Part 2 - Running Tide Carbon Removal and System Architecture

Mass transfer is a challenging engineering problem. Gigatons of mass must be fixed in the fast cycle and then transferred to slow cycle reservoirs. Running Tide’s thesis is that fully engineered systems will struggle to scale to the magnitude of the climate emergency because enormous mass transfer places enormous demand on energy, construction materials, and supply chain logistics. We therefore pursue interventions which amplify naturally-occurring mechanisms of carbon removal.

We frame our solutions from the perspective of mass transfer ratio – the ratio of mass that has to be manipulated by artificial systems to the gross mass of CO2e removed from the fast to slow cycle. In order to quantify carbon removal and generate commercial carbon credits, gross CO2e removal must be converted to net CO2e removal by deducting all emissions across the lifecycle of the carbon removal technology.

2.1 Running Tide Carbon Removal

Running Tide has developed a global CDR system that integrates and amplifies three natural carbon pathways. Running Tide processes sustainably-sourced, carbon-rich forestry residues into carbon buoys: drifters on which to grow macroalgae. Carbon buoys are coated with calcium carbonate that stimulates a process known as enhanced alkalinity, a recognized CDR approach to sequestering CO2. Macroalgae growing in the surface of the open ocean fixes carbon through photosynthetic growth, then sinks, transporting the embodied fast carbon from the atmosphere-ocean interface into the deep ocean, a slow carbon reservoir.

2.1.1 Terrestrial Biomass Sinking

Terrestrial forestry systems photosynthetically fix fast cycle carbon, storing it in primary biomass, soils, and their supported secondary biomass such as mycelium networks. As forest systems mature, the ecosystem becomes more adept at recycling carbon, and the rate of novel carbon drawdown slows.

Oceans and rivers transport fixed carbon from these terrestrial systems and , thereby creating a removal. This process can be amplified by deliberate harvest, release, and sinking. If biomass is removed from forests at a rate which does not harm its rate of growth, we can preserve the fast-cycle fixation function of the forest while amplifying the removal mechanism to the slow cycle. Forest carbon cycling is fast carbon cycling. In creating an intervention which siphons this cycling carbon into slow cycle reservoirs, we actuate the forests as a carbon removal pump.

Roughly speaking the gross mass transfer ratio of terrestrial biomass sinking is about unity: a ton of forestry residue is a ton of CO2e.

2.1.2 Surface Ocean Alkalinity Enhancement

Dissolution of alkaline minerals, such as calcium carbonate, into the surface ocean alters the balance of the carbonate system in seawater and increases the surface water’s capacity to dissolve carbon dioxide from the atmosphere.

Alkalinity enhancement increases the capacity of seawater to hold inorganic carbon. It achieves carbon removal by reallocating fast cycle carbon to the larger and more stable bicarbonate reservoir. It simultaneously amplifies the fast cycle transfer of atmospheric CO2 into surface waters as the dissolved CO2 that was reallocated to the bicarbonate reservoir is .

The gross mass transfer ratio of ocean alkalinity enhancement will vary seasonally and regionally between 0.26-0.95 with .

2.1.3 Surface Ocean Macroalgae Cultivation

Growing and sinking macroalgae at the surface of the ocean is not the same as coastal blue carbon seaweed afforestation. Because photosynthetic fixation occurs in the open ocean, and because the entire intact organism later sinks to the deep ocean, this pathway achieves carbon removal in a manner similar to the biological pump.

Photosynthetic carbon fixation by phytoplankton in the open ocean is known to be nutrient-limited. Macroalgae species generally have more favorable Redfield ratios, or ability to fix carbon per available nutrients, than phytoplankton. Macroalgae cultivation in the open ocean may therefore amplify the fast cycle fixation of the biological pump.

Carbon fixed by phytoplankton is eaten by higher tropic members of its food web. They metabolize and exhale much of that carbon back into the surface ocean, only sinking a portion of it by aggregating it into fast-sinking fecal pellets. By contrast, non-buoyant species of macroalgae will sink rapidly as an intact organism. Macroalgae cultivation in the open ocean may therefore amplify the fast-to-slow transfer of the biological pump.

Because of this dual amplification, open ocean macroalgae cultivation may present a powerful pathway for carbon removal technology. However, macroalgae is a coastal organism that has evolved to thrive in nutrient-rich and low-energy waters and cultivating macroalgae in the open ocean will be challenging. The good news is that macroalgae present as a diverse array of species with myriad lifecycles and they are highly plastic and adaptable organisms. Open ocean macroalgae cultivation should therefore be seen as a biotechnology frontier which deserves our attention and investment.

Macroalgae grows from microscopic starting material to a macroscopic bulk organism; its mass transfer ratio is essentially infinite from the perspective of moving macroalgae seed to the open ocean. However, macroalgae, and especially non-buoyant macroalgae, must be grown on a floating substrate to remain in the photic zone. Coastal macroalgae aquaculture regularly achieves a biomass to substrate mass ratio in excess of 20:1. These yields may not be attainable in the open ocean, but Running Tide further defrays this mass transfer ratio by synthesizing carbon buoy substrates out of terrestrial biomass and ocean alkalinizing materials.

2.2 Running Tide System Design

Running Tide has designed a system around the purpose of amplifying ocean based carbon removal. Because carbon removal is the transfer of carbon from the fast to the slow cycle, we deploy our system far from coasts and focus on pathways which already remove carbon: ocean transport of terrestrial biomass, dissolution of CO2 in surface waters, and photosynthetic fixation and sinking of marine biomass.

The core principle of Running Tide’s architecture is the development of simple, modular, mass-producible components (carbon buoys) which can be placed in ocean currents, will remain buoyant for a tunable period of time, disperse in ocean currents, and then rapidly switch buoyancy and descend to the seafloor.

While drifting, the carbon buoys deliver an intervention to the chemistry of the surface ocean: they either alkalinize it through mineral dissolution or remove carbon from it by growing macroalgae. Upon sinking, they deliver both terrestrial and marine biomass to the ocean floor. Because these carbon buoys disperse over a vast geographic area, the spatial density of individual carbon buoys is dilute. This minimizes acute environmental exposures and allows the intervention to be delivered as a perturbation to ocean-scale carbon cycling dynamics.

Because carbon removal is a mass transfer problem, the commercial viability of carbon removal technologies will depend on the net, rather than the gross, mass transfer ratio. The architecture of numerous passively drifting carbon buoys provides Running Tide’s system with a powerful mass transfer lever. Engineered systems reliant on anthropogenic energy inputs are only required to move the seeded carbon buoys offshore and place them in surface currents. The fixation of carbon from ambient seawater, and the relocation of biomass from regions of growth to regions of durable storage, is powered through sunlight, ocean currents, and gravity.

This system is being specifically developed to amplify carbon removal technologies to a climate-relevant Earth-scale operation. Bringing it into full existence will require key breakthroughs in applied science as well as investments to rewire the logistics chains of the industrial economy.