For Carbon Removal, Scaling is More Critical than Permanence

June 28, 2023

Why an understanding of carbon flux is essential to a viable carbon market

The first trees nearly froze the earth. They towered above the understory thanks to their evolutionary ability to produce lignin, which gives trees their compressive strength. When these first great trees fell, no organism had yet evolved that could digest lignin, as fungi and terrestrial herbivores do today. As a result, these trees lay buried deep underground until humans excavated them as the world’s first coal deposits. At Living Carbon, we aim to enable natural and scalable carbon removal on par with these ancient evolutionary events by enhancing photosynthesis and slowing the decay rate of plants. Three hundred million years from now, a similar story could be told about today; an era of carbon sequestration so massive the rocks will tell our story. How do we achieve this? Here’s a hint: every major ice age over geological history is associated with an evolution in photosynthesis, coupled with other contributing factors such as continental positions, ocean currents, and enhanced weathering. At Living Carbon, we believe that intentionally recruiting large-scale biospheric and geological processes to assist human effort holds the most potential to draw down carbon in the quantity and time frame we need, to preserve the world we know.

A concept often encountered in carbon removal is that of “permanence,” the idea that the quality of carbon removal is best measured by the duration of storage of removed carbon out of the atmosphere. We believe the carbon removal industry is increasingly becoming hyperfocused on permanence without adequately valuing scalability. Fluent in commodity markets, it is easier to grasp an idea of fixed exchange: one ton of carbon is bought, and one ton is sequestered forever. While durability is important, the permanent removal market will not grow to be large enough by 2050 to achieve gigaton-scale removals. Scalability is under-discussed in both carbon removal markets and public policy.

The miscorrelation of permanence with efficacy in carbon drawdown has serious implications, as noted in a recent United Nations publication on carbon removal:

“The permanence of being chemically fixed (e.g. in rocks or in geological storage) is physical permanence (or physical irreversibility) and has no economic value beyond the time horizon. If we were to value carbon storage independently of any time horizon of interest, 1 tCO2 removed and stored through carbon mineralization could be considered to have a value 100, 1000, or 10,000 times greater than the value of 1 tCO2 removed and stored for 100 years. This leads us to an absurd conclusion that we know is not true.” - United Nations Article 6.4 Supervisory Board; Removal Activities Under the Article 6.4 Mechanism, May 2023

Carbon removal is often thought of as the accumulation of a commodity in a storage vault, where the focus is primarily on the rate of return to the atmosphere as a determinant of quality. However, in our current situation, the rate of removal is far more important than the rate of return. The removal needs to happen within the next few decades to preserve our global biospheric systems. The production of carbon with thousand-year permanence is irrelevant if the scaling factor is a hundred times lower over a relevant time frame. 

Using biotechnology, we are working with plants to increase the rate of photosynthesis, decrease the rate of decay, and clean soils made toxic by industrial activity. In addition to their improved rate of carbon uptake, our trees are more resilient to the negative effects of climate change; initial results show drought tolerance and resilience to increased temperatures, and they have the potential to grow on land where climate change is already degrading the ability of other trees to thrive.

Leveraging biological carbon drawdown enables us to avoid the high starting costs, and requirements for ongoing and intensive management seen in methods such as direct air capture. At Living Carbon, we focus on high-quality nature-based carbon projects by planting multiple species of trees, including our climate-smart seedlings, on land degraded by human activity. We are especially focused on abandoned minelands as sites of former fossil fuel extraction. The US alone has 133 million acres of reforestable land, capable of capturing 333 Mt of carbon annually. [Source: American Forests]

The Keeling Curve with Enhanced Biology

2°C warming threshold

We looked at the impact of increasing biomass (photosynthesis enhancement), reducing the decay rate, and increasing the total percentage of managed forests that utilize enhanced trees and carbon-optimized practices. These may include coppicing (felling trees at their base and letting them regrow) protecting biomass from near-term decomposition. For more information on our dataset, click here.

