Miraculously well-preserved leaves from a 23-million-year old forest are revealing to scientists how Earth’s plant species might respond to rising levels of carbon dioxide (CO₂). Published in the journal Climate of the Past, the new study looked at leaves from the early Miocene when CO₂ levels were believed to have been high and discovered that these plants were able to harvest the increase, effectively booming as the concentration got higher. This is the first time high levels of atmospheric carbon have been linked to increased plant proliferation, which the researchers say has unpredictable ramifications as it could put more strain on already struggling species while others thrive.

The researchers took samples from a 2009 drill core that penetrated 100 meters (330 feet) into a now dry lake bed in Dunedin, New Zealand. Dubbed Foulden Maar, it sits inside a small, long-extinct volcanic crater and is recognized as a scientific gold mine for research into Earth’s past. Its layers of sediment are made up of silica-rich algae and alternating blackish layers of organic matter that fell in during other seasons, essentially trapping time-capsules of data into Earth’s past climate in its layers.

These layers contain countless leaves from a subtropical evergreen forest that are so well preserved scientists can still see microscopic details such as the veins and stomata, pores that take in air and release water in photosynthesis. Leaves are particularly informative when it comes to the environment they “lived” in as they maintain their original chemical composition unlike fossilized bones.

The team analyzed the carbon isotopes within the preserved leaves from several tree species that were found at differing layers in the sediment deposit, which gave an indication of how much atmospheric carbon existed at the time. They also compared the leaves' anatomical features against those of modern leaves and determined that the atmospheric carbon was around 450 parts per million. This result is a good match for the temperature data for the early Miocene, which predicts it was around 5-6°C (9-10.8°F) warmer than conditions today.

The leaves revealed these trees were super-efficient at absorbing carbon through the stomata without losing too much water in the process, enabling them to grow in areas that otherwise would have been too dry for forests. The researchers believe this adaptation was likely mirrored in forests across the northern temperate latitudes.

Atmospheric carbon is currently around 415 parts per million and is expected to reach 450 by around 2040 thanks to human-caused emissions. This means that plant species may begin to behave in the same way the ancient species did in Foulden Maar, potentially indicating that a global greening event is on the horizon.

"It all fits together, it all makes sense," said study co-author William D'Andrea, a paleoclimate scientist at Lamont-Doherty,  in a statement. In addition to showing how plants might react directly to CO₂, "this should give us more confidence about how temperatures will change with CO₂ levels.”