Three to four billion years ago the sun shone with only 70 to 80 percent of its current intensity. Nevertheless, the temperatures on the young earth were high back then, which researchers conclude from the fact that there was hardly any glacier ice. The reason for this was apparently a strong greenhouse effect. A new study now shows that the greenhouse gas responsible for this was likely carbon dioxide. On the one hand, a high CO2 content in the atmosphere explains the high temperatures on the young earth in the model. On the other hand, it also sheds new light on calculations of the temperature of the primeval oceans.
How could there be liquid water on the young earth when the sun shone up to 30 percent less than it does today? This so-called paradox of the young, weak sun has preoccupied scientists for decades. Greenhouse gases were suggested as an explanation before, but previous model calculations remained contradictory. One unanswered question, for example, was the temperature of the oceans. Calculations of the water temperature at that time are based on the ratio of different oxygen isotopes in very old limestone and silica rock. The heavy oxygen-18 isotope indicates higher water temperatures than the lighter oxygen-16 isotope. According to classic calculations, the oceans must have been around 70 degrees Celsius three to four billion years ago.
New model for the climate of the young earth
A team led by Daniel Herwartz from the University of Cologne has now come to different results based on a new model. Accordingly, it would be plausible that the isotopic composition of seawater has changed in such a way that previous calculations based on it are falsified. Such a change was previously considered unlikely. If the atmosphere at that time contained an extremely large amount of CO2, which was gradually stored on earth in the form of lime, the heavy oxygen-18 isotope in particular would have been bound into the rock during this process, according to the authors.
“High CO2 contents would thus explain two phenomena at the same time: on the one hand, the warm climate on earth and, on the other hand, why the geothermometers that are often used indicate apparently hot sea water. If you take into account the other oxygen ratio in seawater, the result is more likely a temperature of 40 ° C, ”explains Herwartz. The high temperatures on earth can theoretically also be explained by other greenhouse gases such as methane. However, these would have had no influence on the oxygen isotopes in seawater. A greenhouse effect from other gases would not explain why the oxygen geothermometer delivers too high temperatures. “Both phenomena can only be explained with a lot of CO2,” says Herwartz.
A lot of CO2 in the atmosphere
Using their model, the authors also estimated the total amount of CO2 in the atmosphere at that time. According to this, the CO2 gas pressure in the atmosphere of the young earth was about one bar – as much as if our entire atmosphere today consisted of CO2. “Today CO2 is just a trace gas in the atmosphere. Compared to that, a bar of CO2 sounds like an absurd amount. But if we look at our sister planet Venus with about 90 bar CO2, that is put into perspective, ”explains co-author Andreas Pack from the University of Göttingen.
In the further course of the earth’s history, the CO2 content of the atmosphere and the oceans gradually decreased. The emerging plate tectonics played an important role. Large storage facilities for CO2 were mainly possible on land – for example in the form of coal, oil, gas, black shale and limestone. However, the young earth was largely covered by oceans, so that the storage capacity for carbon was also limited. “This also explains the enormous amount of CO2 in the young earth from today’s perspective,” says co-author Thorsten Nagel from Aarhus University in Denmark. “Because roughly three billion years ago, plate tectonics and the development of land masses, in which carbon could be stored for a long time, were only just picking up speed.”
Plate tectonics reduced CO2 levels
The plate tectonics subsequently ensured that large land masses with mountains emerged. When the rocks weathered, CO2 was converted into limestone and thus permanently bound. When plates pushed on top of each other, bound CO2 was also transported deep into the earth’s mantle. Such processes significantly reduced the CO2 content in the atmosphere and the greenhouse effect decreased. This was partly offset by the fact that the sun’s radiance increased at the same time. Nevertheless, it became significantly colder on earth and ice ages occurred more frequently. Scientists used to see precisely this cooling as an argument against the thesis of the high CO2 content of the atmosphere at the time. Because if the CO2 level had only fallen slowly, it would have had to stay warm on earth much longer.
In the new model, however, the existing data come together to form a consistent picture, in which the CO2 initially enabled high temperatures on earth and then fell significantly with the advent of plate tectonics. “Earlier studies had already indicated that the limestone content in old basalts indicates a strong decrease in the atmospheric CO2 content. That goes well with an increase in the oxygen isotopes at the same time, ”says Herwartz. “Everything indicates that the CO2 content of the atmosphere decreased rapidly after the beginning of plate tectonics” – whereby “fast” in this context means several hundred million years.
Source: Daniel Herwartz (University of Cologne) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2023617118