Preserving ‘carbon crops’ with salt and storing them in dry landfill sites to stop them decomposing could be a low-cost, scalable climate solution, according to physicist Eli Yablonovitch.
The jaw-dropping ancient hill fortress of Masada in Israel is an awe-inspiring place to visit. Situated on a towering plateau overlooking the Dead Sea, the fortress was built between 37 and 31 BC as a stronghold for Herod the Great, and it was later occupied by Jewish rebels resisting Roman rule.
Masada fortress in Israel, where the salty conditions have inspired a potential climate solutionBut even if ancient history isn’t your bag, there’s another reason why Masada should be an interesting stop-off – particularly for anyone remotely interested in the future of the planet.
Back in the 1960s, a handful of date seeds were found by archaeologists in King Herod’s Masada storerooms, preserved in an ancient jar. Carbon dating revealed those seeds dated from between AD 64 and 155 BC, and had been perfectly preserved thanks to the shelter of the jar and the arid climate of the Judean desert.
So perfectly preserved, in fact, that one of the seeds germinated in 2005 and is now a thriving date palm growing on an Israeli kibbutz.
This unlikely tale of a 2000-year-old date seed has helped to inspire a plan for a new form of carbon storage, which its creators say could easily absorb half the world’s annual carbon dioxide emissions.
Salted biomass
Physicist Eli Yablonovitch at the University of California, Berkeley, is best known for his pioneering work on photonic crystals and “strained” semiconductor lasers. But in a paper published earlier this month, he turns his attention to the problem of carbon sequestration.
Plenty of people have already proposed growing biomass – from trees to seaweed – to bolster the planet’s carbon-storage capacity. The problem is how to stop that biomass decomposing at the end of its life, releasing the stored carbon back into the atmosphere. If we can crack that, plants could be the answer to all our carbon storage needs.
Yablonovitch’s idea is to interrupt the carbon cycle by growing “carbon crops” and storing them in huge, dry landfill sites to stop them decomposing. Inspired by the salty, arid climate of the Dead Sea, Yablonovitch says the crops will be salted before they are stored, drawing out water to ensure that they will be well preserved. “The one thing that every living thing needs is moisture,” he says. “If you deprive it of moisture, gradually all life comes to a halt.”
The Masada seeds were the perfect natural experiment for this hypothesis, he says. “It’s about as good a proof as you’ll get that biomass, if kept dry, will be preserved for over 2000 years.”
The paper proposes growing vast quantities of fast-growing plants such as miscanthus, switchgrass and loblolly pine on fallow or marginal farmland.
Farmers harvesting the crops would dry the plants, then chop and salt the biomass to suck out as much moisture as possible. The preserved crops would then be entombed in a “biolandfill” lined with thick plastic sheeting, where they would remain indefinitely. “If the biomass is sealed off in that way, it could stay preserved for thousands of years,” says Yablonovitch.
It’s a carbon-negative solution, according to the paper: for every tonne of dry biomass, about 2 tonnes of CO2 is sequestered.
A cheap silver bullet?
There are plenty of novel carbon sequestration ideas doing the rounds of university campuses and Silicon Valley investor compounds. But what sets this one apart is its potential to be a low-cost, scalable solution, says Yablonovitch.
Farmers already know how to grow the plants in question – there are about 50 of them listed in the paper as being suitable – while the design of the biolandfill is pulled almost directly from existing municipal landfills, which are already lined with thick plastic to prevent groundwater contamination.
The paper suggests that by using known techniques and technologies in this way, the cost of carbon sequestration via salted biomass could be as low as $60 per tonne, compared with between $250 and $600 per tonne for direct air capture today. That equates to an added cost of only $0.53 per gallon of gasoline (based on US prices) to make petrol carbon neutral, says Yablonovitch.
Based on these figures, using salted biomass to trap 50 per cent of the world’s annual emissions is “quite reasonable”, he argues. Doing so would require the use of land equivalent to around one fifteenth of the area currently occupied by cropland, pasture and forest. “We’re looking at costs that are rather modest and that are suggestive that we should really do this right away,” he says.
That isn’t to say that the idea is without challenges. One of the main barriers is designing and building a landfill so it remains completely dry. “The one thing we have not answered is the exact time sequence of constructing the landfill,” says Yablonovitch. “You have to dry the crops before sequestering them, and you don’t want to be upset by rain while you’re building the landfill. So, I think you’d have to imagine some type of canopy or a tent during that time that would keep the biomass dry.”
Finding enough salt could also be a challenge, the paper explains, and could stretch world supplies depending on the crops grown. There is also the question of land availability. Yablonovitch says the solution could be scaled up without impinging on productive agricultural land, but designing the right incentives for farmers to ensure that is the case will be a delicate task for policy-makers.
Major hurdles
For many years, Ning Zeng at the University of Maryland has been studying the potential for burying timber in underground “wood vaults” as a means of carbon sequestration.
He says the idea for biolandfills is an “interesting proposal”, but that it may be trickier than expected to actually deliver. For one thing, finding a location for a landfill with easy access both to plentiful biomass and enough salt will be a challenge. Engineering a landfill to be completely dry and leak-proof is another major hurdle. “As a theoretical idea I think it’s interesting, but there are a lot of practical, economic and logistical issues that need to be thought through,” he says.
Still, it’s always cheering when a renowned physicist says they have a solution to the climate crisis – and it seems others also think the idea is worth exploring further. Yablonovitch says he has already been approached by oil firms, start-ups and investors interested in taking the idea on.
Most of all, Yablonovitch is optimistic about humankind’s ability to find a workable solution to the climate crisis. “Climate change has been presented to the public as an unsolvable problem,” he says. “And as an engineer, I am somewhat offended by that because if you put some very smart people to work on it, which is happening now, you will find a solution. I’m not saying that ours is the solution that will win – there could even be less expensive solutions. [But] I would say the way to look upon this is as a problem to be solved, and not as an unsolvable calamity for humanity.”
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