Rocks for crops
The other way to enhance weathering is using so-called “ex situ” methods, which involve grinding up rocks to a fine powder and turning them into carbonate minerals. One way to do this would be in industrial facilities, using heat or acid and running CO2 through the rock powder. “That, we could do very fast. I mean, that, you could do in as long as it takes you to build the facility and dig the rocks up.”
The other ex situ approach involves excavating rocks, grinding them up, and spreading the rock powder on fields. This is colloquially known as “rocks for crops.” The rocks-for-crops idea enhances weathering by increasing the surface area of the material via the crushing and grinding. The biological activity in soils also can speed up weathering. (Again, we witness the wonders of soil microbes!) Ground-up rocks dissolve one to five times faster when plants are around.3 West remarks, “It’s actually a pretty interesting idea because it has the potential benefit of providing nutrients to crops, too, so you can get a double whammy, especially in developing countries where fertilizers are a little more scarce or more expensive relative to the value of the crops.” Right now, a research team from the Leverhulme Centre for Climate Change Mitigation in the UK is conducting mesoscale field trials in Illinois, Australia, and Malayasian Borneo to figure out if this is something that works on a five-to-ten-year timescale.4
There are some respects in which rocks for crops looks like a win-win solution. It is a fairly low-tech approach, and it could be done right now with existing technology. But it would also be a massive mining and transport operation, with its own particular geography. “We have to dig up rock, and so that’s a big mining operation; and another aspect here is that some rocks are more reactive than others, so you can’t just dig up any old rock and expect it to work very well,” West explains. You would have to go where the rocks are—places with minerals like olivine, one of the faster-reacting silicates. Certain tropical areas, with warmth and moisture, look the most favorable. In short, rocks for crops is attractive in that it lets nature do the work, that it could have co-benefits for farmers in terms of fertilization, and that you don’t have to concentrate CO2.
Of ultramafic silicate rock flour, and mountains
Rocks for crops faces questions of scalability, as all these carbon removal techniques do. The scientific literature is full of calculated potentials: for instance, that dusting two-thirds of productive cropland with basalt could extract 0.50 to 4 gigatons CO2 equivalent per year by 2100.5 The aforementioned bestselling compendium Drawdown, on the other hand, suggests that if olivine was applied to one-third of tropical land, it could lower atmospheric carbon dioxide by 30 to 300 parts per million by 2100.6 But Drawdown also notes that some scientists claim the rates in nature are actually ten to twenty times higher than those in the lab. I ask West: What is going on with these estimates?
“There’s this fundamental problem in what’s called weathering kinetics, which is that there’s an enormous discrepancy between lab and field rates,” West explains. Laboratory experiments can’t capture all the effects of biology on mineral dissolution. “If you add even microbes—forget plants and all the assets they excrete—just the little molecules that microbes excrete are enough to dramatically, potentially ten, twenty times, increase the rate that minerals dissolve … So, we know that. We don’t know exactly why. What is it that microbes do that make minerals dissolve faster?” Field trials will help reconcile these discrepancies between lab and field rates of weathering, but they’re not an easy thing to get funded, because they’re considered “applied science” for this particular goal. That is, you’d have to be studying rock weathering specifically for carbon sequestration purposes—something that is beyond basic science. Most funding agencies aren’t quite to that point, save in the UK, where there is some dedicated funding for this research. However, we also need fundamental basic research into areas like reaction kinetics, which will be crucial to understanding the results from field trials.
Is it possible, I ask, that field trials will show that enhanced weathering is a way bigger solution than you read about in, say, assessment reports on carbon removal?
“I think there is certainly the possibility that field trials will show that it’s more effective than we’ve guessed, right? But there’s the possibility that it goes the other way around.”
West then points out a few more complications with enhanced weathering. Olivine—a rock that is more effective than basalt at sequestering carbon—contains trace amounts of nickel and chromium that are toxic, and so spreading it on the food supply, or on tropical forests, might cause it to accumulate in the food chain. This is one reason why researchers are now looking more at basalt, even though it’s less effective for CO2 capture—about 0.3 tons CO2 per ton of basalt, versus 0.8 for olivine.7 Another consideration is that the calcium and magnesium get released from the rock, but then bond to clay minerals—“secondary minerals”—instead of ending up bonding to CO2 to make carbonate and increasing the amount of carbon stored. Finally, the literature points out another potential issue, which is how the application of rocks would impact biodiversity in the surrounding ecosystems—biomes that may already be adapted to nutrient-poor and acidic mature soils.8
Now I’m ready to ask West my dumbest question: If we start drawing massive amounts of CO2 into rock, are we going to end up with too many rocks everywhere?
He laughs. “Oh, no, you can’t do that.”
“Okay. It’s the type of thing that people might wonder about.”
“Okay. I say that glibly, that you can’t do that.” But West recalls a geochemist group expedition to Oman, where there are mountains where the ancient seafloor has been uplifted. “And that ancient sea-floor—there’s huge mountains of it that had been converted into carbonate, and we tried to do some back-of-the-envelope calculations on what’s the size of rock that would be needed to take ten gigatons of CO2 or whatever, some reasonable amount of carbon to offset emissions. It’s enormous. It’s a mountain’s worth, right?”
