In the past few years, carbon dioxide removal (CDR) has transformed from a little-known concept to a generally accepted component of climate action portfolios, with billions of dollars of public support and hundreds of millions of dollars in private spending supporting its growth.

This shift has been driven by the scientific consensus that removing carbon dioxide (CO2) from the air will need to play a critical role to limit global temperature rise to 1.5 degrees C (2.7 degrees F) — this conclusion was initially included in the Intergovernmental Panel on Climate Change’s landmark report on 1.5 degrees C in 2018 and notably underscored in the just-released 2023 Sixth Assessment Report.

CDR will play a critical role in helping meet climate targets, but it cannot be a substitute for drastically reducing greenhouse gas emissions, which must remain a top priority.

The growing interest and investment in CDR is spurring wide range of new CDR approaches and technologies. Each is at different stages of development, varies in where and how carbon is sequestered, and how easy it is to measure how much carbon is removed. While some approaches and technologies may provide unexpected and valuable co-benefits, many also present uncertain environmental and social impacts.

As carbon removal is a comparatively new field, developing a broad portfolio of technologies and approaches will be critical to reducing risks and costs and helping ensure there is capacity to remove carbon from the air at the levels needed in coming decades.

Here are four key things to know about the recent growth and diversification of the CDR field over the past several years.

1. Many New CDR Approaches are Emerging

Just a few years ago the CDR landscape was primarily made up of three companies — Carbon Engineering, Climeworks, and Global Thermostat — focused on direct air capture (DAC), arguably the most advanced and well-understood CDR technology. This portfolio has grown and diversified significantly over the past several years:

Direct Air Capture: Start-up companies, like Noya, are developing new DAC technologies that can reduce energy or resource use and can be integrated into existing infrastructure, which can avoid some of the challenges that come with siting new, standalone infrastructure. For example, DAC can be integrated into a building’s cooling towers, which remove a building’s heat by circulating water and air. The air moving through this cooling system can be redirected into pipes and stand-alone carbon dioxide capture equipment. The system uses small surface areas and could reduce energy needs. Other conceptions of DAC, such as a technology developed by Verdox, may be able to significantly reduce energy usage.

Biomass-based CDR: Instead of combusting biomass and capturing the emissions (as is done with bioenergy with carbon capture and storage, or BECCS), there is growing interest in using biomass — organic material from plants — for direct carbon removal. One iteration of this from Charm Industrial involves converting biomass left over from growing crops into a carbon rich “bio-oil” and injecting that into the ground where that carbon is sequestered. Another approach, developed by Kodama Systems, involves improving forest management practices, including forest thinning to reduce wildfires, combined with burial of residual wood for carbon removal.

MineralizationOcean alkalinity enhancement is a type of carbon mineralization that can be done with certain types of rocks that are reactive with carbon dioxide, such as olivine. Some companies, like Vesta, are looking into mixing this ground rock with sand and spreading it on beaches. In this way, the wave action helps speed chemical reactions that lock away carbon dioxide, while also helping replenish eroded coastlines, potentially reducing storm surges. On land, ground up basalt rock that can be applied to croplands to improve soil quality while it removes carbon. 

Ocean CDR: Some companies plan to farm seaweed such as kelp or sargassum in the open ocean and are testing methods of sinking it to the deep ocean to sequester the carbon it contains. For example, Seafields cultivates and sinks sargassum by bursting its air sacs. Others, like Seaweed Generation, are collecting existing Sargassum and sinking it with automated vessels. In the past decade, sargassum has grown at explosive and invasive levels in the Central Atlantic, which could be linked to fertilizer runoff into the ocean. This approach could help alleviate that nuisance while removing carbon.

Crop Enhancement: Researchers and companies are exploring the use of enhanced photosynthesis for carbon sequestration. This is a process already used in agriculture to genetically modify, or breed, hybrid plant species to increase carbon dioxide uptake and sequestration in the soil. Some new CDR companies, like Living Carbon, are developing trees with enhanced photosynthesis capacity through gene editing so that they can grow faster and absorb more carbon dioxide. In one case, a metal accumulation trait can help these trees absorb metals through their roots, which can slow down the decay of wood, and allow these trees to be planted on land contaminated with heavy metals. Others, like Recapture, are using hybrid tree species, which are not genetically modified but are bred to absorb more carbon dioxide and grow faster. The tree’s wood is then used for building materials, storing embodied carbon.

