton grazing (top-down control) as an important contributor to changes in phytoplankton variance. The results of this dissertation highlight the influence of internal climate variability on marine phytoplankton in a warming climate.

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