Over the course of decades, centuries and millennia, the steady skyrocketing of redwoods, the tangled march of mangroves along tropical coasts, and the slow soaking of carbon-rich soil into peat bogs have blocked billions of tons of carbon.
If these natural vaults are broken through, through deforestation or swamp dredging, it would take centuries before those redwoods or mangroves can return to their former fullness and recover all that carbon. Such carbon is “Irrecoverable” in the time scale, decades and not centuries, necessary to avoid the worst impacts of climate change, and keeping it under lock and key is essential.
Now, through a new mapping project, scientists have estimated how much sunk carbon resides in bogs, mangroves, forests and other parts of the world and which areas need protection.
The new estimate puts the total amount of sunk carbon at 139 gigatons, the researchers report on November 18 in Nature Sustainability. This is equivalent to about 15 years of emissions human carbon dioxide at current levels. And if all that carbon has been released, it’s almost certainly enough to push the planet beyond 1.5 degrees Celsius of warming above pre-industrial levels.
“This is the carbon we must protect to avoid climate catastrophe”says Monica Noon, an environmental data scientist at Conservation International in Arlington, Virginia. Current efforts to keep global warming below the ambitious 1.5 degrees C target require achieving net zero emissions by 2050 and that the carbon stored in nature remains stationary. But agriculture and other development pressures threaten some of these carbon deposits.
To map this carbon at risk, Noon and his colleagues combined satellite data with estimates of how much total carbon is stored in ecosystems vulnerable to human incursion. The researchers excluded areas such as permafrost, which stores a lot of carbon but is not likely to develop (even if it is melting due to warming), as well as tree plantations, which have already been modified.
The researchers then calculated the amount of carbon that would be released from the earth’s conversions, like the clearing of a forest for agricultural land. That land could store varying amounts of carbon, depending on whether it becomes a palm oil plantation or a parking lot. To simplify, the researchers hypothesized that the reclaimed land was left alone, with saplings free to grow where giants used to be.
Carbon: a vital resource
This allowed the researchers to estimate how long it might take for the released carbon to be replenished into the soil. Much of that carbon would remain in the air by 2050, the team reports, as many of these ecosystems take centuries to return to their former glory, rendering them unrecoverable in a time frame that matters to tackle climate change.
Releasing those 139 gigatons of irrecoverable carbon it could have irrevocable consequences. For comparison, the United Nations Intergovernmental Panel on Climate Change estimates that humans can emit only 109 gigatonnes more of carbon to have a two-thirds chance of keeping global warming below 1.5 degrees C. “These are the places that we absolutely must protect” says Noon.
About half of this irrecoverable carbon is found on only 3.3% of the Earth’s total area, roughly equivalent to the area of India and Mexico combined. The key areas are the Amazon, the Pacific Northwest and Borneo’s tropical forests and mangroves. “The fact that he is so concentrated means that we can protect him”says Noon.
About half of the carbon is irrecoverable already falls within protected areas existing or lands managed by indigenous peoples. Adding another 8 million square kilometers of protected area, which is only about 5.4 percent of the planet’s land surface, would bring 75 percent of this carbon under some form of protection, Noon says.
“It is really important to have spatially explicit maps of where these irrecoverable carbon stocks are located”says Kate Dooley, a geographer at the University of Melbourne in Australia who was not involved in the study. “It’s a small percentage globally, but it’s still a lot of land”. Many of these dense stores are located in high-risk locations, he says.
“It’s so hard to stop this drive to deforestation”he says, but these maps will help focus the efforts of governments, civil society groups and academics on the places that matter most to the climate.
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