Cenozoic CO2: Chance history or inevitable outcome?

Abstract

Earth’s climate has undergone dramatic change, but as far as we know this change has never been severe enough to make the planet uninhabitable, despite what might be considered as close calls. External climate drivers (those which are caused by factors unrelated to climate) are known to be operating on long timescales, but it is not yet clear what the impact of these drivers has been due to the complex behaviour of the Earth system. To address this, I have developed GECCO (Geological Evolving Carbon Cycle + Ocean model), a new carbon cycle box model specifically designed to represent the carbon cycle and climate interactions on multimillion year timescales.
To equip GECCO with the tools necessary for modelling of the long term carbon cycle and climate systems, a picture of the carbon cycle has been built up from previous work and then supplemented with novel features where certain aspects had not previously been considered. Key components of the carbon cycle that are novel to GECCO, or are an improvement on previous efforts, include the representations of plate tectonics, ice dynamics, the terrestrial carbonate reservoir and ocean carbonate chemistry. In Chapter 2 each component of GECCO is described in detail, including a discussion of the simplifications and limitations of each part of GECCO in its current state, and potential future improvements to GECCO.
Once developed, GECCO was applied to simulate changes in oceanic calcium and magnesium ion concentration, and how these might have affected the carbon cycle and climate over the Cenozoic. Available fluid inclusion data indicates that calcium concentration has approximately halved over the Cenozoic, and magnesium concentration has approximately doubled. Driving GECCO with this forcing initially showed steady state atmospheric CO2 fell by 250ppm, regardless of the timescale over which the change in ion concentrations occurred. Further ensembles runs were performed to estimate the sensitivity of this result to four key climate feedbacks. Results from these ensembles show that both transient and steady state atmospheric CO2 and CCD response to calcium and magnesium forcing is highly sensitive to feedback strength. In terms of the power of forcings, magnesium is shown to be approximately the same strength as calcium as a driver of atmospheric CO2 change, and the effects of calcium and magnesium are found to be synergistic rather than simply additive.
GECCO was also used to explore the potential for changes in some aspects of plate tectonics to drive changes in climate. An ensemble of runs was performed during which the temporal lag between plate subduction and volcanic outgassing (the ‘outgassing lag’) was varied. Another ensemble was conducted during which the variability around that lag (the ‘outgassing spread’) was varied. Finally, the robustness of these results was tested by performing a multitude of runs to estimate the importance of four key feedbacks in regulating climate response to changing outgassing lag. Changes in outgassing lag drive important changes to atmospheric CO2 by altering carbon storage capacity of the subterranean reservoir. The magnitude of carbon cycle and climate response to this tectonic driver is extremely dependent on climate feedback strength. Varying the outgassing spread had a more muted effect, primarily acting to alter the timescale over which atmospheric CO2 variability occurs.
In summary, two external drivers of climate were analysed for their potential to drive carbon cycle change. Ensembles of runs show both drivers are able to cause large transient changes in atmospheric CO2, however neither driver was able to drive climate instability using a sensible range of forcing strengths unless feedbacks are altered from their estimated Precenozoic strength. Feedback strength is shown to be extremely important in determining long term climate evolution, and the response to changes in feedback strength is found to be similar regardless of the nature of the climate driver. Furthermore, the interplay between different climate feedbacks is found to explain most of the observed behaviours, suggesting that the relative strength of climate feedbacks is the dominant control on Earth’s long term climate. In some situations, external drivers are able to drive transient or steady state climate responses, but the magnitude of both transient and steady state change is highly dependent on feedback strength.

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ORCID for Luke Tyrrell: orcid.org/0000-0002-1002-1716

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