On Carbon Dioxide Removal
Tags: Geoengineering, Ocean Acidification, Global Warming, Carbon Dioxide Removal, Earth Science

The world is run by deluded, unintelligent, and incompetent people. The case for decarbonization was solid 40 years ago, and it still isn’t happening. In order to reduce the risk of runaway global warming and ocean acidification, carbon dioxide removal (CDR, also known as “negative emissions technologies”) is necessary.

To get this out of the way, burning fossil fuels is geoengineering. Yes, carbon dioxide removal is geoengineering, but more importantly, it’s counter-geoengineering. If you have used so much as a single joule of fossil energy in your life, then you’re a geoengineer. You have played God. You have blood on your hands.

And don’t even get me started on direct air capture. The costs are way, way too high. Building machines that suck carbon dioxide out of the atmosphere is naïve, laughably on the nose, and severely scale-limited by its capital intensiveness. Of course, that means it’s the only approach that ever gets any media attention.

For what it’s worth, here are my thoughts on three carbon dioxide removal strategies, in ascending order of promise and peril.

Reforestation, Afforestation, and Soil Carbon Sequestration (SCS)

This involves growth of forests and prairies, or agricultural practices like no-till farming, cover cropping, erosion control, and soil conditioning. It’s inexpensive, but not a lot of carbon can be stored this way, and when wetland methane emissions are involved, it’s hard to say with confidence what interventions will achieve a net cooling effect.

Aquatic Bioenergy with Carbon Capture and Storage (ABECCS)

In recent years, fertilizer runoff and nutrient pollution have caused vast blooms of the holopelagic seaweed Sargassum natans in the North Atlantic Gyre, also known as the Sargasso Sea (the home of the North Atlantic garbage patch). This demonstrates that the oceans’ biological deserts will bloom under unusual circumstances, like a lot of deserts in California. The ecosystem is close to a critical point, and can amplify small additions of limiting factors like nitrogen, phosphorus and iron into extremely large quantities of biomass.

When it comes to BECCS, there is no better way to meet the scale of the CDR challenge than by turning the oceans’ biological deserts, which cover a whopping 25,000,000km² of the Earth’s surface, into seaweed ranches. Luckily, deep ocean water contains limiting nutrients, and artificially upwelling it fertilizes surface waters. The upwelling pipes can use organic Rankine cycle heat engines to extract ocean thermal energy, for pumping and dynamic positioning.

The seaweed “mats” (and, incidentally, garbage) are harvested by aquatic weed harvesters, transported to process plants on semi-submersible platforms (perhaps by bulk carriers), converted into a biomass slurry, injected into offshore engineered geothermal reservoirs, and hydrothermally liquefacted. The fluid produced from these reservoirs is a mixture of bio-oil, biogas, carbon dioxide, and nutrient-rich water. The bio-oil is transported by oil tankers, injected into depleted oil and gas wells, and left damn well alone. The biogas is converted to carbon dioxide by oxy-fuel combustion, and the concentrated carbon dioxide is injected into deep geologic reservoirs. The water is used as fertilizer.

There have been proposals to deposit terrestrial biomass on the seabed, or bury it just underneath, in deep sea sediments. Storing marine biomass in this way would be inexpensive, but the biomass may decompose too quickly, and perhaps even cause eutrophication and hypoxia. Also, more artificial upwelling would be required to meet the nutrient demand, offsetting some of the savings.

Artificial Ocean Alkalinization (AOA)

The stock of “dissolved inorganic carbon” (DIC) in the oceans is 50 times larger than the stock of carbon in the atmosphere. There are many chemical species of this DIC; some are neutral, like aqueous carbon dioxide, and some are anionic, like bicarbonate. The molar fractions of these chemical species are in chemical equilibrium, just as the atmosphere and oceans are in chemical equilibrium.

CO₂(aq) + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2H⁺

In aqueous solutions, there are “conservative” and “non-conservative” ions. A conservative ion, such a calcium cation, does not associate or dissociate with a proton or hydroxide anion within the pH range of natural water. On the other hand, a non-conservative ion, such as a bicarbonate anion, might associate with a proton to become carbonic acid, or dissociate with a proton to become a carbonate anion, within that pH range. There is actually a small excess of conservative cations in the oceans, and this “charge imbalance” shifts the chemical equilibrium away from the neutral species, and towards the anionic species. This is how artificial ocean alkalinization works: by increasing the charge imbalance, we can reduce the acidity of the oceans, and in doing so, shift carbon dioxide from the atmosphere (where it causes radiative forcing) to the oceans.

How might we add a biblical quantity of alkalinity to the oceans? As it happens, there are large quantities of alkaline groundwater associated with geologic features called ophiolites. These are “sections of Earth’s oceanic crust and the underlying upper mantle that have been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.” Ophiolites contain large quantities of peridotite, an “ultramafic” igneous rock. Because this peridotite was recently (in the geologic sense) uplifted from the upper mantle, it is far out of chemical equilibrium with Earth’s surface, and represents a vast reservoir of chemical potential energy. This energy drives the non-volcanic hydrothermal activity associated with ophiolites. Exothermic water-rock “serpentinization” reactions, such as…

2Mg₂SiO₄ + 3H₂O → Mg₃Si₂O₅(OH)₄ + Mg(OH)₂

…drive natural convection of the groundwater, and also generate alkalinity, hence the “alkaline springs” associated with ophiolites. This suggests that techniques similar to solution mining or (less charitably) deliberate alkaline mine drainage could be used to cheaply extract alkalinity from ophiolites.

The storage of carbon dioxide in ophiolites has been proposed. Because the ultramafic rock is so reactive, mineralization of the carbon proceeds rapidly. This is probably the most secure way to store carbon. One particularly simple means of doing this is illustrated by Figure 8. The idea is to construct a sort of ‘enhanced geochemical reservoir’ (not unlike an enhanced geothermal reservoir) in places where “peridotite is present beneath a thin veneer of sediment offshore,” so that natural convection of seawater through the reservoir will drive the mineralization of DIC. “The cost would be relatively low,” because no concentration of carbon dioxide or pumping is necessary, but the process is quite slow, because the DIC is quite dilute. Strangely, this paper does not seem to consider the possibility that this hydrochemical circulation would remove carbon dioxide from the atmosphere primarily by alkalinizing the oceans, and secondarily by mineralizing DIC.

There is a catch, however. This process may emit nutrients like phosphorus and iron, heavy metals like chromium, nickel and cadmium, and abiotic methane (a potent greenhouse gas). Interestingly, there are “deep subsurface biospheres” in “hyperalkaline peridotite aquifers,” including methanotrophic bacteria. It’s possible that alkaliphilic, thermophilic hydrogen and methane oxidizing microorganisms could feed on the nutrients, and bioaccumulate the heavy metals, along the lines of in situ bioremediation. These microorganisms could then be filtered out of the alkaline produced water by maricultured filter feeders, such as sabellid worms, or bivalves. The wet, contaminated biomass would be handled in the same way as algal biomass, except that the contaminated produced water would be injected into a deep geologic reservoir.