The crop residues that lie scattered across thousands of farmlands globally could be an unexpected secret weapon against climate change. By turning these leftover husks, roots, and leaves into carbon-sequestering biochar, we could lock away between 3 and 7% of total global greenhouse gas emissions for the next 100 years.
This extraordinary finding from a recent new study shows, in fact, that the potential of repurposed crop residues is so powerful that countries including Bhutan, India, and Ghana could use it to lock away the equivalent of over 40% of their current greenhouse gas emissions.
Largely, crop residues are either left to degrade on farmland, or they are burned, heaving emissions into the atmosphere. And yet the technology does exist to turn this raw material into biochar, through pyrolysis, a process that burns organic material at high heat and with limited oxygen, to create the carbon-rich charcoal-like residue. In this pyrolyzed state, crop residues will seal carbon in for longer periods than fresh plant matter that degrades. And, when mixed into the soil, biochar can keep a lid on this carbon for decades more.
Plenty of studies have shown biochar’s potential on small scales—but until now, the worldwide potential of farm waste, produced at industrial scales, was unknown. Writing in the journal GCB Bioenergy, the team of researchers based their analysis on a large dataset on international crop production, which allowed them to develop a high-resolution worldwide map of agricultural residues. They looked specifically at 34 crops that produce residues which can be turned into biochar. These included everything from wheat straw, to rice husks, and fruit peels.
If we scooped up all that residue and turned it into biochar, the potential could be enormous: the researchers estimate that with this agricultural waste we could lock away one billion metric tons of carbon into the soil each year. Biochar would degrade at different rates depending on how hot (faster) or cool (slower) the local climate: factoring these variables in, the scientists determined that 72% of that one billion metric tons would still be locked into the soil after 100 years. Strikingly, that is an amount equivalent to 45% of global agriculture’s total emissions in 2019, the study found.
But the reality is that a chunk of agricultural residue isn’t just there for the taking: a share of crop leftovers are actually lost during harvest, or, critically, are used to feed livestock and make bedding. So when the researchers accounted for this reality in their analysis, they showed that biochar’s potential carbon-locking power decreases to about 510 million metric tons a year—about 360 million of which would remain locked away in the soil after 100 years.
That may look like a steep decline, but it still signifies a huge amount—and, these benefits become especially apparent on national scales.
If Bhutan transformed its available farm residues into biochar, it would succeed in locking away the equivalent of 68% of its national emissions. For India—the world’s third-largest emitter—that figure is 53%; for Ghana it’s 44%, Bulgaria, 39%, and in Rwanda it signifies more than a third.
So, as a national measure to mitigate greenhouse emissions—perhaps as part of countries’ nationally determined contributions towards our global climate goals—biochar could hold clear promise.
At a global scale, the researchers’ more conservative estimates show that we could lock away at least 3% of worldwide emissions with this one approach, using this widely-available resource that might otherwise just be burned.
The researchers do caution that the available modeling on biochar degradation rates becomes more uncertain as time goes on—so there may be some uncertainty around biochar’s behavior in agricultural soils over the course of 100 years. Furthermore, the pyrolysis method that’s used to make biochar is an extremely energy-intensive one—though that’s changing with developments in the energy-efficiency of this process.
Meanwhile, there are also the added benefits of biochar to weigh in—like the fact that it also improves soil properties for growing crops. And that, unlike other large scale carbon-sequestering approaches like reforestation, biochar doesn’t need vast tracts of new land to deliver its benefits.
We urgently need to draw down excess carbon dioxide, the researchers say: “Making biochar from crop residues is one of the few tools we have that can do this at scale without competing for land.”
Woolf et. al. “Potential for biochar carbon sequestration from crop residues: A global spatially explicit assessment.” GCB Bioenergy. 2023.
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