Scientists studying leaves from a 23-million-year-old forest have for the first time linked high levels of atmospheric carbon dioxide with increased plant growth, and the hot climate off the time. The finding adds to the understanding of how rising CO2 heats the Earth, and how the dynamics of plant life could shift within decades, when CO2 levels may closely mirror those of the distant past.
Scientists retrieved the leaves from a unique onetime New Zealand lake bed that holds the remains of plants, algae, spiders, beetle, flies, fungi and other living things from a warm period known as the early Miocene. Scientists have long postulated that CO2 was high then, and some plants could harvest it more efficiently for photosynthesis. This is the first study to show that those things actually happened in tandem. The findings were published this week in the journal Climate of the Past.
"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," 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 been building that CO2 was high then, but there have been paradoxes."
The so-called "carbon fertilization effect" has vast implications. Lab and field experiments have shown that when CO2 levels rise, many plants increase their rate of photosynthesis, because they can more efficiently remove carbon from the air, and conserve water while doing so. Indeed, a 2016 study based on NASA satellite data shows a "global greening" effect mainly due to rising levels of human-made CO2 over recent decades; a quarter to a half of the planet's vegetated lands have seen increases in leaf volume on trees and plants since about 1980. The effect is expected to continue as CO2 levels rise.
This might seem like good news, but the reality is more complex. Increased CO2 absorption will not come close to compensating for what humans are pouring into the air. Not all plants can take advantage, and among those who do, the results can vary depending on temperature and availability of water or nutrients. And, there is evidence that when some major crops photosynthesize more rapidly, they absorb relatively less calcium, iron, zinc and other minerals vital for human nutrition. Because much of today's plant life evolved in a temperate, low-CO2 world, some natural and agricultural ecosystems could be upended by higher CO2 levels, along with the rising temperatures and shifts in precipitation they bring. "How it plays out is anyone's guess," said Reichgelt. "It's another layer of stress for plants. It might be great for some, and horrible for others."
The deposit is located in a small, long-extinct volcanic crater now located on a farm near the southern New Zealand city of Dunedin. The crater, about a kilometer across, once held an isolated lake where successive layers of sediments built up from the surrounding environment. The feature was recognized only within about the last 15 years; scientists dubbed it Foulden Maar. Recognizing it as a scientific gold mine, they have been studying it ever since. Some have also been fighting an actual mining company that wants to strip the deposit for livestock feed.