6. Solving for Legacy CO2: Carbon Mineralization

Papakōlea Green Sand Beach on the Big Island Hawaii.

A little known fact: Earth naturally takes atmospheric CO2 and transforms it into solid rock, where this carbon resides for thousands or even millions of years. This is the “slow carbon cycle”, which occurs over thousands of years, in contrast to the fast cycling of carbon from the air into plants via photosynthesis.

It all starts with rain, which binds CO2 from the atmosphere as carbonic acid. When rain falls on alkaline rock formations, rainwater slowly dissolves the rock, via a process called chemical weathering. Over time, this weathering releases bicarbonate, calcium, magnesium and potassium, which streams and rivers transport to the sea.

There, marine organisms like algae, coral, sea snails, barnacles and urchins use these mineral building blocks to grow their shells and skeletal structures. When these creatures die, their remains fall to the ocean floor, where they combine with sediments and are compressed by the deep sea into limestone.

Unfortunately for us, carbon mineralization occurs far too slowly in nature to help stem our rapidly rising levels of greenhouse gas emissions. According to MIT Technology Review:

[Carbon mineralization] draws down at least half a billion metric tons of carbon dioxide annually. The problem is that society is steadily pumping out more than 35 billion tons every year. 

However, we can harness the carbon mineralization process and artificially speed it up, so that large amounts of CO2 can be captured within a useful time frame. This artificial process is called enhanced weathering.

Limestone Rock Cracking From Weathering Effects

Enhanced weathering (EW) facilitates increased rates of CO2 absorption by certain mineral rocks, that are known to be proficient at sequestering CO2 from the air. It all starts with crushing the mineral rock into small fragments, to expose as much surface area as possible.

After that, there are a few ways to initiate CO2 capture from crushed mineral rocks. It can be heated and pressurized in a reactor, mixed in with farmland soils or deposited on beaches and shallow seas.

A heated reactor could make sense paired with geothermal power, but would be slow and complex to implement. Soils weathering has potential, but the rate of sequestration is slower than beaches or seas, which may be the most viable option.

Waves, tides and currents provide the power naturally to mechanically mix and agitate the rock fragments, which is necessary to prevent layers of silicate crust forming on exposed edges, which block CO2 absorption. As such, this post will focus on enhanced weathering via ocean action.

One particular type of magnesium silicate rock, called Olivine, has been found to absorb CO2 faster than other minerals. For this reason, olivine is the optimal mineral of choice, and EW projects ought to be located near rock formations with olivine.

Olivine Fragments. Credit: WhatsOrb

It is clear that we have abundant amounts of olivine, enhanced weathering can work quickly to sequester CO2 and could be scaled up to the magnitude of the climate crisis. For perspective, applying olivine to 2% of the world’s beaches could offset all of humanity’s annual CO2 emissions.

That being said, there are challenges. Significant energy would be required, project costs are uncertain, environmental impacts are still being studied, and gaining public approval to turn beaches green or dump crushed rock into near shore seas could prove difficult in many areas. Let’s explore each in turn.

Energy Inputs and CO2 Emissions

Mining olivine out of the ground, pulverizing it into suitably small fragments and transporting it to the final location for deposition entails the use of heavy machinery, which need substantial power and fuel supplies to operate. Currently, these machines run almost entirely on fossil fuels, which would erode the net CO2 benefits of enhanced weathering. One estimate found that 10-30% of total CO2 captured by enhanced weathering projects would be negated as a result.

At these levels of emissions, the net CO2 gain is probably still worth the investment, but fossil fuel use will make the project more expensive per ton of CO2 captured. This metric ought to be measured against the price of other geological storage techniques, such as BECCS and Direct Air Capture, to gauge how enhanced weathering compares equivalent alternatives. More on costs later.

We’ll probably need to accept some fossil fuel emissions from enhanced weathering projects, in exchange for an overall net CO2 benefit. However, we can still aim to reduce mining, milling and transportation emissions as much as possible.

Most diesel engines can run on a 20% biodiesel blend without engine modifications, which could be the minimum fuel blend for all long distance heavy haul trucks, ultra-class dump trucks, backhoes and other mining vehicles.

Heavy Haul Truck Moves Gravel. Credit: McClymonds Supply

Biodiesel can be sourced from the waste cooking oil and animal fats available from restaurants. Over time, long haul trucks could be retrofitted with bolt-on conversion systems, like that of Optimus Technologies, that allows for 100% biodiesel fueling.

Large Kite for Cargo Ship. Credit: Skysails

For marine transit to deposit olivine in shallow seas, new biofuel mixes can again be utilized. ExxonMobil recently ran a successful trial of its partial biofuel blend in the Netherlands, which can be dropped into existing engines and reduces CO2 emissions by 40%. Transport ships could also be fitted with large kite sails developed for cargo ships by companies like Airseas and Skysails. Kites can cut fuel consumption and emissions by another 20%.

Fully electric rock pulverizers could be utilized on the mine site, which would gradually yield cleaner power as the larger grid decarbonizes. Clean power to mill olivine would allow it to be crushed to the optimally small fragment size, in order to yield the best rate of CO2 capture.