Our Keeling curve model takes the current annual measured average increase of 2.37 ppmCO₂/year over the last decade and extrapolates this trend until 2050. While the Keeling curve is best modeled by an exponential rather than a linear function (the rate of increase of atmospheric CO₂ has been itself increasing since the Industrial Revolution), over a time frame of only 27 years, and assuming that the most pessimistic “business as usual” scenarios of human emissions are unlikely to occur, for illustrative purposes a linear model is adequate. The annual periodicity of CO₂ increase and decrease caused by variation in photosynthesis of land plants in the Northern Hemisphere according to the seasons is represented as a trigonometric function superimposed on the linear annual increase. Photosynthesis enhancement is represented as an increase in the amplitude of the falling portion of the sinusoidal wave. Decomposition resistance, increases in soil carbon storage, and other methods to reduce the rate of return are represented by a decrease in the amplitude of the rising portion.

As this model is presented as a conceptual illustration, we used the global existing managed forest area totaling 2.05 billion hectares to show the effect of portions of this area being planted with trees enhanced for carbon capture, decomposition resistance, or both. In reality, the total plantable area is significantly larger if we include reforestable lands degraded by previous human mismanagement.

NOAA Dataset:

The Keeling Curve shows the ongoing change in the concentration of carbon dioxide in Earth’s atmosphere. It was named for the pioneering climatologist Charles David Keeling, who initiated the measurement program at the Mauna Loa Observatory in Hawaii. The Keeling Curve is beneficial for understanding climate change because it shows both the progressive increase in CO2 due to human emissions, and the annual seasonal fluctuation as biomass grows and decays throughout the year. There is variation due to changing rates of terrestrial photosynthesis over the year: the concentration of landmass in the Northern Hemisphere adds to the seasonal flux, with an annual drawdown during the Northern Spring and Summer and return due to decomposition in the Northern Winter.

Any carbon removal method will bend this curve downward in proportion to the removal rate, with the rate of return (re-release of CO2 into the atmosphere) subtracted. Enhanced biology offers significant near-term drawdown that can flatten the curve, expanding our window to reduce human emissions and develop complementary solutions. 

Global Carbon Flux

Carbon flux is the continuous flow of carbon between Earth’s carbon pools, from the oceans to the atmosphere to geological formations and the bodies of living and dead organisms.

Illustrative diagram of the biological carbon cycle.

The Carbon Cycle: Carbon flows between each reservoir in an exchange called the carbon cycle, which has slow and fast components. Yellow numbers represent natural fluxes (in gigatons of carbon per year), red are human contributions in gigatons of carbon per year (or effects on natural fluxes from human contributions), and white numbers indicate carbon storage pools. [Source: NASA]

On average over the last decade, terrestrial plants absorb 123 gigatons (Gt) of carbon per year through photosynthesis, compared to human emissions of 9 Gt annually (NOAA). When we enhance the rate of photosynthesis, decrease the rate of decay, and regenerate degraded ecosystems, we are increasing the standing plant biomass pool (living plants) on Earth. Currently, the standing plant biomass pool is about 550 Gt, assuming a steady state, where photosynthetic drawdown is balanced by plant respiration and microbial decomposition of plant parts. In a given year, both these processes return approximately 60 Gt each of carbon to the atmosphere, with about 3 Gt added to the soil carbon pool (where 2,300 Gt currently reside).

In general, carbon flux between the atmosphere and the surface of the planet can be considered according to the "fast" (timescales of years to centuries) or "slow" (timescales of millennia to gigayears) carbon cycles. As expressed in the recent UN report, any carbon drawdown method must be considered relative to a relevant time horizon to be correctly evaluated for its efficacy.

“The value of removals, and indeed of emissions reductions or any climate action, is relative to our climate goals and our time horizon. If our goal was to tackle the next ice age, we might have set a time horizon of 25,000 years. But given the situation we are in, a time horizon of 100 years might be more appropriate. Of course, one could argue that it should be 200 or even 300 years.” - United Nations Article 6.4 Supervisory Board; Removal Activities Under the Article 6.4 Mechanism, May 2023
Illustration of simplified funnel metaphor.
Illustration of simplified funnel metaphor.

We can consider the biological carbon cycle as a funnel which carbon passes through. On one side, the intake of the funnel represents the fixation of atmospheric carbon dioxide. The body of the funnel represents the time carbon spends in living or dead biomass, and the output represents the return of carbon dioxide to the atmosphere through the respiration of decomposing organisms.

