Greenhouse Warming on Earth’s Past, Present, and Future

Carbon dioxide in Earth’s atmosphere provides an important regulator of climate. Without it, or with too little, Earth would be completely frozen. But the rapid rise in carbon dioxide in recent times due to fossil fuel consumption and changes in land use has caused unprecedented warming with consequences to human civilization.

Understanding how Earth’s climate responds to atmospheric carbon dioxide is an important problem not only for anthropogenic climate change today but also for understanding Earth’s distant past (when the sun was fainter than today) and distant future (when the sun becomes brighter than today). In a paper by Eric Wolf, Brian Toon, and myself titled “Evaluating climate Sensitivity to CO2 across Earth’s history” and published in Journal of Geophysical Research – Atmospheres, we calculate the expected warming for early-, modern-, and future-Earth scenarios across a much wider range of carbon dioxide levels than typically considered for present-day climate change. We show that a doubling of atmospheric carbon dioxide would have caused a greater amount of warming on early Earth (when the carbon dioxide fraction of the atmosphere was high) compared to today (when carbon dioxide is a trace constituent). In general the amount of warming to be expected from such a carbon dioxide doubling (known as the “climate sensitivity”) depends upon the amount of solar energy received, the starting carbon dioxide budget, and the mean temperature of the planet.

Did respiration evolve before photosynthesis?

Animals today stay alive by breathing in oxygen-rich air through a process known as oxygenic respiration, which consumes oxygen (O2) and releases carbon dioxide (CO2) as a byproduct. Most plants, on the other hand, convert sunlight and carbon dioxide into energy through a process known as photosynthesis, which consumes CO2 and releases O2 into the atmosphere. Because photosynthesis is a source of oxygen, it seems intuitive that photosynthesis evolved first: once enough O2 was in the air, then respiration would be able to arise in the newly oxygen-enriched atmosphere. However, some biologists have argued since the 1970’s that respiration in fact evolved first. There are many reasons that this might be the case, and new measurements of bacterial respiration at very low levels of O2 have revived this “early-respiration” hypothesis.

In a recent paper written by myself and my two graduate advisers, we argue that small quantities of O2 could have reached the surface of early Earth through transport by atmospheric dynamics. This transport would primarily occur in the Wintertime hemisphere, where a “polar Winter vortex” develops near the polar region, because the lack of sunlight in Winter would allow for greatest amount of O2 to accumulate. Our calculations show that enough dissolved O2 could have accumulated in polar Winter waters to allow early forms of marine life (i.e. microbial life) to develop and use respiration–without needing to wait for photosynthesis to oxygenate the atmosphere. Although our model calculations cannot prove that respiration did in fact evolve first, they least demonstrate a proof-of-concept that the “early-respiration” hypothesis is in fact viable.

Our paper is titled “Availability of O2 and H2O2 on pre-photosynthetic Earth” and appears in the May issue of the journal Astrobiology.