A new study published in Nature Communications has found that Earth’s plant life between 2002 and 2014 has absorbed so much carbon dioxide that the buildup of CO2 in the atmosphere has slowed down, despite humans pumping out more CO2 than ever before. The study also found that between 1982 and 2009, “about 18m square kilometers of new vegetation had sprouted on Earth’s surface, an area roughly twice the size of the United States.” The Economist reports: In 2014 humans pumped about 35.7 billion tons of carbon dioxide into the air. That figure has been climbing sharply since the middle of the 20th century, when only about 6 billion tons a year were emitted. As a consequence, the concentration of CO2 in the atmosphere has been rising too, from about 311 parts per million (ppm) in 1950 to just over 400 in 2015. Yet the rate at which it is rising seems to have slowed since the turn of the century. According to Dr Keenan, between 1959 and 1989 the rate at which CO2 levels were growing rose from 0.75ppm per year to 1.86. Since 2002, though, it has barely budged. In other words, although humans are pumping out more CO2 than ever, less of it than you might expect is lingering in the air. Filling the atmosphere with CO2 is a bit like filling a bath without a plug: the level will rise only if more water is coming out of the taps than is escaping down the drain. Climate scientists call the processes which remove CO2 from the air “sinks.” The oceans are one such sink. Photosynthesis by plants is another: carbon dioxide is converted, with the help of water and light energy from the sun, into sugars, which are used to make more plant matter, locking the carbon away in wood and leaves. Towards the end of the 20th century around 50% of the CO2 emitted by humans each year was removed from the atmosphere this way. Now that number seems closer to 60%. Earth’s carbon sinks seem to have become more effective, but the precise details are still unclear. Using a mix of ground and atmospheric observations, satellite measurements and computer modeling, Dr Keenan and his colleagues have concluded that faster-growing land plants are the chief reason. That makes sense: as CO2 concentrations rise, photosynthesis speeds up. Studies conducted in greenhouses have found that plants can photosynthesis up to 40% faster when concentrations of CO2 are between 475 and 600ppm.
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