Climate change is a serious enough problem, but it could be a lot worse. About half of the carbon dioxide we’ve pumped into the atmosphere by burning fossil fuels has been absorbed by plants and oceans, rather than staying in circulation to drive up temperatures. Scientists are convinced this can’t go on forever — but a new study in Nature shows that we haven’t come to the danger point yet. Over the past 50 years, says the report, humans have quadrupled our emissions, but the planet has kept up by doubling the amount of CO2 it absorbed.
That comes as something as a surprise: several earlier, small-scale studies have suggested we might be on the verge of a tipping point where the planet can’t absorb any more carbon dioxide. “So we decided to take a step back and ask, ‘do we see this at a global scale?’” said Ashley Ballantyne of the University of Colorado and lead author of the new report, in an interview, “and the answer is no.”
Tropical rainforests are one part of the answer for helping the Earth to absorb CO2 emissions.
To get that answer, Ballantyne and his co-authors used what Ingeborg Levin of Heidelberg University, writing in a Naturecommentary, called “a strikingly simple approach.” They took estimates of how much CO2 humans have been pumping out over the past half-century and subtracted the amount that has stayed in the atmosphere. Whatever’s left over must have been absorbed by the land (or more accurately, by plants growing on land) or by the ocean; there’s nowhere else it could have gone.
The calculation is so obvious, it probably could have been done long ago, but, said, Ballantyne, “we [scientists] can become too focused on details, and lose sight of the big picture.” It wasn’t quite as easy as it might sound, however. “Our ability to measure CO2 in the atmosphere has gotten a lot better over the years,” Ballantyne said, “but our ability to measure emissions has actually gotten worse.”
The reason, he said, is that nobody measures carbon dioxide emissions directly. Instead, they use economic activity as a proxy — reasonable enough, since economies run on energy, and that energy comes largely from fossil fuels. In developing countries like China and India, he said, “growth is happening really fast, and emissions accounting isn’t necessarily keeping pace, so there’s more error.”
Indeed, said Ballantyne, “10 percent of our work went to making the calculations, and 90 percent was scratching our heads over the uncertainties.” In the end, the scientists combined emissions estimates from three different sources to ensure they had the best possible information.
What the new study doesn’t answer is where, exactly, the CO2 is being absorbed. One possibility is the lush vegetation in the tropics, where plants take in CO2 for growth, and where, said Ballantyne, very little data is available. Another is the deep oceans — again, a place where scientists and their instruments haven’t gone.
Knowing where the carbon is going is important because it could give scientists a better handle on how much capacity is left. Sooner or later, however, that capacity will disappear. Plants take in more CO2 if there’s more in the atmosphere, but only up to a point. The oceans will ultimately stop absorbing carbon dioxide as well, in part because plankton and other sea-based plants will reach their own limits, and also because sea water gets less and less able to take in CO2 as it warms (in some ways, this will be a good thing).
When the Earth finally does reach its absorption limit, all of the CO2 humans emit will stay in the atmosphere, and that will turbo-charge the pace of global warming. “We don’t know exactly when we’ll reach the limit,” Ballantyne said, “but our models suggest things will turn around on land, at least, sometime in the coming century, maybe even by 2030-2050. I would really hope,” he said, “that we can cut back on fossil fuel emissions before that.”