Soil Carbon Sequestration: Myths, Realities, and the Biden Administration’s Proposals
Soil carbon sequestration has gained traction within the Biden administration as a way for farmers to reduce or even reverse U.S. agriculture’s greenhouse gas (GHG) emissions. To advance this technology, Congress proposed the bipartisan Growing Climate Solutions Act, which is intended to help farmers participate in voluntary markets that pay them to store carbon in the soil. Despite bold claims, the science is mixed on whether soil carbon sequestration is a viable climate solution, particularly in terms of the quantity of carbon that can be stored and for how long.
Q1: What is soil carbon sequestration?
A1: In simple terms, soil carbon sequestration happens when plants capture and store, or “sequester,” atmospheric carbon dioxide (CO2) in the soil, increasing the quantity of soil carbon stocks. Decaying plant matter, along with the carbon it contains, becomes part of the soil for a period of time before it is broken down by microbes, releasing the carbon back into the atmosphere.
The length of time carbon stays in the soil before returning to the atmosphere varies significantly based on climate, soil composition, and other factors. For example, changing the soil structure, such as by converting forests and grasslands to farmland, can speed up the process and release much of the sequestered carbon back into the atmosphere. On the other hand, practices like no-till farming and planting cover crops may slow the rate of soil carbon loss and potentially even increase soil carbon levels.
It is estimated that the past 12,000 years (and especially the past 200 years) of agriculture have released 133 billion metric tons of CO2 (GtCO2) from the soil into the atmosphere. That is more than three times as much carbon as all human activities emitted in 2019 (43.1 GtCO2). In some places, cultivated soils have lost up to 70 percent of their original organic carbon. Advocates of soil carbon sequestration, sometimes called “carbon farming,” say that changing the way we farm can restore at least some of that carbon to the soil.
Q2: What are the potential climate benefits?
A2: Since carbon is the primary GHG emitted through human activities, sequestering carbon in the soil could contribute to climate mitigation goals. Estimates for how much carbon could be stored vary widely. The most commonly cited number is from the Intergovernmental Panel on Climate Change (IPCC), which estimates that by 2030, global soil carbon sequestration has the technical potential to mitigate up to about 5.3 GtCO2 per year. Between 2000 and 2010—the latest period for which the IPCC has reported total sectoral GHG emissions—agricultural production emitted around 5.0–5.8 GtCO2-equivalents per year (including the CO 2 equivalents of other GHGs, like methane), so reaching the top end of the IPCC’s estimates could make agriculture a net-negative emissions sector.
However, there is widespread skepticism about whether carbon sequestration can live up to its touted potential. For starters, the scientific community is uncertain about how long carbon can be stored in the soil, how much carbon can be sequestered by different practices, and how to effectively measure and track the carbon that is sequestered. Understandings of soil science have changed over the past decade, with new studies contradicting the idea that the IPCC’s estimates are based on—that stable, carbon-rich molecules can remain in the soil for hundreds or thousands of years. Even based on previous assumptions, other forms of carbon sequestration, like planting trees, are much easier to quantify and verify than sequestering carbon in soil, due to the dynamic and impermanent nature of soil. And there is growing evidence that some practices, such as no-till agriculture, may not store as much carbon as previously believed. Another area of concern for long-term climate impacts is there will eventually be a limit on how much carbon soil can store. These saturation levels could be reached within a few decades,according to some estimates, at which point further mitigation would have to come from other sources.
Even if the IPCC’s current estimates of sequestration potential hold, technical potential does not necessarily equal realistic potential, since cost and other factors are not considered in the calculations. The IPCC says that it is only cost effective for farmers to implement carbon sequestration technologies for up to about 3.8 GtCO2 per year (at a cost of US$100 per ton of CO2, which is much higher than most carbon prices today). This estimate still does not account for social and political factors, though, so the realistic maximum is much lower if farmers do not universally adopt carbon farming practices. Many farmers live season-to-season with very little margin of error, so they may not have the resources or be willing to take the risk of investing in soil carbon sequestration technologies, particularly those that reduce near-term yields. This fact further calls into question the IPCC’s estimates of soil carbon sequestration’s climate potential.
Despite these limitations and uncertainties, a compelling argument can be made for continuing to encourage soil carbon sequestration. Many of the same practices that are believed to store carbon have other beneficial environmental and economic effects. For example, improving overall soil health can increase agricultural yields while reducing the need for agricultural inputs, saving farmers money and reducing nitrous oxide (N2O) emissions—another potent GHG—from synthetic fertilizer application.
Q3: What is the role of carbon markets in soil carbon sequestration?
A3: In the U.S. agriculture sector, people are primarily discussing voluntary markets (as opposed to regulatory markets, like cap-and-trade systems), in which farmers or ranchers sell a credit to investors for every metric ton of carbon that is sequestered on their land. Proponents of voluntary carbon markets say that they create new revenue streams for farmers, offsetting the cost of transitioning to sustainable farming practices and incentivizing more farmers to adopt regenerative agriculture. On the buyer side, investors such as companies, governments, and other entities typically purchase carbon credits for around $15–$20 per metric ton of carbon to offset their own emissions and meet their emissions reduction goals.
