Fossil leaves prove elevated CO2 triggered greening 23M years ago

The links between rising carbon dioxide levels, global warming and greening trends have been confirmed by fossilized leaves from a 23 million-year-old forest.

The leaves were found preserved in the sediment layers that once formed the bottom of a New Zealand lake.

Paleontologists previously unearthed a diversity of plants, algae, spiders, beetle, flies and fungi from the ancient lake bed, found inside an ancient crater called Foulden Maar.

Scientists previously postulated that ancient increases in atmospheric CO2 during the early Miocene allowed plants to perform photosynthesis more efficiently. But the latest research, published Thursday in the journal Climate of the Past, is the first to confirm the link between CO2 and greening in the fossil record.

"The amazing thing is that these leaves are basically mummified, so we have their original chemical compositions, and can see all their fine features under a microscope," lead author Tammo Reichgelt, an adjunct scientist at Columbia University's Lamont-Doherty Earth Observatory, said in a news release.

"Evidence has been building that CO2 was high then, but there have been paradoxes," said Reichgelt, an assistant professor of geosciences at the University of Connecticut.

Lab experiments have shown increases in CO2 can boost photosynthesis, and recent satellite surveys suggest rising CO2 levels are responsible for greening patterns across the planet, including Arctic and drylands ecosystems.

The latest research suggests that greening trends are likely to continue as CO2 levels approach those recorded during ancient period of warming.

But not all plants can easily increase photosynthesis rates, scientists warn. Greening patterns are also dictated by the availability of water and nutrients.

According to the new study, increases in photosynthesis rates won't be able to keep up with current rates of human-caused carbon emissions. In addition, previous studies suggest increases in rates of photosynthesis can prevent staple crops from absorbing calcium, iron, zinc and other minerals important for human health.

Until now, scientists have been confused conflicting paleoclimate signatures found in Miocene deposits. Though previous studies showed temperatures during the period were 3 to 5 degrees C warmer than today, the chemical signatures trapped among the remains of marine organisms showed CO2 levels were just 300 parts per million -- less than expected.

Leaves from the ancient lake bed sediment cores showed CO2 levels were significantly higher. By comparing the fossilized leaf structures, including microscopic veins, stomata and pores, to those of modern leaves, researchers designed a model to more accurately predict CO2 levels.

Their analysis showed atmospheric CO2 levels rose as high as 450 parts per million during the Miocene.

The analysis also showed trees in the Foulden Maar forest were able to absorb larger amounts of CO2 without requiring the same levels of water absorption, allowing them to grow in marginal areas -- places previously too dry to host large plants.

Today, CO2 levels in the atmosphere measure 415 ppm. By 2040, they will likely reach 450 ppm. Researchers suggest the latest study will help climate scientists improve the accuracy of their models.

"It all fits together, it all makes sense," said study co-author William D'Andrea, a paleoclimate scientist at Lamont-Doherty. "This should give us more confidence about how temperatures will change with CO2 levels."

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