Since the Industrial Revolution, humans have emitted more than 2,000 gigatons of carbon dioxide into the atmosphere. (A gigaton is one billion metric tons.)
This concentration of CO2 and other greenhouse gases in the air causes the climate change impacts we’re experiencing today, from forest fires to stifling heat waves and damaging sea level rise — and the global community is still emitting more each year. Unless we make serious changes, climate impacts will only continue to intensify.
The imperative for combating climate change is to curb emissions rapidly — for example, by ramping up renewable energy, boosting energy efficiency, halting deforestation and curbing super pollutants like hydrofluorocarbons (HFCs). The latest climate science tells us, however, that these efforts alone aren’t enough.
To keep global temperature rise to less than 1.5 degrees C (2.7 degrees F), which scientists say is necessary for preventing the worst impacts of climate change, we’ll need to not only reduce emissions but also remove and store some carbon that’s already in the atmosphere.
What Is Carbon Dioxide Removal?
Carbon dioxide removal (or simply “carbon removal”) aims to help mitigate climate change by removing carbon dioxide pollution directly from the atmosphere. Carbon removal strategies include familiar approaches like growing trees as well as more novel technologies like direct air capture, which scrubs CO2 from the air and sequesters it underground.
Carbon removal is different from carbon capture and storage (CCS), which captures emissions at the source — like a power plant or a cement producer — to prevent them from entering the atmosphere in the first place. Carbon capture is a form of emissions reduction rather than carbon removal.
How Important Is Carbon Removal in the Fight Against Climate Change?
The latest climate model scenarios show that all pathways that keep temperature rise to 1.5 degrees C (with little or no overshoot) require carbon removal. The amount ultimately needed will depend on how quickly we can reduce emissions in the near term and whether — or by how much — we overshoot climate targets. Estimates, including both natural and technological carbon removal approaches, range from 5 to 16 billion metric tons per year globally by 2050. (For context, the United States emitted just over 6 billion metric tons of greenhouse gases in 2021.) The faster the world reduces its emissions in the near term, the less it will have to rely on carbon removal.
While enhancing natural carbon removal through reforestation and forest management has long been of interest, efforts to develop and deploy novel technologies and approaches have ramped up only recently. In just five years, carbon removal has grown from a niche concept to a well-accepted component of climate portfolios and has received billions of dollars of federal funding and hundreds of millions of dollars in private investment.
This expansion was largely driven by a 2018 report from the Intergovernmental Panel on Climate Change which concluded that hundreds of billions of tons of carbon removal will be needed by the end of the century to meet global climate goals. The level of carbon removed today remains far below what we expect to need in the coming decades, indicating a need for investment in the public and private sectors to continue growing.
How Is CO2 Removed from the Atmosphere?
Carbon removal can take numerous forms, from new technologies to land management practices. The big question is whether these approaches can deliver carbon removal at the scale needed in the coming decades.
Each carbon removal approach involves tradeoffs, including considerations around costs, resource needs (such as energy, land and water usage), the extent of local benefits or negative impacts, and technology readiness, among others. WRI’s series of working papers explores the possibilities and challenges of using carbon removal to combat climate change and recommends a priority set of U.S. federal policy actions to accelerate their development and deployment.
Here are six options for removing carbon from the atmosphere:
1) Trees and Forests
Plants remove carbon dioxide from the air naturally, and trees are especially good at storing CO2 removed from the atmosphere by photosynthesis. Expanding, restoring and managing tree cover to encourage more carbon uptake can leverage the power of photosynthesis, converting carbon dioxide in the air into carbon stored in wood and soils.
Some management approaches that can increase carbon removal by trees and forests include:
- Reforestation, or restoring forest ecosystems after they’ve been damaged by wildfire or cleared for agricultural or commercial uses.
- Restocking, or increasing the density of forests where trees have been lost due to disease or disturbances.
- Silvopasture, or incorporating trees into animal agriculture systems.
- Cropland agroforestry, or incorporating trees into row crop agriculture systems.
- Urban reforestation, or increasing tree cover in urban areas.
WRI estimates that the theoretical carbon-removal potential from forests and trees outside forests in the United States alone is more than half a gigaton per year, equivalent to all annual emissions from the U.S. agricultural sector. What’s more, approaches to remove CO2 through forests can be relatively inexpensive compared to other carbon removal options (generally less than $50 per metric ton of CO2) and yield cleaner water and air in the process.
Infographic showing five tree-based approaches for carbon removal such as planting and restoring trees in forests and cities.