“So, each year,” he continues, “you have to produce a mountain’s worth of carbonate somehow, and that could be in the oceans. The oceans are big, right? So, several mountains in the ocean … it sort of disappears under the scale of the ocean; but if you’re thinking about doing this industrially, it’s actually a lot of carbonate that you have to make.” West explains, again, that you have to start with some matter—you have to dig up rock, because you’re taking one rock and converting it to another. “There’s a conservation of mass. You can’t just make new rock, so we’re not gonna bury ourselves under rock by doing this. But that said, the calculation that one year’s worth of CO2 emissions amounts to a couple of big mountains’ worth of material, and I’m not talking, like, just piles. I’m talking a big mountain. I don’t remember what the numbers were, but I remember many kilometers. That, I think, serves to illustrate, at least in my mind, what a huge challenge it would be.”
“The fossil fuels industry has an enormous footprint, and we don’t think about it on a daily basis. But you just go to a refinery in LA and it’s huge—and they’re tiny compared to refineries in the Middle East and so on, and then you add them, and it’s just this enormous industry. Effectively, if we want to offset that in an industrial way, we have to have an industry that is of equivalent proportion, and I think that’s sort of lost.” The engineering side of it is pretty eye-opening to think about, West says. “I say that we could do this in five years if we wanted to. Well, sure, but are we really gonna build an industry the size of the fossil fuel industry in five years?” If there’s an abrupt change in climate, we might be stimulated to do that, he speculates, but gradual climate change might be hard-pressed to kickstart that kind of industry. At any rate, the scale of mineral carbonization is enormous—“but we won’t bury ourselves. Does that make sense?”
I’m glad we won’t bury ourselves under mountains of waste carbon, but my questions about scalability are lingering. One aspect I’ve be
en wondering about is the competition between technologies: If we’re going to build out a fossil-fuel-sized industrial infrastructure, which is what we talked about earlier with BECCS and direct air capture, would we choose to build it out for direct air capture or BECCS, rather than for enhanced weathering? Interestingly, there are some ways that enhanced weathering is complementary to other carbon removal practices. For one, biochar is proposed as a carbon sequestration measure that would have fertilization co-benefits in the tropics; rock dust could be complimentary to it, because biochar alone doesn’t provide enough nutrients. Second, enhanced weathering could be used on bioenergy croplands, if those were expanded for BECCS. And third, enhanced weathering might be beneficial for large-scale tropical reforestation programs.
So there are some overlapping spaces where enhanced weathering could complement other drawdown approaches. When geographical factors are included, however, they shrink the potential. One study points out that 80 percent of agricultural commodities are consumed locally, and that areas with limited exports wouldn’t have the transport infrastructure to import basalt for spreading on their fields. Some basalt is held in outcrops without arable land (e.g., Siberia, drylands of Ethiopia), and there’s a carbon cost to moving the rock around. The authors argue that the initial deployment of weathering on croplands would take place in areas with good road access, heavy machinery, and basalt nearby, like North America or the UK.9 And it’s not just road and transport infrastructure that’s needed—labor is also required. Crop, tree, and rubber plantations managed by large-scale agribusinesses that already apply crushed limestone and fertilizer would likely be the first adopters. Small-scale farmers may not have the resources. But the counterargument is that as agriculture develops toward large-tract mechanized farming rather than smaller-scale shifting cultivation, and as roads are developed to reduce yield gaps and bring new cash crops to market, agricultural systems will transform into forms more compatible with enhanced weathering.10 To what extent is this vision incompatible with smallholder, agrocological, polycultural food systems? Even if mechanical methods are available, will women still be bearing bags of fertilizer, as they are in Indonesian plantations?
I once attended a presentation by some modelers who showed a world map of the areas that would be promising for implementation of rocks-for-crops. The best spot, it seemed, centered in the Democratic Republic of the Congo. I raised my hand and asked how you might get people in the DRC to use mineral weathering on their fields. Their answer: “We’ll just pay them.” This might seem like a reasonable approach to one who is unfamiliar with the operational realities of the DRC. But I will leave this here.
As with many climate engineering ideas, one could see this concept as either promising or terrible, depending upon how it is implemented. Mining is often deeply disastrous to both ecosystems and communities. Spinning up a new mining industry could alter the social fabric for the worse—but on the other hand, it could be a source of employment for people with expertise in other types of mines we might want to be phasing out. We could imagine silicate mines being hazardous, dust-inducing places if they were not built out well. One paper flatly states, “Especially in areas where agriculture is not managed by agribusiness, this would require a pan-tropical investment in education, safety equipment and protocols.”11 Like with most other geoengineering ideas, so much depends upon how it is done, and by whom.