100% Electric H4e Mobile Crusher by Keestrack. Credit: Equipment Journal

These steps can all be taken today, without new infrastructure, fuel sources or carbon pricing. In time, diesel may be phased out by electric motors and hydrogen fuel cells, along with a clean electricity grid and carbon pricing.

Environmental Impacts

Deploying enhanced weathering at scale requires depositing thousands of tons onto beaches and into shallow seas. Loading such concentrations of minerals raises the prospect of negative impacts in local marine habitats, even as the planet as a whole could benefit.

Summarizing from this paper, here are some potential environmental impacts:

• While ocean acidification is a major problem globally, certain marine environments are naturally more acidic. In these areas the alkalizing effect of olivine deposits could harm species that have adapted to higher acidity levels.

• Olivine sheds trace amounts of dissolved silicate, nickel and iron, which could be magnified due to the high volume of rock being applied. Increases in suspended particles could cause clogging and smothering in near shore habitats.

• It’s possible that nickel in particular could cause internal harm to marine species, since it is toxic at certain thresholds.

However, there are indications that nickel is less harmful in saline environments like the ocean, and problems from suspended particles would be temporary, since they will gradually get dispersed by waves, tides and currents. Enhanced weathering projects could avoid naturally acidic marine environments.

EW may also benefit areas where it is applied. Dissolved silicate and iron are linked to increases in primary production by phytoplankton and could prevent harmful algae blooms, which are occurring more often due to warming and water pollution. 

Any examination of local impacts from EW must be grounded in a bigger picture understanding of the impacts on marine life from an overheating climate. Earth’s oceans have absorbed 31% of CO2 emissions from human activity and 93% of the excess heat produced by our emissions, leading to ocean acidification and overheating.

The top layer of the ocean is warming 24% faster than it was decades ago, causing harm and disruption to the many species that live in this layer, including key species like phytoplankton and coral reefs.

By absorbing excess atmospheric CO2, global oceans have become 30% more acidic since the Industrial Revolution, which robs carbonate species like coral, oysters, crabs and tiny pteropods (a key species in the marine food web) of the carbonate atoms that they need to grow and maintain their shells. 

Pteropod Shells Disintegrate Due to Acidity Levels. Credit: National Geographic

For the ocean as we know it, warming and acidification pose an existential threat, which EW could protect against by drawing down CO2 and boosting pH levels.

Still, local concerns must be addressed, and real world study is required to better understand what impacts (or benefits) there might be from EW. Fortunately, the first study of the local effects of olivine enhanced weathering is set to begin this year (2021) in the Caribbean, by the nonprofit group Project Vestas.

Cost

Cost estimates range widely for EW. Olaf Schuiling, a professor of geoengineering at University of Utrecht in the Netherlands, claims total costs to mine, crush and spread olivine would be only $10/ton of CO2 captured. 

However, a 2020 study published in Nature estimated EW would cost $80-$180 per long ton of CO2. This is much more expensive than tree planting or soil restoration, but the true test is to compare EW against other geological storage methods, which offer similar scalability and permanence of CO2 capture.

By this measure, EW is quite competitive: the same paper estimated BECCS at $100-$200/ton and direct air capture (DAC) at $100-$300/ton. Real world projects are needed to gain a more accurate idea of cost, which should happen in the coming years, as each of these methods are being explored in pilot projects.

In any case, geological storage is expensive, and will require strong carbon pricing supports. Particularly, a premium needs to be placed on geological methods, since they offer permanent CO2 sequestration that tree planting or soil restoration cannot offer. Thus, governments must recognize the value of removing legacy CO2.

Social Acceptance: Beaches and near shore seas are cherished environments, and the idea of dumping tons of crushed rock could prove contentious. Rigorous study of impacts will be important to assuage certain concerns, but aesthetic objections are unlikely to go away.

Papakōlea beach on Hawaii’s big island has a novel appeal for its green sands, yet it was made that way by natural processes, not people. Artificially changing the color and quality of an existing beach may evoke the perceived stain of human intervention. Or, perhaps as the climate crisis becomes impossible to ignore, people will embrace this particular solution in between their toes.

The safest best is probably to focus on olivine application in remote areas. Considering the overlay of potential olivine mines, warm climates (in which olivine weathering is optimal) and long stretches of remote coastlines, west Africa, Mozambique, South Africa and Oman look most promising.

If marine shipping emissions could be reduced sufficiently, the Saharan coast could be an option, being one of the longest stretches of coastline on the planet with both low population density and a warm climate.

Saharan Coast from Space. Credit: NASA

The colonial legacy of rich northern nations exploiting Africa and the Middle East may make this idea seem controversial. Enhanced weathering projects should only occur if they :

(A) Do minimal environmental harm

(B) Do not exploit workers or communities

(C) Economically benefit the host nation

The just approach would see the world’s wealthy nations - who are largely responsible for legacy emissions - pay for EW projects, as part of their carbon debt from centuries of industrialization and growth. For the host countries, EW projects could yield long term foreign investments in exchange for using deserted areas and abundant minerals. All the while, this mysterious green sand would be drawing down carbon with each rising and falling of the sea.