One major challenge is ensuring that forest expansion in one area doesn’t come at the expense of forests somewhere else. For example, taking farmland out of production would reduce the supply of food. This could necessitate converting other forests to farmland — resulting in more greenhouse gas emissions — unless improvements in farm productivity could fill the gap. Similarly, not harvesting timber from one forest may result in overharvesting in another. These dynamics make restoring and managing existing forests, and adding trees to ecologically appropriate lands outside of farmland, especially important.
2) Farms and Soils
Soils naturally sequester carbon, but agricultural soils are running a big deficit due to frequent plowing and erosion from farming and grazing, all of which release stored carbon. Because agricultural land is so expansive — encompassing more than 900 million acres in the United States alone, or approximately 40% of the country’s land area — even small increases in soil carbon per acre could be impactful.
There are many practices that can increase the amount of carbon stored in soils, although the amount and duration of the carbon sequestered depend on regional climate and soil type, among other factors.
Planting cover crops when fields are otherwise bare can extend photosynthesis throughout the year; using compost can improve yields while storing the compost’s carbon content in the soil; and scientists are developing crops with deeper roots, making them more resistant to drought while depositing additional carbon into the soil. Many of the practices that increase soil carbon also improve soil health and can make agricultural systems more resilient to climate change.
Managing soil for carbon at a large scale, though, is a tricky proposition. Natural systems are inherently variable, and that makes it a real challenge to predict, measure and monitor the long-term carbon benefits of any given practice on a given acre. More research is needed to understand how these practices affect carbon sequestration in different soil types and different climates, and how long the carbon remains stored.
The efficacy of some soil carbon sequestration practices — such as cover crops and grazing management — is also subject to continued scientific debate. Furthermore, changing conditions or management practices from year to year could erase prior gains. And because climate-smart farming practices would need to be adopted over large tracts of farmland to remove a significant amount of carbon, governments and market systems would need to incentivize landowners to implement these measures.
3) Biomass Carbon Removal and Storage
Biomass carbon removal and storage (BiCRS) includes a range of processes that use biomass from plants or algae to remove carbon dioxide from the air and then store it for long periods of time. These methods aim to leverage the carbon storage capacity of plants beyond their natural lifecycles: Whereas trees remove and store carbon only until they die and decompose, biomass carbon removal and storage aims to sequester the CO2 that plants capture more permanently.
There are many different methods for removing carbon using biomass. These include the creation of biochar, which is made by heating biomass in low-oxygen environments to produce a charcoal-like soil additive that sequesters carbon; bio-oil, which uses a similar process to produce a liquid that gets injected underground; and permanent storage of carbon-rich biomass in vaults. Bioenergy carbon capture and storage (BECCS) is another carbon removal pathway which involves generating energy using biomass and then capturing and sequestering the resulting CO2 emissions. One type of BECCS that features prominently in many economy-wide decarbonization scenarios is converting biomass to hydrogen, which could result in a carbon-negative fuel.
While biomass carbon removal and storage can offer long-term CO2 removal, however, not all processes necessarily provide a net carbon benefit.
If BiCRS processes use biomass sources that don’t compete with food crops or ecosystems for land — such as algae or waste materials — they can provide net carbon removal. For example, many forestry and agricultural wastes, such as tree bark, nut hulls, and corn husks and stalks, are burned or left to decompose; using those materials instead for biomass carbon removal and storage can be beneficial from a climate perspective.
But it’s not always straightforward to determine whether biomass is truly sustainable. For example, if crops are grown specifically to be used for biomass carbon removal, they could displace food production or natural ecosystems. This can cause cropland expansion and destruction of forests and grasslands, both of which release carbon, and can erase the climate benefits of BiCRS while also exacerbating food insecurity and ecosystem loss. To fully harness the carbon removal potential of BiCRS pathways, policy and market incentives are needed to encourage the use of waste biomass and disincentivize the use of purpose-grown crops that could undermine the natural ability of forests and soils to sequester carbon.
4) Direct Air Capture
Direct air capture is the process of chemically scrubbing carbon dioxide from the ambient air and then sequestering it either underground or in long-lived products like concrete. This technology is similar to the carbon capture and storage technology used to reduce emissions from sources like power plants and industrial facilities. The difference is that direct air capture removes excess carbon that’s already been emitted into the atmosphere, instead of capturing it at the source.
It is relatively straightforward to measure and account for the climate benefits of direct air capture, and its potential scale of deployment is enormous. However, the technology remains costly and energy-intensive today.
Cost estimates vary but generally range from around $100 up to more than $600 per metric ton of CO2 removed; voluntary purchases of carbon removal credits from direct air capture range from $225 to more than $1,000 per metric ton of CO2 where data is available. These costs are expected to come down significantly in the next decade and beyond as projects are built and technologies improve.