These markets depend on third-party entities who can verify (1) that the amount of carbon claimed to be sequestered actually was, (2) that the carbon would not have been sequestered had it not been for the carbon credits, and (3) that the carbon will remain in the ground for an established amount of time. Each of these criteria has proven difficult to assess when it comes to soil carbon, leading to huge variability in the market for agricultural carbon credits as each third-party verifier establishes their own verification protocols (with mixed scientific validity).
Currently, carbon prices are too low to make the transition to regenerative agriculture profitable for most farmers. Many of the technologies and practices used to generate carbon credits are only cost effective when done on a massive scale, excluding some small farms from joining the market and potentially leading to the further consolidation of U.S. food production in the hands of a few large companies. Those who rent farmland also may not benefit from carbon markets, since carbon farming typically requires significant up-front investments but may not generate profits until years later. Given that almost 40 percent of U.S. farmland is rented, this is a major limitation. Finally, due to the requirement of additionality—that the GHG reductions would not have occurred had it not been for the carbon credits—many regenerative farmers who have already implemented sustainable practices are unable to sell carbon credits, even if their soil has sequestered large amounts of carbon.
Q4: What has the Biden administration proposed with respect to soil carbon sequestration?
A4: Biden has made agriculture a central part of his climate plans, with his campaign website stating that “Soil is the next frontier for storing carbon.” Since he took office, most public and legislative support in this regard has centered around the use of voluntary carbon markets for the agriculture sector. The bipartisan Growing Climate Solutions Act, which is intended to make it easier for farmers, ranchers, and forest landowners to participate in voluntary carbon markets, passed the Senate without amendments in late June and was subsequently introduced to the House, where it awaits further action. If passed, the bill would establish a U.S. Department of Agriculture (USDA) certification program for third-party verifiers and provide technical assistance on how to produce and sell carbon credits. These measures would help standardize the growing agricultural carbon market, making it easier and less risky for farmers to participate.
In April, Agriculture Secretary Vilsack announced that the USDA will also expand the Conservation Reserve Program (CRP) with new incentives and higher payment rates. This expansion includes targeted climate change mitigation elements, like a new Climate-Smart Practice Incentive that aims to increase carbon sequestration; research by the Natural Resources Conservation Service into establishing a baseline for soil carbon on land enrolled in CRP; and a program to measure and monitor soil carbon over the life of new CRP contracts. All these updates are intended to contribute to the goal of sequestering more carbon in agricultural soils across the country.
In the past year, another idea that some advocacy groups, trade associations, the USDA, and others have floated is a USDA-run carbon bank, though this would be a much longer-term solution. This could take many different forms, and depending on its design, it might pay for carbon credits, guarantee a particular price for carbon credits, or finance the farm improvements necessary for carbon sequestration. Secretary Vilsack has said that a carbon bank could be set up through the Commodity Credit Corporation, which was created to stabilize and create agricultural markets.
Q5: What does all this mean for the Biden administration?
A5: Soil carbon sequestration has gained popular and political attention as a possible net-negative emissions technology; however, the science is mixed on whether it can live up to the projections, particularly in terms of how much carbon can be stored and for how long. Relying on soil carbon sequestration to meet GHG emissions reduction goals is risky given the ongoing uncertainties, particularly through programs like CRP that set aside land for only 10–15 years, after which it can return to cultivation and any carbon that was sequestered might be released back to the atmosphere.
A common critique of voluntary carbon markets more broadly is that they do not require buyers to reduce their own emissions. For instance, a company could buy carbon credits for a nominal price instead of changing their business practices to reduce emissions from the company’s transportation, energy use, and other GHG-emitting processes. Many critics have stated that this can be a form of greenwashing that allows businesses (and individuals) to continue business-as-usual without reducing overall emissions.
Since agriculture generates around 10 percent of U.S. GHG emissions, there are many opportunities to reduce emissions in the sector. For example, agricultural soil management activities like fertilizer application are the largest source of U.S. N2O emissions, a GHG that has almost 300 times the global warming potential of CO2. While there are some programs in place to address fertilizer application and other GHG-emitting activities, they deserve more attention and funding rather than relying solely on soil carbon credits to offset the industry’s emissions.
Despite their limitations, agricultural carbon markets have the potential to incentivize some large producers to grow food more sustainably. However, it is important for the Biden administration to use carbon markets as one piece in a suite of climate approaches for the agriculture sector. There are a range of federal programs that incentivize farmers to use climate-friendly practices, whether or not they sequester verifiable amounts of soil carbon, and expanding these programs can both reduce GHG emissions and improve farm resiliency in a changing climate. Creating a more holistic set of agriculture sector policies will help maximize GHG emissions reductions and ensure that all U.S. farmers are able to benefit from contributing to national climate change goals.
Jamie Lutz is a research associate with the Global Food Security Program at the Center for Strategic and International Studies (CSIS) in Washington, D.C. Caitlin Welsh is the director of the CSIS Global Food Security Program.
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