Plants buy us time to slow climate change—but not enough to stop it

Forests and similar ecosystems are one of the most important carbon sinks on the planet because they absorb carbon dioxide from the atmosphere and turn it into food. The Paris Agreement’s carbon dioxide mitigation framework has nature-based solutions as a key component. This is the case for the United States and other countries that took part in the UN Climate Change Conference last week.

Human activities are causing more carbon dioxideTo be emitted into atmosphereScientists have debated whether plants are responding by photosynthesizing less and sucking up more. carbon dioxide than they already do—and if so, is it a little or a lot more. An international team of researchers led jointly by Lawrence Berkeley National Laboratory (Berkeley Lab), UC Berkeley and UC Berkeley used a novel method that combined remote sensing and machine learning with terrestrial biosphere models to show that plants are actually photosynthesizing higher, up to 12% more globally. photosynthesisFrom 1982 to 2020. Global carbon dioxide concentrations increased by 17% during the same period, from 360 parts per Million (ppm) up to 420ppm.

The 12% increase of photosynthesis results in 14 petagrams more carbon being removed from the atmosphere each year by plants. This is roughly equivalent to the carbon emitted globally from fossil fuels. in 2020 alone. While not all carbon removed from the atmosphere through photosynthesis is stored within ecosystems, some is later released back into the atmosphere through respiration. The study however reports a direct connection between increased photosynthesis, and increased global carbon storage. The studyPublished in Nature

“This is an extremely large increase in photosynthesis but it’s nowhere near removing the amount carbon dioxide we’re putting in the atmosphere,” said Trevor Keenan from Berkeley Lab, the study’s lead author. “It’s not going to stop climate change in any way, but it is helping to slow it down.”

Photosynthesis measurement

It is crucial to reduce carbon dioxide, as it remains in the atmosphere for longer periods of time than other greenhouse gases that are driving global warming. Photosynthesis and soils capture around a third the carbon dioxide that is released into the atmosphere every decade from the burning fossil fuels.

Plants open tiny pores on the leaf surfaces during photosynthesis to absorb carbon dioxide from the atmosphere and make their own food. Scientists can place a leaf inside a closed chamber to measure the amount of carbon dioxide that is being released. However, it is much more difficult than measuring how much carbon dioxide a whole forest absorbs.

Initiatives like AmeriFluxA network of measurement sites coordinated in part by the Department of Energy’s AmeriFlux Management Project Berkeley Lab, scientists have built over 500 micrometeorological buildings in forests and other ecosystems around the globe to measure the exchange of greenhouse gasses between the atmosphere, soil, and vegetation. Although these flux towers can be used to estimate photosynthesis, they are costly and therefore limited in their geographic coverage. And, very few have been installed long-term.

This explains why scientists rely upon satellite images to map how green the Earth is and what plants are covering it. This allows them infer global photosynthetic activity. However, these estimates are no longer based on greenness.

History in the picture

Satellite images can capture extra green to account the increased number of leaves plants produce due to their rapid growth. They don’t always account for the increased efficiency of each leaf to photosynthesize. This efficiency does not increase at the same pace as carbon dioxide builds up in atmosphere.

Previous attempts to determine how increased carbon dioxide concentrations affect photosynthesis rates have produced wildly varying results. These ranged from very little to no effects at the low end to large effects at the high end.

Keenan, an assistant professor at UC Berkeley’s Department of Environmental Science, Policy and Management, said, “That magnitude is really important for understanding.” “If the increase [in photosynthesis]If the carbon sink is not large enough, we may not be able to achieve the carbon sink we desire.”

Keenan and his team of researchers tried a new approach. They looked back at almost three decades worth of carbon sink estimates by the Global Carbon Project. They compared them with predictions from satellite imagesTaken between 1982-2012 and models using carbon exchange between land and atmosphere to calculate carbon sink estimates.

He said, “Our estimate of a 12 percent increase is right in the middle” of all the other estimates. “And it allowed us to re-examine other estimates and understand why they were too large or small during the process of creating our estimate. This gave us confidence and helped us to be confident in our results.

This study highlights the importance to protect ecosystems that are helping to slow down climate change. However, Keenan points out that it’s not clear how long forests will continue to provide this service.

“We don’t know what the future holds as far as how.” plantsHe said that the body will continue to react to rising carbon dioxide.” “We expect it to saturation at some point, however we don’t know when nor how high. At that point, land sinks will have a much smaller capacity to offset our emission. We currently have no other solution than land sinks to combat climate change. climate change.”