The rate of movement of carbon through the funnel is the carbon flux rate, which can be modeled by the differential in size between the input and output of the funnel. Analogizing carbon movement as a fluid flow in this way gives productive insights on how to best detain carbon as long as possible in the body of the funnel. Photosynthesis enhancement is analogous to increasing the width of the input, and decomposition resistance is analogous to constricting the output. Keeping this analogy in mind, let us look at how global carbon flux operates.

Human emissions result in an annual excess of about 4 Gt of carbon, averaged over the last decade and including the reduction in emissions in 2020 associated with the COVID-19 pandemic (source: NOAA), which accumulates in the atmosphere as roughly 14.68 Gt of CO2 per year. Atmospheric CO2 concentration is usually measured in parts per million (ppm), and 1 ppm represents an addition of 7.82 GT of CO2. This should therefore result in a year-on-year increase of about 1.88 ppm. The directly measured increase is more like 2.37 ppm/year, showing how our carbon cycle models still underestimate the degrading effects of human societies on the ability of the biosphere to retain fixed carbon. Future projections likely need to assume a baseline annual excess of just over 5 GT of carbon in the near-term. Efforts to absorb this excess and bend the curve of carbon accumulation in the atmosphere can take one of two general paths:

  1. Fast carbon cycle: We can leverage the fast carbon cycle by increasing the total carbon drawn down by photosynthesis in a given year, while ensuring that the rate of carbon release by decomposition either remains constant or increases more slowly than the photosynthetic drawdown (see “Keeling Curve with Enhanced Biology”). We are increasing the pool of standing plant biomass and the rate of carbon uptake, and the impact is a downward bending of the curve. Enhanced biology is essential because it allows us to draw down more carbon on less land. By choosing to work with long-lived woody plants (trees), we have the additional benefit of the relatively constant decomposition rate of wood. This results in more carbon in wood, both living and dead, remaining in the body of the funnel and an increase in carbon storage in the landscape proportional to the increase in photosynthetic rate. If we add active slowing of the decomposition rate, either through traits that enhance the intrinsic resistance of wood to decay, or methods to protect the wood from short-term decay, such as burial, we can increase the rate of carbon removal to likely double that of photosynthesis enhancement alone.
  2. Slow carbon cycle (often called “permanent carbon storage”):
    1. We can add carbon to long-term storage pools by producing carbon-rich minerals using direct carbon dioxide extraction from the atmosphere via industrial processes, such as direct air capture. These solutions aim to introduce carbon into extremely long-term lithic storage directly, without the intervention of the biosphere. It is important to note that these solutions are energetically front-loaded; they expend great energy in industrial processes to produce sufficiently stable carbon forms that are unlikely to return easily to the atmosphere.
    2. There are also forms of biological permanence, which work with biological drawdown and compound sequestration effects. We don’t need to choose one method over the other, but we do need to focus on scalability from an energetics perspective.
      1. Sporopollenin is a biological polymer that has the permanence attributes of a mineral carbon storage method.
      2. Increasing oxalate production in soils is another biospheric mineralization process.
      3. The production of biochar or bio-oil and injection underground are also effective paths, with the advantage (at the cost of increased input energy) of producing highly recalcitrant forms of carbon, which are directly added to the slow carbon pool.

We have a few decades to rebalance the carbon cycle before global processes with immense energetic weight behind them (such as albedo changes, ocean current rerouting, and polar methane clathrate release) begin operating at scale and propel the planet into an ice-cap-free equilibrium, as they have many times before. Therefore, we need fast, scalable, and energetically favorable carbon solutions that increase biomass globally and enhance the ability of the biosphere to hold onto fixed carbon. 