What on earth would motivate the creation of a new industry for removing carbon? It’s the same question we considered for CCS in the previous chapter—but its potential proponents are quite different. When it comes to enhanced weathering, some of the research is done by geologists such as Dr. West, who are not advocates of going out and doing this recklessly or at large scale, but rather are interested in the science, and what we can learn about our earth through exploring it. Then there are modelers, who come up with amazing maps and calculations about the potentials and costs, but are perhaps overly driven by the need to produce papers. Enhanced weathering is quite like BECCS in this regard: it could be an artifact from a community of modelers who are required to create interesting and important projections to keep their jobs. But enhanced weathering lacks obvious champions to operationalize it. One interesting move is a research project by diamond multinational De Beers to implement accelerated weathering to store carbon in kimberlite rock at its mines, given that it has all these mine tailings lying around. A scientist working with the corporation suggests that by using this method they could become a carbon-neutral company.12 Using mine tailings for carbon removal would be one way to reduce emissions from mining while learning more about how weathering works.
Enhanced weathering also has a tiny grassroots following, under the rubric of “remineralization.” The organization Remineralize the Earth was founded in 1986 as a network newsletter, evolving into a magazine in 1991 and eventually becoming a nonprofit in 1995. The movement has roots in agroecology, but can also be traced to the work of a German nutritional biochemist, Julius Hensel, who wrote Bread from Stones in the 1880s. The technology didn’t exist to grind rock dust for soil remineralization at that time. But in the last few decades, some European rock companies have begun research into it again, and Remineralize the Earth draws from their work as well as the current climate crisis. They position soil remineralization as part of a new paradigm in agriculture: “The agenda for SR is clear: to create abundance in an era of diminishing resources and lead us away from fossil fuels. Remineralization is nature’s way to regenerate soils. We can return the Earth to earlier interglacial Eden-like conditions through appropriate technology.”13
Whether Edenic dream or industrial mining hell, two challenges are clear. One is that most people don’t know anything about this idea. Geology is not an everyday subject, and the writing on enhancing mineral carbonization as a carbon removal strategy is dense. The second challenge is that we don’t yet know enough to assess these ideas. The estimates for enhanced weathering’s potential are so variable that they are practically useless, in my view, though scientists may disagree. The estimated implementation costs are $60–600 trillion dollars “for mining, grinding and transportation, assuming no technological innovation, with similar associated additional costs for distribution”14—a variation so wide that it strikes me as nearly meaningless.
I’m not bashing these technologies simply because they carry unknowns: they hold a lot of intriguing possibilities. For instance, enhanced weathering makes water more alkaline. Could this make it useful for reducing ocean acidification in coral reef regions? One study indicates that it could reverse ocean acidification under a low- to mid-range climate change scenario and restore global mean surface pH by 2100.15 That would be a pretty big deal, and worth investing in. Another new paper, among the weirdest peer-reviewed science I’ve read, suggests that life in the seabed could make coastal ecosystems a good place to try enhanced weathering. In this scenario, microorganisms and invertebrate fauna could act as weathering agents as part of a “benthic weathering engine.” Long filamentous microbes called “cable bacteria” transport electrons, making the top few centimeters of the coastal sediment acidic, and this causes carbonates to dissolve faster. There are also areas where the entire top fifteen centimeters of sediment passes through the guts of large-deposit feeders several times per year, and these “gut transits” can increase dissolution rates.16 Potentially, the approach constitutes yet another way that life could do the work of rebalancing our carbon burden—but it’s all at a very conceptual stage, right now.
I ask West what he thinks is important for people to know about weathering, and he muses for a second. “I think it’s important to recognize that it’s a natural process that’s happening all the time, but that it’s happening very slowly, and that there is this potential to make it happen faster. But then I think the other thing that’s important … and I think I would say the same is true for any of the other carbon dioxide removal—to actually implement [weathering] in a meaningful way, in my persona
l opinion, would be enormously challenging, and so I guess it comes back to the moral-hazard question that I raised at the beginning.” People shouldn’t view discussion of these things as meaning that we have a technology we can rely on. “Scientists are working on these things because we’ve got to turn over every stone, and maybe if we combine many different technologies, they can start to play a meaningful role. But like I said, I think it was very sobering for me to do the calculation of what the scale is that would be required for something like this. And it is maybe not technologically impossible, but it seems incredibly daunting.”
This is the case with virtually all the technologies we’ve explored: it’s not technologically impossible to scale them up, but it’s daunting, for a whole host of reasons. Some of the parameters, like thermodynamics, are fixed. Some are a bit more malleable, like the qualities of plants. What could make the most difference, and really give these approaches wings, are changes in our politics, economics, and culture. In Part III of this book, we’ll speculate on what a society dedicated to carbon removal might be like.
Sketch: Mountain
The peridot earrings glint green in the light. I’m holding them up when Christa calls.
“Mom?” she asks. “Are you ready?”
“Almost. I received the ticket. Or invitation. Pass? Whatever you call the thing that lets me get into the reception. And those earrings you sent. Your assistant was very kind.”
“Okay, that’s good,” she replies. A long pause.
“What’s wrong?”
“New York. They’ve just announced. They’re building a mountain too. A hundred meters taller than ours. That’s the part the press keeps repeating. Nothing about the mountain’s concept, or the carbon sequestration.”
“Well, the one in Qingdao is also taller.”
After Geoengineering Page 16