High CO2 Levels in the Air were Responsible for High Temperatures on Young Earth

The temperature on the young Earth was warm despite that radiation of the sun was comparatively low. An international group of geoscientists has discovered essential clues that high levels of carbon dioxide (CO₂) in the air led to such high temperatures.

Image Credit: Dima Zel/Shutterstock.com

It got cooler with the start of plate tectonics because CO₂ was slowly captured and stored on the newly forming continents.

Three to four billion years ago, very high atmospheric CO₂ levels caused the high temperatures on the still-young Earth. During that time, the Sun shone with only 70%–80% of its current intensity. However, the climate on the young Earth was quite warm since there was barely any glacial ice.

This phenomenon is called the “paradox of the young weak Sun.” With the absence of efficient greenhouse gas, the young Earth would have frozen into a chunk of ice. Whether methane, CO₂, or a completely different greenhouse gas heated up planet Earth is a matter of discussion among researchers.

A new study led by Dr Daniel Herwartz from the University of Cologne, Professor Dr Andreas Pack of the University of Göttingen, and Professor Dr Thorsten Nagel from the University of Aarhus (Denmark) currently indicates that high CO₂ levels are a believable explanation. This would also resolve another geoscientific issue: ocean temperatures that were too high. The study recently appeared in Proceedings of the National Academy of Sciences.

A controversial question in earth science is related to the temperatures of the early oceans. There is proof that they were very hot. Measurements of oxygen isotopes on ancient limestone or siliceous rocks, which act as geothermometers, denote seawater temperatures more than 70 °C. Lower temperatures would only have been feasible if the seawater had altered its oxygen isotope composition. But this was considered implausible for a long time.

Models developed as part of the new study demonstrate that high CO₂ levels in the air may offer an explanation, as they would also have altered the composition of the ocean.

High CO₂ levels would thus explain two phenomena at once: first, the warm climate on Earth, and second, why geothermometers appear to show hot seawater. Taking into account the different oxygen isotope ratio of seawater, we would arrive at temperatures closer to 40 °C.

Daniel Herwartz, Institute of Geology and Mineralogy, University of Cologne

Moreover, it can be imagined that a lot of methane was present in the air. However, that would not have had any impact on the ocean composition. Therefore, it would not elucidate why the oxygen geothermometer denotes very high temperatures.

“Both phenomena can only be explained by high levels of CO₂,” Herwartz added.

The researchers calculate the total amount of CO₂ to have summed around one bar. That would be as if the existing entire atmosphere is made up of CO₂.

Today, CO₂ is just a trace gas in the atmosphere. Compared to that, one bar sounds like an absurdly large amount. However, looking at our sister planet Venus with its approximately 90 bar of CO₂ puts things into perspective.

Dr Andreas Pack, Professor, Centre for Geosciences, University of Göttingen

CO₂ on Earth was ultimately eliminated from the ocean and the air and stored in the form of gas, oil, coal, and black shales, as well as in limestone. Such carbon reservoirs are mostly situated on the continents. But the young Earth was mostly covered by oceans and there were barely any continents, so the carbon storage capacity was restricted.

“That also explains the enormous CO₂ levels of the young Earth from today’s perspective. After all, roughly three billion years ago, plate tectonics and the development of land masses in which carbon could be stored over a long period of time was just picking up speed,” explained Thorsten Nagel from Aarhus University.

As far as the carbon cycle is concerned, the start of plate tectonics modified everything. Huge landmasses with mountains offered quicker silicate weathering, which transformed CO₂ into limestone.

Moreover, carbon was efficiently trapped in the Earth’s mantle due to the subduction of oceanic plates. Therefore, plate tectonics made the CO₂ content of the air fall sharply. Recurrent ice ages exhibit that it became considerably colder on Earth.

Earlier studies had already indicated that the limestone contents in ancient basalts point to a sharp drop in atmospheric CO₂ levels. This fits well with an increase in oxygen isotopes at the same time. Everything indicates that the atmospheric CO₂ content declined rapidly after the onset of plate tectonics.

Daniel Herwartz, Institute of Geology and Mineralogy, University of Cologne

But in this context, “rapidly” denotes several hundred million years.