Enhanced Weathering - A primer

Look at a rock. Look at it again in an hour, in a month, or maybe even in 10 years. Chances are it will look rigorously the same. It’s “just there”. And yet, rocks might hold part of the solution to a stable and liveable climate. (i.e The case for carbon capture)

If “immobile” or “everlasting” are good words to describe rocks on a human timescale, “ever changing” and “fertile” might be better words if you look at them on a geologic time scale. Rocks are essential to keep the complex cycles of nutrients and gases flowing in and out of our soils and atmosphere. They’re essential to life on earth. Thank you rocks.

How weathering works in practice

Take the right rock, put it in the right conditions and through the process of weathering, it will release nutrients and capture CO2 - carbon dioxide - from the atmosphere at the same time.

Capturing CO2 from the atmosphere is what we call negative emissions. They are the opposite of emitting greenhouse gases and exactly what we’re looking for to fight the worst consequences of climate change.

Rocks disintegrate under the combined physical and chemical assault of water, air and microorganisms that live on their surface. Their core minerals - such as magnesium or calcium - dissolve through chemical reactions that also capture CO2 from the air and store it in carbonates. These are stable and harmless minerals. Once they’ve formed, they can follow two paths. They can stay dissolved in the soil and groundwater until enough of them accumulate, at which point they precipitate - the carbonates turn into solid crystals -. The other path sees the carbonates wash out to sea with rainwater. Here again, they might follow different paths. They might end up absorbed by micro-organisms and eventually trickle to the bottom of the ocean. They might also just stay there, dissolved in seawater. Whatever the outcome, we think the vast majority of the carbon absorbed from open air ends up safely sequestered for centuries or even millions of years.

The natural processes of rock weathering capture CO2 and store it securely on geological time scales. Finally some good news, as this sounds suspiciously like a list of requirements for the perfect Negative Emission Technologies.

Unfortunately the process, from rock and CO2 to stable carbonates, is clunky. Today, minerals capture a pitiful 3,5% of human-made emissions. No one is blaming the rocks. But it would be nice to give this entire enterprise a nudge. Or a boost rather.

Giving weathering a boost

Our digestive tract is 5m long, more or less. But, to give a chance for more of the nutrients to pass from the chocolate cake you just had to your bloodstream, evolution has found a way to pack a huge contact area in your guts. The nooks and crannies of our intestines add up to a contact surface of about 30m2 - that’s enough to park 3 SUVs, or 25 bikes -, at least 60 times larger than if our innards consisted of simple, smooth tubes.

We can boost natural weathering processes in the same way, by increasing the contact area between the minerals and ambient air. More surface area equals more space for the chemical reactions to happen.

A simple way to seriously increase the surface area is to crush the rocks into much finer grains and disperse them over large areas. This can be a crop field - for Terrestrial Enhanced Weathering -, a coast line - for Coastal Enhanced Weathering - or the ocean - you get it, for Ocean Enhanced Weathering, which is not technically weathering, but is very similar -.

We can encourage things further by using the best possible rocks for the job. Basalt or peridotite are good contenders. They’re widely available and contain minerals, such as olivine, that quickly dissolve thanks to their high magnesium or calcium contents. We could even consider using non-toxic mining waste, crushed recycled concrete or waste from steel blast furnaces or cement kilns to do the job.

Chemistry, geology and small scale lab experiments tell us enhanced weathering works and has no side effects that might be truly catastrophic. Rough estimates suggest we could capture 2 to 4GtCO2 - Gigatons of CO2 - each year this way, that’s 5 to 10% of today’s emissions of greenhouse gases. Not bad. But the real range of estimates across the scientific literature is 0 to 100GtCO2 per year, at costs ranging from 50 to 200 US dollars per ton of captured CO2. These ranges show how far we need to go to understand the full potential, or lack thereof.

Enhanced Weathering outside the lab

The idea of Ocean Enhanced Weathering, with fleets of boats pumping minerals on the ocean’s surface, has been around for over 20 years. The difficulties involved in the logistics of such an operation seem to have deterred even the most curious and enterprising researchers from trying them out in the open. That, and the fact that dumping materials overboard from a ship is illegal according to international law.

Things are slowly moving forward for Terrestrial Enhanced Weathering, and its cousin Coastal Enhanced Weathering, though.

Project Carbdown - Terrestrial Enhanced Weathering

Project Carbdown is one of the very few projects taking Enhanced Weathering out from the lab and into the field. A team of German entrepreneurs launched the project in 2021, the scientific endorsement coming from professors at Hamburg and Wageningen universities, among others.

They’ve selected three fields, two in Germany and one in Greece, and equipped them with pretty much any type of sensor you can imagine. They’ll measure how much CO2 they can absorb from the atmosphere - they’re aiming for somewhere between 4 and 40 tons of CO2 per hectare, in normal talk that’s 1 to 10 Berlin to New York flights for each 100m by 100m field - and how fast.

