From Trees to Tech and Beyond: Carbon Dioxide Removal (CDR) in All Its Variations

CDR approaches are more diverse than many realize. To achieve the scale of carbon removal the world will need, it’s essential to explore many options in many places.

In our first blog, we defined carbon dioxide removal (CDR) as any human activity that removes carbon dioxide from the atmosphere and durably stores that carbon in geologic, terrestrial, or ocean reservoirs, or in products. We also articulated why we will likely need to remove billions of tons of carbon dioxide from the atmosphere each year, while also reducing emissions dramatically, across the global economy.

Scaling CDR to meet the demands of the climate crisis is a daunting challenge. But there is good news. There are many CDR approaches, and they rely on different inputs. They can be performed in different locations. They can also benefit from different applications of science and technology. This suggests possibilities for a portfolio of CDR solutions, comprised of different approaches in different locations, that is diversified enough to scale.

In this blog, we highlight the range of known CDR approaches, differences in what they require, and why we must consider all of them to achieve the scale of removals we need.

CDR is more than trees or technology

The best-known CDR approaches are likely trees and direct air capture (DAC). Trees serve as natural carbon sinks by removing carbon dioxide from the atmosphere through photosynthesis and storing that carbon dioxide as biomass. DAC, which pulls carbon dioxide out of the air using fans and materials that selectively bind with carbon dioxide, has recently garnered a lot of media and investor attention.

But there are many other approaches. Carbon dioxide naturally reacts with and is stored by materials that the earth has in abundance, including rocks, water, and a variety of plants besides trees. These reactions can be accelerated in many ways. New approaches and companies are continually emerging, and there is still runway for more innovation.

Figure 1 below illustrates the variety of approaches emerging.

These approaches rely on different inputs, to different extents. They take place on different types of land or bodies of water, require different levels of energy, and utilize different types of technologies.

Three examples below illustrate the wide variety of inputs, locations, and technology applications that different CDR approaches employ.

Example 1: Macroalgae and microalgae sinking

Macroalgae (seaweed) or microalgae (phytoplankton) take up carbon from the atmosphere through photosynthesis as it grows. Algae can be grown in the ocean or other open bodies of water, or in controlled environments. Experiments are now underway to then sink algae to a depth where it will not decompose, durably storing carbon.

The main inputs required are sunlight, nutrients, and suitable growing conditions. Most of the ocean is deficient in the necessary nutrients to grow algae at scale. But, when these nutrients are made abundant, these aquatic plants have the potential to capture more carbon per square meter than terrestrial ones.

Macroalgae and microalgae are typically grown in the ocean. This setting is critical for both the cultivation of these water organisms, which require large water areas to achieve scale, and the ultimate sequestration of captured carbon dioxide, which requires depths that prevent decomposition.

Several technological interventions could be used to optimize and accelerate algae formation, enable sinking, and measure outcomes. Innovations in biotechnology could be used to stimulate faster algae growth, and the ability to customize algae production by species and location could potentially unlock rapid learning rates for algae-based approaches.

Example 2: Terrestrial enhanced weathering

Terrestrial enhanced weathering (TEW) takes advantage of natural weathering processes by crushing and spreading rocks, more specifically alkaline minerals, and spreading the ground rock on agricultural land or forest floors where these minerals react with carbon dioxide and water. During these reactions, carbon dioxide is converted into dissolved bicarbonate: a stable form of carbon that will not be re-released into the atmosphere. Reactions between weathered minerals and carbon dioxide occur naturally; terrestrial enhanced weathering approaches speed up and scale up this process.

The main inputs for TEW are suitable minerals and the energy required to grind and transport those minerals. The earth has abundant natural minerals, so supply of those minerals is theoretically not a limiting factor in the long term. And although it may seem like grinding and transporting minerals would consume significant energy, the total energy consumption of TEW is small compared to its removal potential. As a result, TEW should be relatively scalable, based on its inputs.

TEW does however require significant land on which these alkaline materials can be safely spread. The most common setting for TEW is on agricultural fields, but there are also plans to disperse these minerals safely in forests or on non-arable lands. In either case, land-use conversion, which would be necessary for other forms of CDR, is avoided. If sufficient land is made available for TEW, it could support widespread deployment.

Technology can also be used to improve several stages of the TEW process. The most important areas for technological improvement are in the sourcing, processing, and dispersal of minerals, and in the measurement of carbon dioxide uptake. Different ways of deploying alkaline materials, mixing them, and measuring them can increase the rate of carbon dioxide uptake, and each of these steps will affect the rate at which TEW occurs. If sufficient land is made available for TEW, it could support widespread deployment.

Example 3: Electrochemical water capture

Electrochemical water capture approaches pump water through a facility that uses electricity to either remove carbon dioxide from the water or change the chemistry of the water, so that it can absorb more carbon dioxide from the air. These approaches enhance the natural ability of our oceans and waterways to absorb carbon dioxide from the atmosphere.

The main input for electrochemical ocean approaches is electricity. These approaches use considerably more energy than either of the two examples above. More than the other two examples, our ability to scale these approaches responsibly and affordably will depend on the speed at which low-carbon energy ramps up around the world.

Because electrochemical water capture approaches rely on the passive absorption of carbon dioxide from the atmosphere into water, they are typically sited near large bodies of water such as oceans or lakes. They can be deployed as stand-alone facilities or as add-ons to other facilities that already move water, such as water treatment or wastewater treatment facilities.

Given that the consumption of electricity is the most significant input to these approaches, it is also the biggest opportunity for gains through technology. Technological advances in membrane materials, design, flux, and durability can all improve the energy efficiency of achieving carbon removal through these methods.

Conclusion

Clearly, CDR is more than just trees or DAC fans. There are a range of approaches that require different inputs, locations, and advances in science and technology. This is good news for the global effort to scale CDR to over a billion tons per year, because it suggests that we can build a global portfolio that is diversified enough to reach that scale.

RMI is currently tracking 32 distinct CDR approaches. We expect new ones will emerge. In our next blog, we will introduce the full set of 32 approaches and our approach to categorizing them for coordinated and efficient research, development, and deployment, as we seek to achieve our climate goals.