Creating a Diversified Solutions Portfolio to Meet the Scale of the Carbon Removal Challenge

RMI’s approach to classifying 32 unique carbon dioxide removal approaches by key resource inputs, to advance a collectively diversified and scalable portfolio

RMI’s assessment of the carbon dioxide removal (CDR) space and path forward has candidly addressed the challenge of achieving the massive scale of emissions removals the IPCC has identified as essential to achieving long-term climate goals.

Alongside that sobering realization lies reason for hope. There are many different CDR approaches using different inputs, environments, and technologies. This suggests the possibility of a portfolio of CDR options diversified enough to meet the scale of removals we need.

In this article we will present a taxonomy that covers all known forms of CDR, and organizes them into categories based on which primary resource input and — as a result — which common challenges they share, as it relates to scaling.

We encourage people working on CDR to make use of this taxonomy, because it sheds light on all the known ways that people are trying to answer this fundamental question: what is the most effective and efficient way to remove carbon from the atmosphere using plants, minerals, or energy?

The primary inputs for CDR approaches are either plants, minerals, or energy

CDR approaches can be sorted into three categories based on whether their primary input is plants, minerals, or energy:

• Biogenic CDR (bCDR) approaches rely on plants. They use naturally occurring biogenic carbon fixation mechanisms to capture carbon dioxide from the atmosphere. The most important of these mechanisms is photosynthesis.
• Geochemical CDR (gCDR) approaches rely on minerals. They use naturally occurring neutralization reactions between acidic forms of carbon and alkaline minerals to convert carbon dioxide from the atmosphere into solid carbonate minerals or dissolved bicarbonates.
• Synthetic CDR (sCDR) approaches rely on energy. They use engineered systems powered by low-carbon energy to directly separate carbon dioxide from the air and capture it, or to alter water chemistry to indirectly remove carbon dioxide from the air.

To these three taxonomic categories, we have added a fourth category for the storage of carbon dioxide. As its name implies, it is not comprised of CDR approaches but of approaches for storing captured and concentrated streams of carbon dioxide through trapping, mineralization, or other physical or chemical processes​.

Within these four categories there are sub-categories, defined below.

Biogenic, geochemical, and synthetic CDR can be further disaggregated based on key process differences

As shown in Exhibit 1, the three main categories of CDR can be further disaggregated into sub-categories based on key process differences.

In biogenic CDR and geochemical CDR, the sub-categories are based on the ways in which the carbon is ultimately stored. For synthetic CDR, the sub-categories are based on the way in which the carbon is removed from the atmosphere.

1. Biogenic CDR (bCDR) sub-categories are based on the way the carbon is ultimately stored:
   1. Living biomass: Approaches that maintain or increase living biomass in forests, agriculture, grasslands, and wetlands. This definition is based on the Natural Climate Solutions (NCS) categorization. This includes enhanced ecosystem techniques such as forestry, afforestation/reforestation, land management, ocean ecosystem restoration, and soil management.
   2. Stabilized biomass: Approaches that use photosynthesis as the carbon dioxide capture step, then store the captured carbon dioxide in a stabilized biomass form. This includes actively stabilized forms of biomass, such as biochar, bio-oil, ocean biomass (microalgae and macroalgae) sinking, long duration wood storage, and wood burial.
   3. Sequestration of CO₂: Approaches that use photosynthesis as the carbon dioxide capture step but then rely on combustion, fermentation, or gasification to convert that biomass to carbon dioxide, which is then stored. This includes bioenergy with carbon capture and storage (BECCS) and other biomass that is converted to CO₂ and stored with carbon capture and storage (CCS), including from waste feedstocks.
2. Geochemical CDR (gCDR) sub-categories are based on the way carbon is ultimately stored:
   1. Solid carbonate minerals: Approaches that seek to enhance the formation of carbonate minerals in carbonate minerals through the acceleration of natural reactions of alkaline materials with carbon dioxide. This includes surficial mineralization.
   2. Dissolved bicarbonate sequestration: Approaches that seek to react alkaline materials with carbon dioxide to produce dissolved bicarbonates that are ultimately stored in the ocean or other bodies or water. This includes coastal and terrestrial enhanced weathering as well as mineral alkalinity enhancement.
3. Synthetic CDR (sCDR) sub-categories are based on the way the carbon is removed from the atmosphere:
   1. Indirect water capture: Approaches that indirectly remove carbon dioxide from the air by altering water chemistry. This includes approaches that strip carbon dioxide out of water and those that produce alkalinity.
   2. Direct air capture: Approaches that use machines to capture carbon dioxide from the atmosphere in a concentrated stream.

Our carbon dioxide storage category does not include any sub-categories.

Categories spotlight risks and opportunities across CDR approaches

The advantage of breaking down the CDR space by key inputs is that it clarifies where there are likely to be shared constraints and shared opportunities.

In biogenic CDR, all approaches will be constrained by the availability of plant-based biomass. Increased availability of land, for example from efficiency improvements in food production or shifts in land use policy, would benefit all approaches. Breakthroughs in enhanced cultivars could increase the production frontier for all relevant approaches.

In geochemical CDR, all approaches will be constrained by the availability of mineral feedstocks. All approaches could benefit from improved processes for recovering minerals from industrial activities and for sourcing, extracting, grinding, distributing, and reacting minerals for CDR.

In synthetic CDR, all approaches will be constrained by the availability of low-carbon energy. All approaches would benefit from more abundant and affordable low-carbon energy, and from advances in process design and efficiency.

Within each of these three categories, the most efficient approaches are likely to win out.

All forms of CDR have advantages and challenges. We will need a portfolio approach

In the long run, the most viable and competitive CDR solutions will be those that source and transform their respective key inputs into durable CDR most efficiently. However, CDR approaches also have a range of other advantages they can harness and challenges they will need to overcome, as they move towards more deployment.

When we step back and look at the CDR space, it’s clear that there will be pros and cons to the approaches in each of our taxonomic categories and sub-categories. Some are more easily and cheaply deployed, but harder to measure. Some are expensive, but highly durable. All may be challenging to scale, depending on the availability and price of their primary inputs: plants, minerals, and low-carbon energy.

As a result, achieving CDR at the scale we need will likely require the smart and strategic deployment of different approaches in different settings. A broad portfolio that incubates a range of CDR approaches can mitigate resource constraints, address varied community preferences, and diversify risks.

Rather than narrowly focusing on only a few approaches such as forestry or direct air capture, we should keep the big picture goal — billions of tons per year of CDR — in mind, think broadly about what is possible, cultivate new approaches, and invest widely in the field of CDR.

Governments, nonprofits, funders, investors, and researchers should encourage this portfolio approach. As CDR markets and policy regimes evolve, the rules of the game should be configured to enable discovery, testing, and — if approaches are validated upon testing — scale-up of the full set of potential CDR approaches. We need an open playing field for innovation, with rules flexible enough to maximize the overall level of CDR within our resource constraints, and with robust environmental and social safeguards in place. If one approach emerges as dominant, that could put CDR on a virtuous cycle of falling costs as production increases – an ideal outcome. But if no technology emerges as dominant, which looks more likely, future generations will thank us for spreading our bets.