They’ve divided their parcels in smaller plots, 3m by 12m. Each one received a different quantity or mix of rock and biochar dust - crushed basalt or olivine rich rocks mixed with a bit of organic carbon - before the start of the planting season. They’re also leaving some plots bare, as a control group. Acidity, temperature, weather, water content and other interesting data points are measured continuously, every 20 minutes. And a frequent sampling schedule will help understand how plants are growing, how much CO2 is captured and how the adjuvants are influencing the soil’s chemistry and microorganisms.

They’re running growing house experiments in parallel as well. There, it’s easier to control parameters and untangle the web of complex flows between soil, minerals, ambient air, water, plants and other living creatures.

The team will look to validate some of the expected positive side effects: does the method encourage plant growth by increasing the soil’s nutrient content? Does it help the soil retain more moisture? And they’ll pay special attention to how the concentration of metals in the soil - especially nickel and chromium, that might be released from the rocks as well - evolves. These metals are toxic at high doses. And while their concentration is not expected to soar, we’re still unsure. We also don’t know how the local microbial population will react to all this.

Project Vesta - Coastal Enhanced Weathering

Another team of entrepreneurs supported by marine ecologists and other breeds of researchers is behind Project Vesta. They’re starting work on their first test site. This is the first open air trial to replicate how rainwater naturally weathers olivine-rich volcanic rocks, washing carbonates out to sea. In the human-enhanced version of this natural process, olivine dust is spread on a beach and the chemical reactions that pump carbon out of the air are spurred by the mechanical action of crashing waves.

Lab experiments and investigation have shown the process works. We have a good idea of how fast things will happen, which side effects to expect and we know where to source the olivine minerals.

Only a field test will help dissipate the remaining uncertainties. The test site is a small beach, somewhere in the Caribbean. It’s being rigged, measured, documented and simulated to establish the baseline against which the team will assess the impact of adding olivine to the system. Once the experiment starts proper, they will measure how fast the olivine gravels dissolve, observe the impact on local flora and fauna and check how the seawater quality changes.

Enhanced weathering will feed new mineral nutrients in the beach’s ecosystem and help fight ocean acidification - another side effect of climate change - . These side effects might turn out to be a boon for local ecosystems, or not.

The team will also use the data they collect to hone their full life cycle assessment. They’re aiming to capture twenty times more CO2 than what is released during the process, once you factor in the environmental impact of mining, crushing, transporting and spreading the minerals.

And since no one has ever done anything of this scale, there are also plans to work with non-profits and develop a code of conduct for enhanced weathering projects and plans to improve public and community engagement in these projects.

What we know today

These trials won’t close the case once and for all.

Clearly, we can’t decide based on two field trials if Enhanced Weathering can play a role in reducing our climate footprint. That’s because the flows these projects are trying to map and quantify are complex and ultra local. No two places on this planet are much the same, so Enhanced Weathering might look very different from site to site.

We’ll need to gather more data points and optimise the methods. For example, we’ll need to find the optimal size of the dust particles and the amount we can add to the soil or coast line each year. We’re also going to need reliable, trusted methods to measure how much CO2 is actually removed from the atmosphere. And we’ll need proper regulations to frame these interventions.

Still, today we know that Enhanced Weathering holds more promises than many other Negative Emissions Technologies :

  • It’s scalable - the minerals are widely available across the surface of our planet and the method does not compete against agricultural land use.
  • It’s permanent - Once the carbon is sequestered as carbonate minerals, it cannot easily leak back into the atmosphere. It stays put for centuries, at least.
  • It’s low tech - The methods rely only on technologies we’ve mastered for a while: mining, crushing rocks, spreading them. Easy.

We will need to build a whole new industry, from scratch

If the first field trials point in the right direction and if we want to scale these methods to make a dent, we’ll need to build, from scratch, an industry that’s as least as big as the coal industry today.

The challenge ahead of us is enormous.

While that dirty industry grew over centuries with the help of the invisible hand of the market, we’ll need to ramp up Enhance Weathering within a decade, and without clear prospects of anyone ever making heaps of profit with it.

The challenge ahead of us is colossal.

In her book Bird by Bird, Anne Lamott recalls the words of wisdom her father had for her older brother one evening, as he sat at his table, petrified by the biology school assignment that was due the next day. He had had three months to write it, and had only just begun. Her father sat down, put his arm around his shoulders, and said, “Bird by bird, buddy. Just take it bird by bird.”

We need to take it bird by bird as well.

We’ll start with more high-quality field trials, validate real-world application scenarios for Enhanced Weathering, then we’ll move on to building the next block of this new industry. Or else.