“We have forgotten half of the story up to now,” said Karl-Heinz Erb, the lead study author and a researcher with the Institute of Social Ecology in Austria.
Using a series of detailed maps derived from satellite information and other types of ecological measurements, Erb and his colleagues estimated how much carbon is contained in Earth’s current vegetation. The number is massive: 450 billion tons of carbon, which, if it were to somehow arrive in the atmosphere as carbon dioxide, would amount to over a trillion tons of the gas. (The mass is greater due to the addition of oxygen molecules.)
Here’s what that looks like mapped, according to the researchers’ image:
But the study also presented an even larger and perhaps more consequential number: 916 billion tons. That’s the amount of carbon, the research calculated, that could reside in the world’s vegetation — so not in the atmosphere — if humans somehow entirely ceased all uses of land and allowed it to return to its natural state. The inference is that current human use of land is responsible for roughly halving the potential storage of carbon by that land.
Here’s how the researchers see that very different, more hypothetical map:
The research was published in the journal Nature by Erb and 12 colleagues from institutions in Austria, Germany, Portugal, Sweden and the Netherlands.
The impact calculation is so large because humans have done far more than just bring about deforestation, which Erb said accounts for about half of the loss of potential vegetation.
“Half of this effect, half of this halving, is already well described,” he said. “This is the deforestation signal. But the other half, in most studies, is completely missing.”
The study found that there are two far-less-recognized components of how humans have subtracted from Earth’s potential vegetation — and that in combination they are just as substantial as deforestation. Those are large-scale grazing and other uses of grasslands, as well as forest “management.” With the latter, many trees and other types of vegetation are subtracted from forests — often the larger and older trees due to logging — but the forests as a whole don’t disappear. They’re just highly thinned out.
“This effect is quite massive, more massive than we expected actually,” Erb said.
The finding has implications for the human past, present, and future.
First, the past: It suggests, in line with the thesis of University of Virginia professor William Ruddiman, that humans have been changing the surface of the planet and putting greenhouse gases in the atmosphere through land use for millennia.
“Our finding is in line with the statement that the impact of humans on the climate was quite considerable also before the industrial times,” Erb said.
However, he added, those past changes were unlikely to account for 450 gigatons or so of lost carbon. That’s not only because deforestation and other land use changes have continued since the industrial revolution, but also because the research suggests that Earth’s potential vegetation was somewhat lower in the past than it is in the present, thanks to changes in the climate.
As for the present: The research means that so-called degraded land — not fully deforested but not “natural” or whole, either — is a phenomenon to be reckoned with.
“It suggests that the amount of carbon released to the atmosphere from land use is approximately equal to the amount still retained,” said Tom Lovejoy, an ecologist at George Mason University who was not involved in the work. “That means the restoration agenda is even more important than previously thought and highlights the enormous amount of degraded land in the world.”
And then there’s the future. The research simultaneously suggests that restoring land could do a lot to fight climate change but also that the highly popular idea of using biomass to withdraw carbon dioxide from the air may need some rethinking.
“Scenarios that limit global warming to 1.5 or 2 degrees [Celsius] require not only rapid cessation of greenhouse gas emissions but also removal of somewhere between about 100 and 300 billion tons of carbon from the atmosphere,” Phil Duffy, president of the Woods Hole Research Center, said in an email.
“This paper suggests that restoring vegetation around the world could in principle achieve that,” Duffy continued, noting that if all the potential vegetation were restored it would offset some 50 years of global carbon emissions. While “the full theoretical potential will never be realized in practice … this paper indicates that restoring vegetation could make an extremely important contribution to controlling global climate change.”
At the same time, the research raises big questions about a strategy called Bioenergy with Carbon Capture and Storage, or BECCS, which many models of future energy and climate systems have relied on to show how we could actually use land to remove carbon from the atmosphere. Those scenarios involve burning biomass to create electricity and then capturing the resulting emissions and burying them in the ground rather than letting them escape to the atmosphere.
The biomass would eventually grow back to pull further carbon from the atmosphere, representing a full reversal of the process by which humans, by burning fossil fuels, put underground carbon in the air in the first place.
Erb suggests that, due to the large carbon losses from managed forests identified by his study, BECCS could be a problem. “I’m a little bit skeptical that, with the carbon footprint of BECCS, it will ever be a large-scale technology that will save the climate.”