For decades, we’ve been pumping billions of tons of harmful greenhouse gases into the air, and for decades plants have been obligingly sucking it back out again. In fact, a good 45% of the carbon dioxide we emit is absorbed back into the biosphere by the world's vegetation and oceans.
But when climate scientists looked into this process, known as carbon sequestration, they noticed something surprising.
Between 2002 and 2014, plants appeared to have upped their game, pulling more carbon dioxide out of the atmosphere than in previous decades. The below chart, released this month by researchers at Lawrence Berkeley National Laboratory, shows the growth rate of atmospheric carbon. As you can see from the red line, levels rose between 1960 and 1990. The blue line, however, shows them stalling significantly following 2002.
The experts were puzzled. Human activity was still polluting the air, but the amount of man-made carbon that lingered there appeared to be in decline. “That portion that stays in the atmosphere – that’s called the airborne fraction," said Trevor Keenan, co-author of the report, "and that has reduced by about 20% over the last 15 years.” The reduction is clearly visible in this next chart, where the airborne fraction thins out after 2002, breaking with the historical upward trend.
So what's going on? Humans haven't stopped emitting huge amounts of noxious gases, and carbon dioxide hasn't stopped accumulating in the atmosphere – it's just that lately, strangely, the rate at which it accumulates is slowing down, or at least holding steady.
The reasons for this aren't yet quantified, say the team at Berkeley Lab. One thing we do know is that as global warming drove up levels of atmospheric carbon dioxide over the past century, plants appear to have responded to the increase by photosynthesizing faster.
And it's not just that they're working harder: the world's flora is also spreading further afield. Scientists at Boston University have reported a worldwide "fertilization" effect, after satellite surveillance showed that somewhere between a quarter and half of the world's vegetated areas were becoming significantly greener, most worryingly in hitherto ice-encrusted territories such as the Arctic.
But the recent pause in the growth rate of global greenhouse gas is likely only to be temporary. As levels of carbon dioxide in the atmosphere continue to rise - and they are rising - plants will find it harder to keep up. The global "greening" effect could taper off, along with plants' increased appetite for carbon.
“Unfortunately, this increase is nowhere near enough to stop climate change,” said Berkeley Lab's Trevor Keenan in a statement. “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.”