Grinding green rocks into tiny dust could be very helpful on climate change. By Matthew Yglesias

Grinding green rocks into tiny dust could be very helpful on climate change. By Matthew Yglesias — Read time: 10 minutes

Grinding green rocks into tiny dust could be very helpful on climate change

Let's talk about enhanced rock weatherization

When I’ve written about carbon dioxide removal in the past, I’ve focused on direct air capture — efforts to build, in effect, artificial trees that could suck CO2 out of the air and store it in the ground.

This kind of technology is promising in many ways, but the biggest issue with it is that it’s extremely energy intensive. The CO2 is in the atmosphere in the first place largely because burning fossil fuels is a useful way for humans to obtain energy. Getting the CO2 back out is feasible, but requires more energy than getting it there in the first place. That’s still potentially beneficial because of mismatches in what different energy sources are good for. Right now, we have a lot of pretty good ways of making zero-carbon electricity (hydro, nuclear, wind, solar) but very few good ideas for replacing the use of fossil fuels in the Haber-Bosch process for making ammonia, the foundation of modern fertilizer. So you could imagine a world in which zero-carbon electricity becomes so abundant that electricity is used to offset carbon emissions from non-electricity sectors.

Another idea, which involves less energy use, is to make use of one of several proposals for what’s called “enhanced rock weathering” — essentially grinding up certain kinds of rocks into a very fine powder and spreading it around.

You probably know that rocks tend to wear down over very long spans of time. And I’ve always been dimly aware that this has something to do with rain. But the specific mechanism turns out to be that rainwater is very mildly acidic as a result of atmospheric carbon dioxide. When this slightly acidic rainwater hits rocks, chemical reactions slowly but surely erode the rocks, with carbon atoms rolling downhill and eventually sinking to the bottom of the ocean. The exact pace at which this happens depends on a lot of factors, including the temperature and the actual amount of CO2 that’s around, which for much of geological history helped keep carbon levels in check. When volcanos or other geological activity increased atmospheric CO2, that also increased the pace of rock weathering, which helped restore equilibrium.

The Industrial Revolution unleashed an unprecedented level of human prosperity, but also a much larger and more sustained level of emissions than any volcanic episode. So the natural carbon cycle has been disrupted, and the world is seeking a cost-effective menu of options to contain the downsides.

And one possibility relates to this process of rock weathering because another factor driving its pace is the surface area of the rocks — more surface area means more weathering, which means less CO2 in the atmosphere. Unfortunately, it turns out that the scale of the increased surface area you’d need to have a meaningful impact is quite large. You’d be aiming, ideally, to grind rocks into ultra-fine silt. This is not easy, but it is feasible with existing technology, and it deserves to be part of our climate strategy.

The case for carbon removal in general

Among people who are aware that the most extreme activist rhetoric about climate change isn’t true, there’s an unfortunate tendency to flip all the way around to the conclusion that the whole climate problem is fake or being discussed in bad faith by people whose only goal is to overturn capitalism.

But this, unfortunately, is also not true.

Climate is not an extinction-level menace, and on the current trajectory the future will be richer, on average, than the present. But that is consistent with climate change being really bad. If you look back across human history, you see lots of tragic events (the Black Death, World War II, countless famines) that obviously did not result in the extinction of humanity. But if you had the ability to avoid or significantly mitigate these tragedies, you absolutely would. You wouldn’t pay an infinite cost to avoid them, but you’d pay a lot. In a world of seven or eight billion human beings, the deaths of tens of millions of people is easily survivable, but obviously very much worth attempting to avert. That’s especially true because the problems associated with higher sea levels and shifting weather patterns are more severe in poor countries that have fewer resources available for adaptation and also, in many cases, start with higher baseline temperatures.

So what is to be done? Replace fossil fuels with zero-carbon alternatives.

Replace coal, oil, and gas power plants with hydro, wind, solar, nuclear, and geothermal.

Replace gasoline- and diesel-burning vehicles with battery-electric vehicles.

Replace biomass, gas, and propane cooking fuels with electric induction.

Replace oil and gas home heating with electric heat pumps.

This is all great, but there’s a timeline problem. I’m completely convinced that electric vehicles will displace ICE ones over time, just based on quality and market dynamics. EVs have a lot of intrinsically desirable properties, battery manufacturing will continue to improve, and there’s a virtuous cycle where EV adoption leads to more charging infrastructure which leads to more EV adoption. But these changes take time. California, probably the most ambitious American state in terms of pace, is going to ban the sale of new conventional cars starting in 2035. But they’re not going to force people to give up existing gasoline cars. The average American car is 12.2 years old, so even if California goes forward with this ambitious plan, there will still be a significant minority of gas cars on the road there in 2050. And other states will go more slowly.

You see a similar dynamic with home heat. New York is going to phase out gas hookups in new housing, but New York’s housing stock is on average very old. Some people will get heat pump retrofits, but lots of people won’t.

The fact that even the most aggressive states don’t want to force people to abandon what they’re already using is telling. It’s very hard, politically, to say you’re going to burst into houses and pull out people’s furnaces. It’s also inherently costly to ditch things that still work. When our gas stove broke, we replaced it with an induction stove. I’d like to get an electric car, but our old car works fine and I’m going to keep driving it as long as that’s true.