A permanent solution with high initial energy expenditure, such as direct air capture, offers the reassurance of a negligible rate of return. However, these solutions cannot take advantage of self-scaling processes in the manner of biological solutions, or of operation without continual energy input, as in enhanced weathering. We believe in all solutions on deck, including direct air capture, without focusing on permanence over scale as a measure of quality. DAC is especially efficient on emissions sources, where the high local concentration of carbon dioxide obviates the energetic inefficiency of removal from the general atmosphere. However, we would need 4 GT of carbon sequestered every year to reach net negative emissions with permanent sequestration alone. According to the International Energy Agency (IEA), “In the Net Zero Emissions by 2050 Scenario, direct air capture is scaled up to capture almost 60 MT CO2/year by 2030.” 60 MT of CO2 is equivalent to 16.4 MT of carbon, 0.0041% of what is needed. This is a drop in the bucket, so small that it makes no meaningful difference on the Keeling Curve.

IEA Projection for Annual DAC Sequestration

The chart above fits an exponential function to the International Energy Agency’s projected scale-up of Direct Air Capture by 2030 and 2050. Using this exponential function, the interactive Keeling curve below demonstrates the multiplier needed (50-100x) of the IEA's current projection for Direct Air Capture to have a meaningful reducing effect on the rate of atmospheric carbon dioxide accumulation.

Keeling Curve with Direct Air Capture

2°C warming threshold

We need significant near-term drawdown to flatten the curve and expand our window to both reduce human emissions and develop complementary solutions, including biomass injection, enhanced weathering, and direct air capture, to bring carbon flux as a whole into a state of net drawdown from the atmosphere to the land and oceans.

As an example of biological carbon flux in action, consider Pando, the ancient Aspen Grove, a tree species closely related to the poplars we are working with. Like many other aspen groves, Pando has persisted for millennia, but individual trees come and go over one or two centuries. Carbon is sequestered as new growth and incorporated into the root system as sugar is transported underground. While individual stems are temporary, the grove as a whole increases landscape-scale carbon storage over the longer term, as the persistent root system, where up to half the carbon is stored, continues to add biomass over millennia. Just as the grove can grow even while some trees die, so can we grow pools of carbon even if there is some release back to the atmosphere. We can grow these pools faster on less land with photosynthesis-enhanced and decomposition-resistant trees.

To increase the time that carbon remains out of the atmosphere, we can:

  1. Increase standing biomass (the living carbon pool), and reduce the rate of near-term decomposition of dead biomass.
    1. Afforestation is an example of this first approach, but we must note that a year-on-year increase in standing biomass cannot be maintained indefinitely due to land use requirements. Enhanced management practices such as coppicing can, in certain tree species, significantly increase the biomass production rate.
    2. To address this, we can ensure that all nature-based approaches explicitly consider how to reduce the carbon return rate to the atmosphere. There are simple choices for managed forests, such as burying remaining biomass at least several meters deep after harvest rather than burning it or leaving it on the surface.
    3. Living Carbon is developing a metal accumulation trait that enables trees to accumulate higher levels of metals in their roots and stems, acting as a natural fungicide, slowing the rate of carbon decay and cleaning soils made toxic by industrial activity.
  2. Increase the rate of photosynthesis
    1. The photosynthesis enhancement pathway used by Living Carbon’s treesincreases the rate of carbon assimilation by up to 30-50% in initial studies. We are currently working with hybrid poplar and loblolly pine. However, the pathway is not species-specific. Our goal is to plant local varieties.
    2. While we are also working to reduce the rate of decomposition, increasing the rate of carbon assimilation while the rate of decomposition remains constant in itself reduces the rate of atmospheric accumulation of carbon dioxide(see “Keeling Curve with Enhanced Biology”).
    3. If we assume that field-planted photosynthesis-enhanced trees average a 25% increase in the main stem and root carbon, then all this carbon adds cumulatively over the life of the tree (which can be modeled as 10 to 12 years for a managed forest of photosynthesis-enhanced hybrid poplar but is more like 80 years without harvesting and replacement). This means that photosynthesis-enhanced trees will add about 12.5% to the drawdown potential of a given area of forest land, year on year if planted as 50% of the total trees to avoid a monoculture.

Achieving our climate goals requires us to model relative rates and carbon flux and understand how carbon flows through ecosystems. If we do this correctly and act with precise intention to leverage the relevant energy flows in our favor, we can achieve efficient, scalable carbon drawdown with significant life-generating benefits – clean air, water, food, soil, and built environments. We can also use the products of biological sequestration to launch new markets and wood-based construction technologies.

