Part 1 - Carbon Removal and the Earth System

The human-caused climate emergency is commonly and narrowly understood to be a problem of atmospheric carbon dioxide, as this greenhouse gas contributes directly to warming and destabilizing the biosphere. However, in order to develop effective solutions to our carbon problem we must develop a systems-level understanding of atmospheric carbon within the context of the global carbon cycle.

1.1 Earth System Carbon Cycling

The Earth system stores carbon in several primary carbon reservoirs, namely the marine and terrestrial biospheres, the atmosphere, the ocean, sediments and rocks. The Earth system has two distinct, but coupled, carbon cycling dynamics: the fast carbon cycle and the slow carbon cycle.

The fast carbon cycle consists of carbon that flows in and out of reservoirs continuously or up to a decadal timescale, including the reservoir of atmospheric CO2. It encompasses the movement of carbon via photosynthesis and respiration, as well as the continuous exchange of CO2 amongst the biosphere, atmosphere, and ocean. The fast carbon cycle is dynamic and volatile, and it can be best understood as the flow of carbon through living ecosystems.

The slow carbon cycle consists of the movement of carbon via gravity, pressure, chemical weathering, and ocean overturning circulation. These processes move carbon from living ecosystems into geological and deep ocean reservoirs such as sediments, mineral deposits (oil, gas, coal), and deep waters. Slow cycle reservoirs evolve over centuries to geologic .

The fast and slow carbon cycles are loosely coupled. The global carbon cycle operates through a variety of response and feedback mechanisms which maintain a balance between these cycles, keeping the biosphere in the Goldilocks zone for ecosystems to thrive.

1.2 The Climate Emergency

The problem statement of the anthropogenic climate emergency is that carbon has been artificially moved from the slow carbon cycle to the fast, at . We did this by burning, over the course of two centuries, carbon which the Earth accumulated over eons into slow cycle reservoirs. Barring a Venus-level cataclysm, the Earth system may eventually reverse this anthropogenic carbon imbalance, but this will occur millions of years too late to mitigate the destruction done to ecosystems and civilization.

When carbon is released into the fast cycle, it pools in a number of fast carbon reservoirs. It dissolves into the surface ocean and soil, gets fixed into photosynthetic biomass and, of course, .

Saturating these reservoirs is an insidious ecological problem. The biosphere is famously being destabilized by the greenhouse warming effect of atmospheric carbon dioxide. Similarly the ecologies of shallow seas are threatened by the acidifying carbon concentration in surface seawater, which especially .

1.3 Carbon Removal

Carbon removal is the mass transfer of carbon back from the fast to the slow cycle. Carbon removal occurs naturally in the Earth system over geologic time. The goal of carbon removal technologies is to amplify and accelerate this transfer in order to mitigate the human-caused climate emergency. The task for carbon removal technologies of moving mass cannot be overstated: gigatons of carbon-containing mass have to be transferred to the slow cycle net of the emissions it takes to move them.

Carbon removal technologies achieve carbon removal through a deliberate anthropogenic intervention. To be relevant, these interventions must be additional to the fast-to-slow cycle transfer that is naturally occurring already by way of the Earth system carbon cycle. In order to be commercialized, these interventions must be quantifiable. Carbon removal is durable by definition, as these interventions actuate a transfer to the slow carbon cycle.

Even if the only goal of carbon removal were to remove the anthropogenic perturbation to atmospheric carbon, this cannot be achieved in isolation from the rest of the fast cycle. Fast cycle reservoirs are tightly coupled; adding or removing carbon to one of them will induce a fast cycle rebalancing among the others. Only a portion of the carbon emitted by humans resides in the atmosphere today.

When CO2 is artificially removed from the atmosphere, it will be replaced by CO2 outgassing and other fluxes from other fast cycle carbon reservoirs such as the soils and surface ocean, as these reservoirs are . This is an example of why it is important to define carbon removal as fast to slow removal, not narrowly as atmospheric removal. The only way to mitigate anthropogenic emissions is to remove the totality of those emissions – be they pooled in the atmosphere, soils, or oceans – from the fast to the slow carbon cycle.

1.4 Fast Cycle Carbon Fixation

Carbon fixation means the aggregation of carbon into the non-atmospheric fast cycle reservoirs.

From the perspective of carbon removal, fixation includes not just photosynthetic carbon fixation, but such things as the inorganic dissolution of carbon into the soils and surface oceans.

Fast cycle fixation is not carbon removal. Tree-planting, for example, may amplify fast cycle fixation but it does not remove carbon to the slow cycle. Forests are a fast cycle reservoir, and carbon credit programs which rely on fast cycle reservoirs such as forestry rightly have to be concerned with durability, as forest carbon can burn down or otherwise continue to move through the fast cycle. By the same token, burning wood for fuel is conceptually different from burning coal; only the latter reintroduces carbon from a slow cycle reservoir back into the fast cycle.

