Young Earth was heated by extreme levels of atmospheric CO2

Three to four billion years ago, a young Earth was heated up by extremely high atmospheric CO2 levels, according to a new study from the University of Cologne. The greenhouse gas effect may also explain why ocean temperatures were high at this time. 

Despite the fact that the sun shone on the young Earth with only 70 to 80 percent of its current intensity, the climate must have been quite warm because there was hardly any glacial ice. This phenomenon is referred to as the “paradox of the young weak Sun.” 

If it was not for the presence of a potent heat-trapping greenhouse gas, the young planet would have frozen into a lump of ice. But whether it was CO2, methane, or an entirely different greenhouse gas is a matter of debate among scientists. The new research suggests that high CO2 levels are a plausible explanation.

A long-standing question in Earth science concerns the temperatures of the early oceans. Measurements of oxygen isotopes on very old limestone rocks indicate that seawater temperatures exceeded 70 degrees Celsius. Model simulations in the new study show that high CO2 levels in the atmosphere may be responsible for heating up the oceans.

“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,” said Dr. Daniel Herwartz. 

It is also possible that there was also a lot of methane in the young Earth’s atmosphere, but this would not have had any effect on the composition of the ocean. “Both phenomena can only be explained by high levels of CO2,” said Dr. Herwartz. 

The study authors estimate the total amount of CO2 to have totalled approximately one bar, or the equivalent of today’s entire atmosphere being filled with 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,” explained Andreas Pack from the University of Göttingen.

On our planet, CO2 was eventually removed from the atmosphere and the ocean and stored in the form of coal, oil, gas, limestone, and black shales. Since early Earth was largely covered by oceans, the storage capacity for carbon was limited. 

“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.

The onset of plate tectonics changed everything for the carbon cycle. Large land masses with mountains provided faster silicate weathering, which converted CO2 into limestone. Carbon also became trapped in the Earth’s mantle as oceanic plates were subducted. 

Plate tectonics caused the CO2 content of the atmosphere to drop sharply, and repeated ice ages show that Earth became significantly colder. 

“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,” explained Dr. Herwartz.

“Everything indicates that the atmospheric CO2 content declined rapidly after the onset of plate tectonics.” In this context, however, “rapidly” refers to several hundred million years.

The study is published in the journal Proceedings of the National Academy of Sciences.

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By Chrissy Sexton, Earth.com Staff Writer