We might have to treat our humble house plants to an extra helping of sunlight this week. As human-induced CO2 emissions continue to increase, a new study suggests that the Earth's vegetation has upped its game to help offset that growth. After a 40-year upward trend, the rate at which atmospheric CO2 levels increased hit a plateau between 2002 and 2014, thanks largely to plants plucking more CO2 out of the air than they have previously.

In 1959, the rate of growth of carbon dioxide levels in the atmosphere sat at about 0.75 parts per million (ppm) per year, and over the second half of the 20th century, that figure increased rapidly, up to 1.86 ppm per year in 2002. But in the years since then, the rate of growth has flatlined, despite the fact that human activity continues to pump out more and more CO2. That's not to say the amount of CO2 in the atmosphere isn't still going up – it is – but the rate of increase has slowed, remaining steady on about 1.9 ppm each year since 2002.

Not only that, but the amount of anthropogenic CO2 that stays in the air has dropped by around 20 percent. So where is it all going? New research, headed up by the Department of Energy's Lawrence Berkeley National Laboratory, analyzed data from the Global Carbon Project and ran computer models to determine what changes may be occurring in Earth's carbon cycle.

This graph shows the steady CO2 growth rate over the second half of the 20th century, and an unexpected plateau from 2002 onwards(Credit: Berkeley Lab)

"We believed one of the planet's main carbon sinks had unexpectedly strengthened," says Trevor Keenan, a corresponding author of the paper. "The question was: which one?"

Oceans are considered a carbon sink, but computer models indicate their rate of absorption remains fairly stable. Instead, the team looked to vegetation, which can fluctuate year-to-year in terms of carbon uptake, and tested 10 "global dynamic vegetation models," which simulate different combinations of atmospheric CO2 levels, temperature, soil moisture, and other factors, to see how that carbon cycle might change over time.

From those models, it appears that over the decades these ecosystems have effectively doubled the rate of CO2 they absorb. In the 1950s, the Earth's vegetation absorbed between 1 and 2 petagrams (1 trillion to 2 trillion kg, or 2.2 to 4.4 trillion lb) of CO2 each year, but in the 21st century that number had risen to between 2 and 4 petagrams annually.

Interestingly, the models suggest that the increase in CO2 levels is itself responsible for the improved absorption performance of the planet's plants. With more CO2 in the air, the plants are better able to photosynthesize, meaning they grow better and further increase their rate of photosynthesis. That boom was helped along by the fact that plant respiration, a process which sees them take in oxygen and "exhale" CO2, didn't increase at the same rate.

"These changes decreased the amount of anthropogenic CO2 that stays in the atmosphere, and thus slowed the accumulation of atmospheric CO2," says Keenan. "This highlights the need to identify and protect ecosystems where the carbon sink is growing rapidly."

But as encouraging as these figures may appear, the news isn't quite as rosy as it may seem at a glance.

"Unfortunately, this increase is nowhere near enough to stop climate change," says Keenan. "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."

The research was published in the journal Nature Communications.

Source: Berkeley Lab

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