Plants temporarily halted the acceleration of rising carbon dioxide concentrations in the atmosphere, new research suggests.
From 2002 through 2014, CO2 levels measured over the oceans climbed from around 372 parts per million to 397 parts per million. But the average rate of that rise remained steady despite increasing carbon emissions from human activities, researchers report online November 8 in Nature Communications. After pouring over climate measurements and simulations, the researchers attribute this steadying to changes in the relative amount of CO2 absorbed and released by plants.
The work is the first to clearly demonstrate that plants can affect the growth rate of atmospheric CO2 over long time periods, says study coauthor Trevor Keenan, an earth systems scientist at Lawrence Berkeley National Laboratory in California. Still, human emissions remain the dominant driver of CO2 levels, he says. “If we keep emitting as much as we are, and what we emit keeps going up, then it won’t matter very much what the plants do.”
Each year, land plants and the oceans remove about 45 percent of the CO2 emitted from human activities. The amount of CO2 absorbed by these natural processes has doubled over the last 50 years.
Plants absorb CO2 through photosynthesis, a process that is more efficient when CO2 is abundant. Plants also release CO2 during respiration when they tap into their stored-up energy. Warmer temperatures increase respiration rates.
During the early years of the new millennium, the rate of rising CO2 concentrations outpaced the rate of global warming. This caused plants to absorb more CO2 during photosynthesis than they released during respiration. That imbalance slowed the buildup of CO2 in the atmosphere, the researchers propose. While rising CO2 levels had accelerated from a yearly increase of 0.75 parts per million in 1959 to 1.86 parts per million three decades later, between 2002 and 2014 the rate held at around 1.9 parts per million.
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Carbon dioxide concentrations in the atmosphere rose more slowly from 2002 through 2014 than predicted by climate simulations. Rising concentrations of the greenhouse gas had previously accelerated from 1959 through 1988. Natural variability in the 1990s, including the strong 1997–1998 El Niño, makes establishing a clear trend for the intervening interval difficult.
“Almost certainly this effect has already ended,” says David Schimel, a climate scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Thanks in part to the recent monster El Niño (SN Online: 7/16/15), “the last couple years have been the warmest on record. Whatever was going on is done.” Still, the event will help scientists better forecast how plants will affect climate change in the coming decades, he says.