Hover over each approach to learn more:

a long arrow with natural approaches at the top and technological approacheson the bottom
carbon removal approaches on land group of trees to represent afforestation and reforestation outline of seedling plant for soil carbon sequestration icon outline of sun and field for biomass carbon removal and storage icon outline of a large rock, carbon mineralization icon
carbon removal approaches in the ocean wetland grasses coming out of water outline seaweed and rocks icon nitrogen periodic table symbol icon with waves representing artificial upwelling & downwelling large rock as alkalinity enhancement battery with a lightening bolt

Note: This figure includes carbon removal approaches mentioned in countries' long-term climate strategies as well as other leading proposed approaches. The natural/biotic vs. technological/abiotic categorization shown here is illustrative rather than definitive and will vary depending on how approaches are applied, particularly for carbon removal approaches in the ocean.

2. There are Benefits to Combining CDR Approaches and Technologies

Some projects are working on combining different CDR approaches. For example, Switzerland-based Neustark is coupling DAC with mineralization. In one iteration, DAC-captured carbon dioxide can be sequestered in recycled concrete. Crushed concrete from building demolition can then be combined with captured carbon dioxide where it reacts and stores it. This mineralized material can then serve as aggregate — a key component of concrete, along with cement. Another example, is San Francisco-based Heirloom, a direct air capture company that uses limestone to capture carbon and operates the United States’ first DAC facility. In early 2023 they partnered with CarbonCure to inject that carbon dioxide into concrete, where it is sequestered through mineralization. 

There are also ocean-based CDR projects that combine seaweed cultivation with an approach known as artificial upswelling to enable growth in nutrient depleted surface waters. Upwelling is a natural process where cold, nutrient-rich water from the deeper ocean rises to the surface, replacing nutrient-depleted and warmer surface water. Artificial upwelling speeds up this process and could be valuable in oceanic deserts where the warmer ocean temperatures have disrupted the natural upwelling cycle and reduced surface nutrient levels.

Seaweed can also be grown on offshore satellite-controlled rigs below the surface of the water, recreating upwelling through wave and solar powered pumps. Through the motion of the waves, deep-sea nutrient rich water is pumped through valves into the rig on which seaweed is growing, providing nutrients needed for growth.

3. Quantifying and Tracking Carbon Removal are Important

Different CDR approaches and technologies capture and remove carbon in different ways. For some, it is easy to measure the amount removed, while for others it is more challenging. And, how this amount changes over time also depends on the approach: some provide more permanent, or durable, removal, while others are more prone to reversal. For example, the carbon dioxide removed by DAC can be measured directly. When sequestered in geological formations, it is expected to remain there for thousands of years, and monitoring approaches already exist to track this.

Many other novel CDR technologies and approaches however sequester carbon dioxide in ways that are more difficult to measure, which results in a higher degree of uncertainty in how much carbon is removed. Some also sequester carbon in ways that are not permanent, so the amount removed may decrease over time. Without greater certainty in the amount removed and over what duration, it is difficult to understand the ultimate impact these approaches have on the climate.

These measurements also underpin the buying and selling of carbon credits. Without accurate measurement and verification, carbon credits can overstate, or otherwise inaccurately state, the amount of carbon removed, jeopardizing market integrity.

Range of carbon dioxide removal processes and their estimated timescales of carbon storage.

For example, it can be particularly difficult to measure carbon sequestration of approaches applied outside of contained environments — such as, spreading rock dust over cropland or in the ocean (this is in contrast to a closed system like a DAC facility). In these cases, sampling in different locations and over time will be needed, likely combined with laboratory models, to estimate sequestration, which can require significant effort and cost.

Tracking carbon sequestered over time — often referred to as permanence or durability — is also important to understanding how long carbon is sequestered and out of the atmosphere. Durability can be less certain for approaches that depend on biological processes to sequester carbon, as opposed to geological carbon sequestration. For example, carbon sequestered in organic forms like ocean sediments can be re-released by natural or human-made disturbances, while carbon stored geologically is chemically altered into solid minerals or stored so deeply in the earth that it would take thousands of years to naturally cycle back into the atmosphere.

