Researchers reject 30-year-old paradigm: The emergence of forests did not reduce the amount of CO2 in the atmosphere

Today, a world without trees is hard to imagine. But that was nonetheless the case before forests and humans emerged, back when the land was home to shallow shrub-like plants.

For many years, researchers all over the world have assumed that it was the trees that caused CO₂ levels in the atmosphere to decrease and the climate on Earth to cool.

Until today at least. Now researchers from the University of Copenhagen, using a new method, have learned that the small vascular plants that populated the Earth before the evolution of trees 385 million years ago may have played a greater role than previously assumed.

“A new method has enabled us to calculate the CO₂ level in the atmosphere in the past based on plant fossils. This enables us to understand climate changes when CO₂ levels increase. Among other things, the first results suggest that small changes to the CO2 level may have a greater effect on the climate than previously thought,” says Associate Professor Tais W. Dahl from the Globe Institute at the University of Copenhagen.

The study can help us understand the processes controlling the climate on Earth and e.g. the impact of trees on the climate today.

“We initially applied the method to the time before forests emerged – a time which researchers agreed was characterised by high levels of CO₂ in the atmosphere. We used to think that the emergence of forests reduced the amount of atmospheric CO₂ on Earth. But instead of 4,000 ppm, which is the amount researchers assumed was found on the planet back then, we have shown that the figure is close to 600 ppm, which is not far from the level we are approaching today,” says Tais W. Dahl and adds:

“The fact that the CO₂ level has not changed that much may mean that the climate is more sensitive to changes in atmospheric CO₂ levels than previously assumed. The Earth’s climate sensitivity is a much debated topic, because it is affected by various processes such as cloud formation, water vapour and ocean currents, which may intensify changes in temperature.”

By studying both live plants and ancient plant fossils, the researchers are now able to calculate the amount of CO₂ in the atmosphere at the time where the fossils were alive. The new method makes it possible to calculate CO₂ levels of previous climates more accurately.

We need to focus on “weathering”

Tais W. Dahl and his colleagues do not believe trees are as good at removing CO₂ from the atmosphere as previously believed. The vascular plants with shallow roots that existed before trees emerged do this much more effectively.

“We believe the emergence of primitive vascular plants, not forests, causes the greatest reduction in CO₂ levels. This is because vascular plants have shallow roots which, unlike tree roots, cannot hold well on to nutrients in the ground and therefore need to extract more nutrients from the subsoil through weathering of minerals,” says Tais W. Dahl.

And weathering plays a key role in understanding how we can reduce the level of CO₂ in the atmosphere.

“Plants remove CO₂ from the atmosphere for two reasons. The first is probably the one we are most likely to think of, namely that the plant itself absorbs CO₂ and turns it into sugars. But plants also absorb nutrients from the soil or rock they live on by dissolving rock particles through chemical weathering. For this process, plants need CO₂ to dissolve minerals. This is actually the primary reason why plants and trees reduce CO₂ levels over longer periods of time,” explains Tais W. Dahl.

It is also the reason why trees may not be as significant as Tais W. Dahl and Earth scientists previously thought, because trees are not as good at weathering as small plants.

Finding solutions is key. But to find solutions, we need to know as much as possible about the processes that can potentially remove CO₂ from the atmosphere.

“Early plants needed to extract more nutrients from minerals in the ground than trees, and this led to more weathering. Trees have well-functioning root systems, which helps them hold on to nutrients and create a more closed system. Therefore, over time, primitive plants do more weathering than trees, and it is mainly through weathering that trees and plants help remove CO₂ from the atmosphere,” explains Tais W. Dahl.

A step on the road to more solutions

Does that mean that we should start planting a lot of vascular plants – instead of trees and forests – to curb the amount of CO₂ in the atmosphere? “Not quite”, says Tais W. Dahl.

“Trees are good for other reasons; biodiversity is one. And then we need to remember that the solution to present-day climate changes is not simply more weathering. Much more is needed to counter climate changes.”

The study increases our knowledge of what controls atmospheric CO₂ levels and how sensitive the climate is to changes to the amount of greenhouse gases in the atmosphere. And then it can help give us an idea of how natural processes affect CO₂ levels.

“Finding solutions is key. But to find solutions, we need to know as much as possible about the processes that can potentially remove CO₂ from the atmosphere, e.g. planting trees and increased weathering,” says Tais W. Dahl and adds:

“To understand how this works on a global scale, and what the consequences are, it is a good idea to look at what happened in the past when the Earth saw major changes and these mechanisms changed. And that is what this study does.”

The study, “Low atmospheric CO₂ levels before the rise of forested ecosystems”, was conducted together with an international team of researchers. It has been published in Nature Communications.

Contact:

Associate Professor Tais W. Dahl
tais.dahl@sund.ku.dk
+45 35 32 23 56

Journalist and Press Officer Sascha Kael Rasmussen
sascha.kael.rasmussen@sund.ku.dk
+45 93 56 51 68