As carbon dioxide climbs, photosynthesis ramps up

Land ecosystems, like this forest in Ontario, Canada, can be effective sinks for carbon dioxide in the atmosphere. But many such sites of green growth are also disappearing around the world. Photo: James Wheeler via Flickr
Nov 2016

New research shows that the land “carbon sink,” or the carbon dioxide absorbed by forests, grasslands and other ecosystems around the world, has grown in recent years – helping to slow down the build up of greenhouse gases in the atmosphere.

There may be a bright spot in the global climate change story: New research suggests that the planet’s land ecosystems, from forests to prairies, are absorbing more carbon dioxide from the atmosphere than they did several decades ago.

And it seems to be having an effect on the skies. The study, which combines data taken on the ground and from space, finds that carbon dioxide continues to build up in the atmosphere. But it’s not building up as fast as it should be, especially given the volume of carbon dioxide that humans are dumping into the air on a yearly basis. The reason seems to be that plants are conducting more photosynthesis, and packing on more green material, than they did in the 1980s and 90s – likely because excess carbon dioxide in the air is fueling their growth.

The results come at the same time as the Global Carbon Project, a global research project of Future Earth, released its 2016 Global Carbon Budget. The project found that for the third year in a row, the carbon dioxide emissions of nations around the world haven’t increased, or not by much. Taken together, the two reports may suggest a much needed silver lining for the world.

“Carbon dioxide in the atmosphere is increasing. That’s a given,” says Trevor Keenan, the lead author of the study and research scientist at the Lawrence Berkeley National Lab in California. “That it’s not increasing faster than it used to is novel and quite good news.”

But Keenan also cautions that there’s a limit to how much carbon the planet’s green life can take up. That’s especially true in an age when forests and other ecosystems are disappearing at surprising rates. He published his results in the journal Nature Communications. Researchers from the Global Carbon Project also contributed to the study.

A delicate balance

The study probes the razor-thin balance in how carbon cycles in and out of the atmosphere. Every year, during the growing season, the world’s plants, from deciduous oak trees in high latitudes to creeping vines in the tropics, absorb carbon dioxide as they conduct photosynthesis – like taking in a deep breath. But ecosystems also exhale, or respire. Specifically, microbes in the soil consume organic material that had been stored in places like forests or grasslands and expel it back to the atmosphere as carbon dioxide. 

The gap between these two processes is small, Keenan says. Every year, the world’s land ecosystems absorb about 120 to 130 billion tonnes of carbon dioxide from the atmosphere – more than 20 times the annual emissions of carbon from the U.S. But they also expel about 117 billion tonnes, leaving a small gap that scientists call the land “carbon sink.” A similar sink exists in the oceans.

And they’re critical to the future of climate change, says study co-author Josep Canadell. He is the Executive Director of the Global Carbon Project and a research scientist at Commonwealth Scientific and Industrial Research Organisation (CSIRO) Oceans and Atmosphere in Canberra, Australia. “A little more than half of all anthropogenic carbon dioxide emissions are removed by natural sinks,” he says. “Therefore, the dynamics of these sinks into the future are above and beyond incredibly important.”

And there’s good reason to think that those dynamics are shifting on land. That’s because of a phenomenon that scientists call the “carbon fertilisation effect.” Because carbon dioxide is key to photosynthesis, if you give plants on land more of the gas, they are often able to grow more – why some plant nurseries pump extra carbon dioxide into their greenhouses.

To get a sense of whether that is happening on a global scale, Keenan, Canadell and their colleagues collected data on global plant growth and information from on-the-ground research – small-scale studies that record how carbon cycles in and out of ecosystems in various locations. They then used computer models to merge the two types of data and compared their results to measurements of carbon dioxide in the air up to 2014.

During the second half of the 20th Century, carbon dioxide built up in the atmosphere at a careening pace – each year, on average, added more carbon to the atmosphere than had been added the year before. But between 2002 and 2014, the team discovered, that acceleration seemed to taper off. Carbon dioxide is still accumulating in the air, but it’s doing so at a steady pace, despite the smog blowing out from factories. As Keenan puts it: “The rate of increase is not slowing, but it’s not speeding up, either.”

And that, in turn, is mirrored by what’s happening on the ground. Since 1900, the team estimates, plants have increased how much carbon dioxide they take up by about 18 billion tonnes. Ecosystems have also increased how much they respire, but by less – about 13 billion tonnes. As a result, the gap between what ecosystems are taking in and what their spitting out is widening. Plants, in other words, are helping to make up for humanity’s mistakes.

Reaching saturation

On the surface, that’s good news. But it won’t last forever, Keenan says. Like a glutton at a buffet, there’s only so much carbon dioxide a plant can absorb before it reaches its limits. What that maximum is for entire ecosystems isn’t clear, but the world may hit it at some point. On the other hand, there’s no limit to how much ecosystems can respire. “So it’s not inconceivable that we may reach a point where plants don’t benefit from increasing carbon dioxide anymore, yet ecosystems continue to respire more and more carbon dioxide back into the atmosphere,” he says.

And, in fact, data from the 2016 Global Carbon Budget shows that after 2014, the last year of Keenan’s research, the uptake of carbon dioxide by land plants plummeted – what Josep Canadell of the Global Carbon Project calls an “almost collapse of the land sink.” He credits that dive to the recent El Niño, which brought hot, dry weather to much of the world.

In all, both scientists say that the study underscores the importance of protecting ecosystems that are strong carbon sinks – or that take in much more carbon dioxide than they give off. The problem is that scientists still aren’t sure which forests or grasslands do a better job of absorbing greenhouse gases than others. It’s a critical priority for research, Keenan adds, as many nations have included conserving wild areas in their plans to combat climate change.

“Where are these sinks? We don't really know. … We don’t have predictive power to tell how these ecosystems are going to evolve into the future,” Keenan says. “We have countries all over the world designing mitigation strategies with the best intentions, but designing them with very little science, perhaps.”