Fossil leaves allow high atmospheric carbon to encourage ancient ‘global greening’

Scientists studying leaves from a 23-million-year-old forest have for the first time high levels of atmospheric carbon dioxide associated with increased vegetation, and the hot climate of the time. The finding adds to the understanding of how increasing CO₂ warms the earth, and how the dynamics of plant life could shift within decades, when CO₂ levels can look closely at those of the distant past.

Scientists have picked up the leaves of a unique single-sided New Zealand multi-bed containing the remains of plants, algae, spiders, beetles, flies, fungi and other living things from a warm period known as the Early Miocene. Scientists have long postulated that CO₂ was then high, and some plants could harvest it more efficiently for photosynthesis. This is the first study to show that these things actually happened in tandem. The findings were published in the journal this week Climate of the past.

“The great 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,” said lead author Tammo Reichgelt, an adjunct scientist at Columbia University’s Lamont-Doherty Earth Observatory and Assistant Professor of Geosciences at the University of Connecticut. “Evidence has built that CO₂ was high then, but there have been paradoxes. “

The so-called “carbon fertilization effect” has major implications. Lab and field experiments have shown that when CO₂ levels up, many plants increase their rate of photosynthesis because they can more efficiently remove carbon from the air, and retain water while doing so. Indeed, a 2016 study based on NASA satellite data shows a “global green” effect mainly due to increasing levels of manpower CO₂ in the past decades; a quarter to a half of the plant’s plant land has increased in leaf volume on trees and plants since about 1980. The effect is expected to continue as CO₂ levels up.

This may sound like good news, but the reality is more complex. Increased CO₂ absorption will not come close to compensating for what people go through in the air. Not all plants can benefit, and among those who do, the results may differ depending on temperature and availability of water as nutrients. And there is evidence that as some important organisms rapidly photosynthesize, they absorb relatively less calcium, iron, zinc and other minerals that are essential for human nutrition. Because much of today’s plant life evolved into a temperate, low CO₂ world, some natural and agricultural ecosystems could be increased by higher CO₂ levels, along with the increasing temperatures and shifts in precipitation they bring. “How it plays out is anyone’s guess,” Reichgelt said. “It’s another layer of stress for plants. It can be great for some, and terrible for others.”

The castle is located in a small, long-extinct volcanic crater now on a farm near the southern New Zealand city of Dunedin. The crater, about a kilometer across, once held an isolated lake, on which successive layers of sediments were built from the surrounding environment. The feature has only been recognized within about the last 15 years; scientists called it Foulden Maar. They recognize it as a scientific gold mine and have been studying it ever since. Some have even fought against an actual mining company that wants to strip the bail for livestock.

In the new study, the researchers took samples from a 2009 drill core that ran 100 meters to near the bottom of the now dry mare bed. Boundary between whitish annual layers of silica-rich algae that bloomed every spring for 120,000 years are alternating blackish layers of organic matter that fell in other seasons. These include countless leaves from a subtropical evergreen forest. They are so perfectly preserved that scientists can see microscopic veins and stomata, the pores through which leaves absorb air and at the same time release water during photosynthesis. Unlike most fossils, the leaves also retain their original chemical compositions. It is the only such well-known castle in the southern hemisphere, and has been far better preserved than the few similar ones known from the north.

The Miocene has long been a source of confusion for paleoclimate researchers. Average global temperatures are thought to be 3 to 7 degrees C hotter than today, and ice has largely disappeared near the pulse. However, many proxies, especially derived from marine organisms, have suggested CO₂ levels were only about 300 parts per million — comparable to those of pre-industrial mankind, and not enough to account for such warming. With evidence of high CO₂ misleadingly, scientists have speculated that previous proxy measurements should be disabled.

Based on the new study and a related previous one also at Foulden Maar, the researchers were able to arrive at this conundrum. They analyzed the carbon isotopes inside leaves of half a dozen trees found at various levels in the burrow. This helped them at time zero in on the carbon content of the atmosphere. They also analyzed the geometry of the leaf stomata and other anatomical features, and compared these with modern leaves. By combining all the data in one model, they found that atmospheric CO₂ was not 300ppm, but about 450 — a good match for the temperature data. Secondly, they showed that the trees were super efficient at sucking carbon through the stomata, without leaking much water through the same route – a factor for which all plants should be responsible. This allowed them to grow in marginal areas that would otherwise be too dry for forests. The researchers said that this higher efficiency was probably reflected in forests across the northern temperate latitudes, with their much larger land masses.

Human emissions have now shifted CO₂ levels up to about 415 parts per million, and they will almost certainly reach 450 by about 2040 identical to those experienced by the Foulden Maar forest. Estimates of the resulting temperature increases over decades and centuries vary, but the new study suggests that most are in the ballpark.

“It all fits together, it all makes sense,” said student co-author William D’Andrea, a paleoclimate scientist at Lamont-Doherty. Next show how plants can react directly to CO₂, “this should give us more confidence in how temperatures will change with CO₂ levels, “he said.

Study co-author Daphne Lee, a paleontologist at the University of New Zealand of Otago, dropped the charge to study the rich ecosystem of Foulden Maar after it came to light. More recently, she became an unexpected defender of the lake, when a company with owners in Malaysia and the United Kingdom announced plans to mine the castle for use as a food additive for barges, ducks and other intensive local animals. With many more discoveries likely to be made, scientists were horrified, associating them with locals who were afraid of noise and dust. Dunedin City Council is now considering buying the land to protect it.