CO2 last 15 million years ago as high as today

30 meters higher sea levels in the greenhouse of the middle Miocene

Life picture from the Miocene: Savannahs dominated © Keith Schengili-Roberts / GFDL
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So far it has only been clear that carbon dioxide concentrations in the atmosphere are today as high as never before in the last 800, 000 years. Now, for the first time, a researcher has identified when there was an era of comparably high values: 15 million years ago, a time when there was no Arctic ice and sea levels were up to 30 meters higher. As she reports in "Science", this could blossom again soon.

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When was the carbon dioxide content of the atmosphere last as high as it is today? So far no one has answered this question conclusively. Because ice cores and the air trapped in them, the only more accurate clue for past atmospheric compositions, only reach back some 800, 000 years. During this time, the values ​​varied only between 180 and 300 particles per million parts of air (ppm). Today, however, the concentration of CO2 in the atmosphere is already at 387 ppm - and the trend is still upward.

Element ratio in single cell dishes

To explore the impact of such values ​​on the climate and the environment, looking back at similar conditions in the Earth's history is very helpful. For the first time, Aradhna Tripati, Assistant Professor of Earth Sciences at the University of California, Los Angeles, and colleagues have given such a look back. At the University of Cambridge in England, she had developed a method by which it was possible to accurately determine the relationship between the chemical elements calcium and boron in the shells of fossilized unicellular algae, and to deduce their conditions during their lifetime.

Tripati now used this method for the first time to determine the content of atmospheric carbon dioxide down to 20 million years back. As a test, she first determined the values ​​for the last 800, 000 years and compared them with the results from the ice cores. When they agreed, confirming that the technique worked, she went back further. display

Similar values ​​15 million years ago

"The somewhat shocking discovery is that the only time in the last 20 million years that carbon dioxide levels were similar to those of the modern level is 15 to 20 million years ago - at a time when this planet is dramatically different was as today, "said Tripati. "During the mid-Miocene, CO2 levels were at around 400 ppm for a long time, almost the current level. At that time, global temperatures were five to six degrees higher than today and sea levels were 22 to 30 meters higher. There was no permanent sea ice cover in the Arctic and very little ice in Greenland and the Antarctic.

Greenhouse without ice

The study thus clearly shows that the carbon dioxide in the atmosphere and this already at an increase of around 100 ppm can cause severe climatic and ecological changes. The greenhouse world of the Miocene could, in the opinion of the researcher, soon become ours too, if the current development continues. There are forecasts that CO2 levels could even rise to 600 and more ppm in the next century if human emissions of greenhouse gases continue as before.

In the next step, Tripati will not only continue to forecast the results for the last 20 million years, but is also working to extend the period to 50 million years. At that time, after the end of the Cretaceous, there were no ice caps or glaciers on the entire planet, desert dominated large areas of the subtropics.

(University of California - Los Angeles, 09.10.2009 - NPO)