And broad areas of activity simply have no “good enough” alternative to fossil fuels, including important sectors like aviation, maritime shipping, steel and concrete fabrication, and agriculture. It’s not that it’s impossible to decarbonize these sectors. People are working on it, and a lot of promising ideas are in the mix. Lots of things kinda sorta work. But there’s nothing that’s truly ready to go at large scale and just waiting for deployment. And even if there were, deployment itself is extremely difficult unless you’re willing to let change happen at a fairly gentle pace.

The upshot is that we can expect a level of warming that does significant harm, in part because of political stubbornness but in part because we just don’t have viable solutions for everything — hence the need to also pursue ideas like carbon capture and various forms of carbon sequestration.

The olivine option

Olivine is a greenish magnesium iron silicate whose chemical formula is (Mg,Fe)2SiO4. Who cares? Most of the time, nobody. Olivine is not particularly rare and not particularly useful. This Finnish website says olivine makes an ideal sauna rock, but your mileage may vary on that.

For our purposes, though, the interesting thing about olivine is that it weathers relatively rapidly. And in this context, the fact that it’s not otherwise very useful is a virtue because it means there isn’t a lot of competing demand for olivine that’s keeping the price high. If you want to buy a bunch of olivine and smash it up, that won’t break the bank. A group called Project Vesta proposed taking a bunch of olivine, grinding it down into olivine sand, and then spreading the sand on select beaches. The idea got a big write-up in MIT Technology Review, and Bill Gates tweeted a video about it that Elon Musk will no longer let me embed on Substack but that you can see if you click the link. If you want a lot of technical details, I recommend this National Academies report.

My initial reaction was to wonder why they wanted to put the olivine sand on the beach. On the one hand, yeah, it makes sense that sand would be on a beach. On the other hand, beachfront real estate tends to be pretty valuable and it’s not obvious that people want green sand. But Vesta is counting on the motion of the waves to further grind the sand and thus further increase the pace of weathering. Their argument is that rather than a waste of perfectly good beachfront real estate, there are actually important co-benefits here. Many beaches are being eroded, increasingly so as sea levels rise, so there’s a need for beachfront resilience projects. Giant piles of green sand can kill two birds with one stone.

Campbell Nilsen recently wrote up a different proposal for Works in Progress, arguing that reliance on the beach mechanism is too fiddly and unreliable.

His argument is that rather than grinding the olivine into sand, dumping it on the beach, and hoping the waves will grind it further, we should just suck it up and grind the olivine really, really small to start with. How small? He says “we’d want to process a lot of olivine and break it down into very small particles — not sand, which (with diameters in the hundreds of microns) is too large, but silt (with diameters in the 10-50 micron range).” Once you have olivine silt, you can mix it with water and slide it into the ocean, no beach required. The downside is that while olivine is not rare or expensive, grinding stuff costs money, and the more you grind it, the more money it costs. In part that’s because the capital equipment costs money, but the biggest factor is the ongoing energy cost of running the grinders. Essentially, the sand approach economizes on energy by taking up a lot of land, while the silt approach economizes on beachfront by requiring a lot of energy.

We need more energy

Importantly, while the energy requirements of silt-grinding aren’t trivial, they also aren’t exorbitant. That’s important because generating electricity, of course, produces CO2 emissions.

Nilsen calculates that it requires about 80 kWh of electricity to produce enough silt to sequester one tonne of CO2. According to the Energy Information Administration, 80 kWh of coal-fired electricity emits roughly 180 pounds of carbon dioxide. So even if you’re using the most carbon-intensive electricity source available, you are firmly in net negative emissions territory. Coal also generates a lot of particulate air pollution that is harmful to human health, so I wouldn’t actually recommend burning tons of coal in order to power rock-grinding machines that sequester carbon dioxide. But the point is just to underscore that the emissions math works.

If you go the sand route, the energy requirements are even smaller. And of course we don’t need to categorically choose between the two. To the extent that beaches are available and/or people want the sand as a resilience strategy, we can go for the lower-energy approach. And we can also produce plenty of relatively energy-intensive silt. The big issue with the electricity requirements of the silt isn’t that it doesn’t work climate-wise, it’s that it just costs money — Nilsen says once it’s all up and running, you’d be looking at $1.36 trillion per year to sequester 100 gigatonnes per year. That’s a lot. But it’s not an impossibly large sum of money. If you aggregate across the OECD nations, it comes out to about 2.26% of the rich world’s GDP — a very large sum, but feasible.

And of course, as with everything related to climate, there are no magic tipping points. Sequestering 100 gigatonnes per year would be great, but sequestering 50 or 10 or five or even one would be helpful — you’ve got to start somewhere.

Crucially, the cost of carbon removal is heavily influenced by the cost of energy. This relates to a point I’ve made before, that while increased energy production has some ecological costs, it could also solve many environmental problems. Solar panels and wind turbines consume space, nuclear plants require waste storage, and coal and gas contribute to air pollution, but abundant energy can also solve problems — including the problem of climate change — if we set ourselves up right.