Direct air capture also requires substantial heat and power inputs: Scrubbing 1 gigaton of carbon dioxide from the air could require nearly 10% of today’s total energy consumption. To result in net carbon removal, therefore, direct air capture technology would need to be powered by low- or zero-carbon energy sources.
Investing in technological development and deployment experience, together with increasing availability of cheap, clean energy, could advance prospects for direct air capture at a large scale.
In recent years, direct air capture has seen a significant jump in public and private investment and a growing number of companies are developing the technology. Annual basic research funding for DAC and other carbon removal approaches has grown more than ten-fold since 2019, and the landmark Bipartisan Infrastructure Law and Inflation Reduction Act both provided critical funding and deployment support for direct air capture projects in the United States. The private sector has also begun to step up with a set of new initiatives — for example, a group of companies came together in 2021 and committed to spend nearly $1 billion on permanent carbon dioxide removal, including but not limited to DAC, by 2030 to help spur development by creating guaranteed demand.
As interest and investment in direct air capture continue to mount, attention is shifting to also focus on implementation. It will be important for decision-makers and those building direct air capture projects to focus not only climate benefits, but also on equity and sustainability as this industry develops.
5) Carbon Mineralization
Some minerals naturally react with CO2, turning carbon dioxide from a gas into a solid and keeping it out of the atmosphere permanently. This process is commonly referred to as “carbon mineralization” or “enhanced weathering,” and it naturally happens very slowly, over hundreds or thousands of years.
But scientists are figuring out how to speed up the carbon mineralization process, especially by enhancing the exposure of these minerals to CO2 in the air or ocean. That could mean moving air through large deposits of mine tailings (rocks left over from mining operations) that contain the right mineral composition; crushing or developing enzymes that chew up mineral deposits to increase their surface area; spreading certain types of ground rock on croplands or coastal areas where it reacts with and locks away carbon dioxide; and finding ways for certain industrial byproducts, like fly ash, kiln dust or iron and steel slag, that are reactive with CO2 to sequester it.
Carbon mineralization can also be used to sequester carbon dioxide that’s already been captured by injecting that CO2 into suitable rock types where it reacts to form a solid carbonate, permanently storing it. Other applications could sequester carbon and replace more emissions-intensive conventional production methods — for example, by using mineralization as part of concrete production, which is used at a multi-billion-ton scale globally.
Scientists have shown that carbon mineralization is possible and a handful of start-ups are already developing approaches, including mineralization-based building materials. However, there is more work to be done to map out cost-effective and prudent applications for scaled deployment and improve measurement of carbon sequestration.
6) Ocean-based Approaches
A number of ocean-based carbon removal approaches have been proposed to leverage the ocean’s capacity to sequester carbon and expand the portfolio of options beyond land-based applications. However, nearly all of these strategies are at early stages of development and require more research, and in some cases field testing, to understand whether they are appropriate for investment given potential ecological, social and governance impacts.
Each approach aims to accelerate natural carbon cycles in the ocean. Potential solutions include leveraging photosynthesis in coastal plants, seaweed, or phytoplankton; adding certain minerals to seawater that react with dissolved CO2 and lock it away; or running an electric current through seawater to accelerate reactions that ultimately help extract CO2.
Some ocean-based carbon removal options could also provide co-benefits. For example, coastal blue carbon (carbon stored in mangroves, seagrasses, and salt marshes) and seaweed cultivation could remove carbon while also supporting ecosystem restoration, and adding minerals to help the ocean sequester carbon could reduce ocean acidification. However, much is still unknown about the broader ecological impacts of these approaches and further research is needed to better understand potential risks before these approaches are pursued at any scale.
In the near term, cultivated seaweed can also be used for products like food, fuel and fertilizer, which may not result in carbon removal, but could reduce emissions compared to conventional production and provide an economic return that supports growth of the industry.
The Future of Carbon Removal
Analysis by WRI has shown that the most cost-effective and lowest-risk strategy for increasing carbon removal capacity involves developing and deploying a variety of approaches in tandem.
Moving forward, diverse methods of carbon dioxide removal must be built into climate change strategies around the world to avoid dangerous levels of global warming. The past few years have seen important steps in this direction, but more will be needed to realize national and global climate goals.
It will be critical to keep increasing public and private investment across the portfolio of carbon removal approaches to determine which can become viable options for meeting the scale of removal we expect to need in the coming decades.
By James Mulligan, Gretchen Ellison, Kelly Levin, Katie Lebling, Alex Rudee, and Haley Leslie-Bole
Originally published on WRI website.