Earth’s plants are soaking up more of our CO2 these days

Study suggests carbon uptake by forests has doubled since the 1950s

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Forests have doubled their CO2 uptake since the 1950s, a new study says. @bberwyn photo.

Staff Report

Scientists say they’ve place yet another piece in the complex global plant-carbon cycle, with a new study suggested that atmospheric CO2 levels have plateaued in recent years because forests and grasslands are removing more of the heat-trapping gas. The research was led by a scientist with the Department of Energy’s Lawrence Berkeley National Laboratory .

Fossil fuel burning and other human activities continue to emit increasing amounts of carbon, but the study found that, between 2002 and 2014, the rate at which CO2 increased in the atmosphere held steady at about 1.9 parts per million annually.

And even though the  proportion of the CO2 emitted annually by human activity that remains in the atmosphere declined by about 20 percent, the slowdown can’t keep pace with emissions, so the overall amount of human-caused CO2 in the atmosphere increased, just not as quickly.  The study is based on ground and atmospheric observations of CO2, satellite measurements of vegetation, and computer modeling and was published online in the journal Nature Communications.

“This highlights the need to identify and protect ecosystems where the carbon sink is growing rapidly,” says Trevor Keenan, a research scientist in Berkeley Lab’s Climate & Ecosystem Sciences Division and the corresponding author of the paper.

The scientists said the stalled CO2 growth rate is linked with an increase in land-based photosynthetic activity, fueled by rising CO2 levels from fossil fuel emissions. In the cycle, CO2 levels rise in the atmosphere, photosynthetic activity flourishes and plants take in more carbon, sparking more plant growth, more photosynthesis, and more carbon uptake.

The researchers said plant respiration also plays a role. Plants use oxygen and produce CO2, but that respiration did not increase as quickly as photosynthesis in recent years. This is because plant respiration is sensitive to temperature, and it was affected by the recent slowdown in global warming that was observed most significantly over vegetated land. So, between 2002 and 2014, plants took in more CO2 through photosynthesis, but did not “exhale” more CO2 into the atmosphere through respiration.

“These changes decreased the amount of anthropogenic CO2 that stays in the atmosphere, and thus slowed the accumulation of atmospheric CO2,” says Keenan.

The findings may help explain why the atmospheric CO2 level has remained flat since 2002. This pause has been surprising surprising because it occurred as human activity pumps more and more carbon into the atmosphere. All that CO2 must be going somewhere, so the scientists suspected something about the carbon cycle has recently changed in a big way.

“We believed one of the planet’s main carbon sinks had unexpectedly strengthened. The question was: which one?” said Keenan.

The scientists said computer models show that oceanic carbon uptake has increased steadily in recent years, so they turned to land-based ecosystems, which have more inter-annual variability. To study these influences, the scientists used ten “global dynamic vegetation models” that predict how the terrestrial carbon cycle changes over time.

Their models used satellite data of vegetation cover and plant activity to show global photosynthesis and respiration rates. They validated the model by comparing its results with data from AmeriFlux and FLUXNET, which are networks of eddy-covariance research towers that measure ecosystem carbon, water, and energy fluxes in North and South America. Berkeley Lab manages AmeriFlux for the Department of Energy.

Taken together, the models zeroed in on rising CO2 levels as having the biggest impact on photosynthesis and plant respiration. The result is a boost in terrestrial carbon uptake, particularly in tropical and high-latitude ecosystems.

Specifically, the models suggest rising CO2 levels caused terrestrial ecosystems to double the rate at which they take in carbon, from between one and two petagrams of carbon per year in the 1950s, to between two and four petagrams of carbon per year in the 2000s. For comparison, human activity emits between nine and ten petagrams of carbon per year (one petagram is one trillion kilograms).

The scientists conclude this increase in carbon uptake put the brakes on the growth rate of atmospheric CO2 between 2002 and 2014.

“Unfortunately, this increase is nowhere near enough to stop climate change,” says Keenan, adding that their results answer questions and pose new ones. “We’ve shown the increase in terrestrial carbon uptake is happening, and with a plausible explanation why. But we don’t know exactly where the carbon sink is increasing the most, how long this increase will last, or what it means for the future of Earth’s climate.”

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