 Taking ocean-based CDR approaches as an example of an open system, carbon dioxide can be sequestered in the ocean in many ways. One approach is growing and sinking seaweed, sequestering the carbon it contains in the deep ocean. Depending on where and how the seaweed is sunk, it may be kept out of contact with the atmosphere for decades to centuries. However, monitoring in deep ocean environments is challenging and costly to do across locations and over time, and will involve some amount of uncertainty related to the amount of carbon removed.

One CDR company, Running Tide, has deployed open-ocean monitoring systems that collect geochemical and oceanographic data to track the impacts of ocean CDR interventions and can help reduce these uncertainties.  

While many of these new CDR approaches and technologies hold potential, they are novel and need further research and testing to understand their efficacy in removing carbon dioxide, the timescales of carbon sequestration and expected social and environmental impacts — as well as further research to improve measurement and verification methodologies and frameworks to accurately measure and track all of this. Those in the public and private sectors and others working to advance CDR will all need to work on minimizing these uncertainties as the field continues to develop.

4. There are Tradeoffs to Emerging CDR Methods

 All CDR approaches pose environmental and social impacts, both positive and negative. While some new CDR companies advertise co-benefits of their technologies, it is important to holistically understand the tradeoffs associated with each approach. These must be assessed on a case-by-case basis and consider the impacts and local contexts in which they would be applied.

Some companies provide a benefit by using waste materials within their CDR approaches. Certain types of mine tailings (pulverized rocks that are a waste product from mining activities) can be used to not only remove carbon dioxide from the air through carbon mineralization, but in some cases can also neutralize hazardous materials like asbestos or be used to reclaim useful chemicals in mining waste.

Research is also underway to develop processes that combine critical mineral extraction with carbon mineralization, which is particularly important given the rapidly growing demand for critical minerals in a range of clean energy technologies.

At the same time, mineralization approaches can present negative impacts. Rock dust may contain trace metals like nickel and chromium that can be toxic to ecosystems and people. Large-scale mineralization would require scaling up the mining industry to access sufficient amounts of suitable rock, which could provide jobs, but could also negatively impact communities and the local environment in ways similar to existing mining efforts. 

Many ocean-based CDR approaches also present a range of tradeoffs and could pose negative environmental and social impacts if not done responsibly. For example, ocean alkalinity enhancement can provide a benefit of helping reduce ocean acidification locally but can also present risks of rebound to acidic conditions once alkalinity addition stops, potentially negatively impacting ocean biota, and needing a long-term plan for phase-out for large-scale alkalinity addition.

A kelp farmer stands in a small boat pulling up a rope lined with sugar kelp out of the water.
Blue Dot Sea Farms in Washington’s Hood Canal is currently the only seaweed farm in the state, and officials there and elsewhere say that burdensome permitting systems are holding back ocean farming. Photo by The Pew Charitable Trust (source).

Other ocean-based approaches like seaweed cultivation can provide co-benefits such as reducing acidification locally, reducing storm surges and taking up excess nutrients in coastal areas. At the same time large-scale seaweed cultivation could deplete nutrients in surface waters, decreasing phytoplankton growth. Sinking seaweed to the deep ocean could have negative impacts on seafloor ecosystems that are not yet sufficiently understood.

Lastly, approaches that use biomass need to carefully consider the sourcing of that material to ensure that its use produces a net climate benefit, doesn’t result in indirect land use change, or compete with agricultural production, and doesn’t worsen biodiversity. Focusing on responsible use of waste biomass and avoiding purpose grown crops will be important to ensuring the approach provides a net benefit.

What’s Next for Carbon Dioxide Removal?

CDR is a quickly evolving field. Developing a broad portfolio will be critical to minimizing risks associated with any single approach or technology and helping ensure that CDR can scale to the level needed in the coming decades. Given that the industry is still in its early stages, and many of these companies have emerged very recently, it is unclear which approaches and companies can deliver credible removal while maximizing positive and minimizing negative impacts on the environment and in local communities.

Continued government funding for research, development and deployment is needed for further innovation, along with attention to improving measurement, reporting and verification to better track progress. Strong guidelines and protocols on how projects are implemented and who gets to claim credit for them are also needed.