As expressed in the UN report:

“In addition to the mitigation value of temporary removals in terms of slowed atmospheric warming, temporary carbon removal provides multiple other benefits. In short, deployment of temporary carbon removals:
  1. a) Moderates adverse impacts on biodiversity and allows ecosystems and human socioeconomic systems to adapt over a longer time
  2. b) Buys time for technological developments and economic capacity to address climate mitigation more effectively and for economic opportunities including capital turnover
  3. c) Reduces risk of reaching tipping points such as release of carbon from permafrost or icesheet collapse by smoothing out the path of emissions and avoiding peaks
  4. d) Reduces long-term cumulative climate impacts
  5. e) Reduces costs of meeting temperature targets relative to late mitigation as a slower increase of the damage level lowers the present value of costs
  6. f) Bridges the progress toward the long-term climate target through achievement of near-term benefits."
- United Nations Article 6.4 Supervisory Board; Removal Activities Under the Article 6.4 Mechanism, May 2023

Timing is critical. Near-term deployment of negative emissions technologies is necessary to prevent severe and functionally irreversible tipping points.

IPCC Special Report: Global Warming of 1.5 ºC; Figure 2.5 - Evolution and breakdown of global anthropogenic CO2 emissions until 2100. [Source: IPCC]

A close look at net-zero 2050 scenarios will reveal that many explicitly rely on massive scaling of photosynthetically produced biofuels for fossil fuel and wood burning replacement by 2050. We can unlock this same potential for carbon removal – if we monetize it appropriately. Why are we calling biological, non-permanent removal “low quality” when it is the dominant method by which we avoid crossing the 2-degree threshold under all realistic models?

Furthermore, the carbon removal method assumed under these models is BECCS (BioEnergy production with Carbon Capture and Storage). This is usually considered an engineered, rather than nature-based method (included in the IPCC illustration above as “technological CDR”), but relies on photosynthesis as the carbon drawdown mechanism. Direct air capture from the general atmosphere is not included as a significant drawdown mechanism by the IPCC, for reasons explored above in this post, the distinction is instead between BECCS and AFOLU (Agriculture, Forestry and Other Land Use). In BECCS, plant biomass (for example, switchgrass) is grown under intensive cultivation conditions for carbon capture, and is then burned to generate electricity, with chemical carbon capture devices installed on the exhaust gas stream to recapture emitted CO2, which is then injected underground. This is operationally identical to a CO2 capture system installed on a point emissions source. BECCS is attractive from the perspective of producing a useful energy resource (electricity) in addition to achieving net carbon drawdown, but there are inherent inefficiencies in capturing, emitting, and recapturing carbon. Most notably and often cited, BECCS would require a land area equal to the size of India to cultivate enough biomass to achieve sufficient drawdown by this method alone. This degree of expansion of cultivation is implicitly assumed in models which posit BECCS as our primary drawdown method.

It would be far more efficient from an energetic perspective to decarbonize electricity generation using solar, wind, and other renewables, rather than rely on BECCS. In this instance, photosynthesis enhancement, decomposition inhibition, and similar "engineered nature" approaches can fill the place of BECCS without the disadvantage of massively increasing the land area of the planet under intensive cultivation. This would instead equate to additional carbon capture in the AFOLU category.

More than 80% of scenarios in the IPCC’s Special Report on 1.5ºC overshoot the 1.5ºC temperature threshold before returning to these levels using large-scale carbon removal in the second half of the twenty-first century. This assumes a level of substitutability between emission reductions in the near-term and further-off removals, a notion that is rapidly being institutionalized in net-zero targets. Here we have an institutional framework to incentivize the funding of permanence at the expense of the scaling we need right now. While temperature overshoots might be reversible, this is not necessarily the case for other climate and geophysical dynamics.

When we trace the path of carbon through the biosphere, we are thinking in terms of the movement of energy in living systems. If we take as our goal maximizing the flourishing of the living system of the planet through enhancing large-scale biophysical processes, we can achieve goals and unlock opportunities that are not possible in a linear commodity exchange.

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