Plants helping to slow global warming by sucking up more CO2 as levels of greenhouse gas rise, study finds

Increase in photosynthesis is ‘not stopping climate change by any means’, scientists stress

Andy Gregory
Thursday 09 December 2021 01:00
<p>A tree stands by a highway near Santarem in the Brazilian Amazon rainforest</p>

A tree stands by a highway near Santarem in the Brazilian Amazon rainforest

The extent to which plants have responded to rising levels of carbon dioxide (CO2) in the atmosphere by sucking up more of the greenhouse gas in recent decades has helped slow – but by no means halt – global warming, a new study suggests.

Despite the “very large” increase in photosynthesis reported by scientists this week – an estimated rise of 12 per cent between 1982 and 2020 – they stressed that this is “nowhere close to removing” the amount of carbon dioxide humans are putting into the atmosphere.

“It’s not stopping climate change by any means, but it is helping us slow it down,” said Trevor Keenan of the Lawrence Berkeley National Laboratory, who is the lead author of the study published in the journal Nature on Thursday.

Furthermore, the scientists warn it is unclear for how long forests will continue to perform this service.

But they do expect it to “saturate” in the future, at which point land sinks – currently “the only nature-based solution that we have in our toolkit to combat climate change” – will have “a much lower capacity to offset our emissions”, Dr Keenan said.

Given that we rely on plants to sequester nearly a third of CO2 emissions released into the atmosphere by human activities each year, the question of how plants are responding to increased levels of CO2 with a rise in photosynthesis – and to what extent – has been of importance to scientists for some time.

Previous efforts to estimate how photosynthesis rates respond to increased carbon dioxide concentrations found widely varying results ranging from little to no effects or very large effects.

“That magnitude is really important to understand,” Dr Keenan said. “If the increase [in photosynthesis] is small, then we may not have the carbon sink we expect”.

In their study, which was supported by Nasa and the US Department of Energy, Dr Keenan’s team of researchers used a new method combining remote sensing, machine learning, and terrestrial biosphere models in an effort to better establish the extent to which plants have responded to rising levels of CO2.

They looked back at nearly three decades of carbon sink estimates made by the Global Carbon Project, comparing these with predictions from satellite images of the Earth taken between 1982 and 2012 and models using carbon exchange between the atmosphere and land to make carbon sink estimates.

“Our estimate of a 12 per cent increase comes right in the middle of the other estimates,” Dr Keenan said. “And in the process of generating our estimate, it allowed us to re-examine the other estimates and understand why they were overly large or small. That gave us confidence in our results.”

According to the study, the 12 per cent increase in photosynthesis – which was accompanied by a 17 per cent rise in levels of atmospheric CO2 over the same period – translates to 14 petagrams of additional carbon taken out of the atmosphere by plants each year. This is roughly the equivalent of the carbon emitted worldwide from burning fossil fuels in 2020 alone.

During photosynthesis, plants suck water and CO2 from the atmosphere, which they convert respectively into water and glucose.

But not all of the carbon taken out of the atmosphere through photosynthesis is stored in ecosystems – much of it is later released back into the atmosphere through respiration. The study did, however, report a direct link between the increase in photosynthesis and increased global carbon storage.

The findings highlight the importance of protecting ecosystems that are currently helping slow down the rate of climate change, the scientists said.

But not only do the droughts and extreme heat brought by climate breakdown make plants less effective at absorbing CO2, they also bring the possibility of widespread destruction, as most obviously evidenced by increasingly devastating wildfires around the world in recent years.

And Dr Keenan warned that it is highly unlikely that plants will continue to respond to rising CO2 levels as they have done in the past.

“We don’t know what the future will hold as far as how plants will continue to respond to increasing carbon dioxide,” he said. “We expect it will saturate at some point, but we don’t know when or to what degree.

“At that point land sinks will have a much lower capacity to offset our emissions. And land sinks are currently the only nature-based solution that we have in our toolkit to combat climate change.”

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