CO2, O2, and the History of the Greenhouse Effect: Select Problems in the Evolution of Earth's Atmosphere and Climate

Open Access
- Author:
- Payne, Rebecca
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 11, 2020
- Committee Members:
- James Kasting, Dissertation Advisor/Co-Advisor
James Kasting, Committee Chair/Co-Chair
Christopher Howard House, Committee Member
Julie Genevieve Cosmidis, Committee Member
Raymond Gabriel Najjar, Jr., Outside Member
Mark E Patzkowsky, Program Head/Chair - Keywords:
- geoscience
climate
paleoclimate
astrobiology
photochemistry
climate modeling
Archean
Phanerozoic
Earth history
Earth climate
geology - Abstract:
- To understand the history of life on Earth, it is essential to understand the evolution of the global climate that has maintained a relatively stable and habitable environment for life over the last 4.6 billion years. Earth’s climate itself is the result of a delicate balance between the composition of the atmosphere and the processes on the surface—in the oceans, and on the continents—that interact with it. Prior to the rise of atmospheric O2 at ~2.45 Ga, Earth’s climate relied on CO2 as a major atmospheric constituent in order to keep Earth at a habitable temperature in spite of lower solar luminosity. While climate models have estimated that at least several percent of CO2 in the atmosphere would have been needed to maintain a habitable climate, geochemical data from paleosols have largely predicted CO2 one or two orders of magnitude lower, though there is considerable disagreement between paleosol studies. We argue instead that oxidized iron micrometeorites from ~2.7 Ga offer a new point of comparison that is not subject to the same potential biases as paleosols. Abundant CO2 could have oxidized iron micrometeorites during atmospheric entry, meaning that preserved micrometeorites may offer a means of estimating atmospheric CO2 in an atmosphere largely devoid of other potential oxidants. We estimate that an atmosphere of at least ~23% CO2 would be sufficient to oxidize the micrometeorites and keep surface temperatures warm. Using a climate model, we demonstrate that the new CO2 constraint from micrometeorites also supports the idea that the partial pressure of N2 (and overall surface pressure) was lower on the early Earth, in order for high CO2 to not cause surface temperatures to conflict with evidence of glaciation in the late Archean. The emergence of atmospheric O2 after ~2.45 Ga, and its role as a major constituent during the last ~541 Ma in particular, resulted in a fundamental shift in the greenhouse effect and atmospheric CO2. Using a coupled one-dimensional photochemical-climate model, we find that atmospheric O2 equal to or higher than 21% (its present-day abundance) slightly enhances warming by other greenhouse gases like CO2 and H2O, by broadening their absorption lines to increase effectiveness. But O2 exerts a minor effect on surface temperature at most; our findings support the classical interpretation that changes in solar luminosity, CO2, and global geography are the primary controls on global climate in the Phanerozoic. We apply this analysis specifically to the early Cretaceous, at ~100 Ma, and argue that higher atmospheric O2 would support both the hot temperatures of the Cretaceous as well as the needs of the massive terrestrial life (namely, dinosaurs) that mark that time in geologic history. We also consider carbon cycling on long timescales, using a box model of the atmosphere-ocean-seafloor to study changes in CO2 and in C sources and sinks over the last 100 Ma. We demonstrate that both seafloor weathering and continental weathering are important to climate change since the Cretaceous. Seafloor weathering is closely tied to seafloor temperatures, and so was likely to have been a more important C sink during the Cretaceous when global temperatures were higher than today. We agree with recent analyses that have suggested that continental weathering—especially an increase in continental weatherability over the last ~100 Ma due to tectonic activity and uplift, as well as sea level change—has driven much of the CO2 cycling.