Fast cycle fixation does not achieve carbon removal but plays a critical role in enabling it. Because carbon removal is a mass transfer proposition, the role of fast cycle fixation is to change the density and phase of carbon such that it is easier to transfer the fast carbon mass back to slow. Collecting ambient atmospheric gaseous carbon is a thermodynamically onerous task and before carbon can be removed to the slow cycle it must be fixed in the fast cycle. For this reason, restoration and improvement of fast cycle reservoirs are critically important to re-establishing fast cycle system equilibrium, promoting a stable climate, and reducing disruption to natural ecosystems and human communities.

1.5 Ocean-Based Carbon Fixation and Removal

The ocean actuates several pathways of fast cycle carbon fixation and carbon removal.

One such pathway is inorganic: fast-cycle fixation occurs when the surface ocean dissolves atmospheric CO2. The ocean is particularly well-suited to retain large amounts of carbon. Carbon dioxide performs specific reactions with water, combining to form carbonic acid and further dissociating into carbonate and bicarbonate. The result is a buffer pool of dissolved inorganic carbon, allowing a volume of seawater to dissolve many times more carbon than might be .

Carbon removal occurs in the inorganic pathway when the surface ocean mixes and sinks down into deeper water. .

Other pathways involve more complex biogeochemical leverage. The biological pump is the name given to one such phenomenon – phytoplankton photosynthetically fix dissolved inorganic carbon from the surface ocean, allowing it to in turn . Grazing on phytoplankton by higher trophic members of the ecological web, such as zooplankton and fin fish, packages this carbon into fecal pellets which sink rapidly thereby removing biologically fixed carbon to the deep sea.

Coastal macroalgae forests, like terrestrial forests, participate in fast cycle fixation but do not contribute to carbon removal except insofar as . Much has been made about and its consequences for . This is another ambiguity that can be resolved by understanding carbon removal as the transfer from fast to slow cycles.

1.5.1 Additionality of Biologically Sunk Carbon

The biological carbon, such as phytoplankton, that sinks into the deep ocean was likely already destined to be removed to the slow cycle because it was already inorganically fixed in surface ocean waters, which mix and sink to deeper water. Biological pumping of organic carbon causes additional removal when the inorganic carbon content of surface water is replaced by additional fast cycle fixation of atmospheric carbon into dissolved carbon.

When dissolved inorganic carbon is removed from surface water via biological fixation, it creates capacity for additional dissolution of atmospheric carbon. Dissolution will continue until the surface water either becomes saturated with carbon, or else . Carbon removal technologies which leverage these biological pumping pathways remove the quantity of carbon represented in the novel dissolution of carbon they induce, not the quantity of biological carbon which they sink.

1.5.2 Durability of Carbon Removal Via Sinking

Dissolved inorganic carbon may arrive to deep ocean water via downward mixing of surface water, or else via . Either way, it will remain out of contact with fast carbon reservoirs, including the atmosphere, for hundreds to thousands of years given and . If sunken carbon makes its way into ocean sediments, either through inorganic or biogeochemical pathways, it will . From the perspective of the anthropogenic climate emergency, either outcome can be considered durable removal to the slow cycle.

1.6 The Bicarbonate Reservoir

When carbon dioxide dissolves in the ocean, it undergoes a series of reactions with seawater resulting in a rebalancing of different chemical species of dissolved inorganic carbon: aqueous carbon dioxide, carbonate, and bicarbonate. The balance of these species is governed in a given region of water by thermodynamics (temperature, pressure, salinity), and pH. Because water at the ocean surface can freely exchange acidifying carbon with the atmosphere, the pH in this region is ultimately parameterized by alkalinity. Alkalinity is the imbalance in the total concentrations of cations and anions in seawater. Because these alkalinity-governing solutes are stable in seawater, the bicarbonate reservoir is stabilized for a given thermodynamic condition.

A key characteristic of the bicarbonate reservoir is its enormous quantity (~90%) compared to the amount of carbon dioxide (~1%) in the general seawater system. The bicarbonate reservoir is mechanistically tied to the fast carbon system through marginal exchange with the smaller CO2 reservoir. But because the magnitude of the bicarbonate reservoir is so large compared to the aqueous CO2 reservoir, the residence time of carbon within the bicarbonate reservoir is comparably longer than the residence time of carbon in the CO2 reservoir.

Both the relatively long residence time and stability of the bicarbonate reservoir render it functionally consistent with a slow carbon reservoir rather than a fast one.

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