The Potential and Limitations of Carbon Farming

The Carbon Sequestration Opportunity

Soil stores vast quantities of carbon—more than is held in the atmosphere and all vegetation combined [1] —and plays a key role in the carbon cycle and climate change. Over centuries of agricultural conversion, enormous amounts of carbon [2] have escaped from soils [3] into the atmosphere. However, specific agricultural management practices have been shown to restore lost soil carbon, pulling planet-warming MTCO2e from the atmosphere and combatting climate change. One striking estimate suggests that restoring soil carbon on croplands to natural levels could remove the equivalent of more than a decade of global greenhouse gas emissions.[4]

Carbon-farming practices such as cover cropping, adding organic wastes to soil, no-till farming, and agroforestry offer a win-win opportunity,[5],[6] simultaneously combatting climate change while improving soil health and agricultural yields. Carbon-rich soils bring numerous additional benefits, including improved water-holding capacity, greater resilience to extreme weather, and crops with improved nutritional quality. [7],[8]

Implementation Challenges and Tradeoffs

Despite its significant potential, carbon farming faces real implementation challenges. The capacity to sequester carbon varies dramatically across different geographies,[9] depending on factors like local climate and soil types. This means carbon farming programs won't be equally effective everywhere.

Measuring sequestered soil carbon also presents difficulties. Field data collection can take years, while computer models can instantly predict carbon sequestration from farming practices.[10] However, skipping field trials in favor of modeling alone involves tradeoffs; soil-carbon models come with high uncertainty in their estimates.[11]

A final consideration is that soil carbon sequestered through carbon farming is not necessarily permanent. Soil carbon is dynamic, and long-term storage of soil carbon depends on continued good land management.[12] Reverting back to farming practices that release carbon can gradually undo prior gains.[13]

To ensure carbon farming is widely adopted as a climate solution, public policy plays a key role. Most farmers won't adopt carbon farming practices without external incentives, since the average yield increases from improved soil carbon tend to be modest—providing little economic motivation on their own.[14] Well-designed policies can boost adoption through financial assistance, technical support, and education about the broader benefits for farm operations and soil health.[15]

This case study examines two different county-level approaches:

1. Marin County, California: A partnership-based model built upon successful field trials and integrated into the county's Climate Action Plan.
2. Santa Clara County, California: A grant program featuring an innovative reverse auction funding system that relies on USDA carbon-sequestration models rather than local field trials.

Marin County—Field Trials to Partnership Model

The Marin Carbon Project began in 2007 when local landowner John Wick partnered with Jeff Creek, a PhD rangeland ecologist, to investigate whether improved management practices could enhance soil carbon sequestration.[16] They enlisted UC Berkeley to conduct trials on Wick’s ranch, comparing four treatment approaches (standard grazing, plow plus compost, compost and grazing, and a control plot).

The results exceeded expectations, with the compost and grazing regime resulting in significant increases in soil carbon and water-holding capacity.[17] The success of the field trials led to the formal establishment of the Marin Carbon Project, a partnership that provides technical and financial assistance to farmers and ranchers for planning and implementation.

Collaborative Partnership Model

The project engages a range of partners, solving the fundamental challenge that no single organization possesses all the expertise needed to implement the program:[18]

• Marin County Agricultural Commissioner
• Local Natural Resources Conservation Service (NRCS) office
• Marin Agricultural Land Trust
• Marin Resource Conservation District (RCD)
• University of California at Berkeley (UC Berkeley)
• Marin UC Cooperative Extension
• Carbon Cycle Institute
• Silver Labs

Marin RCD leads the implementation and coordination. UC Berkeley led the pilot research and continues to collect data. The partnership with NRCS enables Marin RCD to use modeling software to predict carbon sequestration, reducing the need for expensive field measurements at every project site. The Marin Agricultural Land Trust monitors carbon-farming project sites alongside its easement monitoring.

Implemented Practices and Process

The Marin Carbon Project supports the implementation of various carbon-farming practices, including:[19]

• Compost application (primarily on grazed rangelands)
• Stream restoration and riparian planting
• Hedgerow establishment, conservation cover, and tillage management
• Cover cropping and multistory cropping
• Prescribed grazing and range planting

The implementation process follows a structured approach:[20]

1. Farms and ranches apply for a carbon farm plan.
2. Marin Carbon Project helps develop carbon farm plans for awarded applicants.
3. Technical and financial assistance is provided for implementation, particularly for projects requiring substantial permitting.[21]
4. Marin Carbon Project monitors carbon farming projects with assistance from the Marin Agricultural Land Trust.

Implementation and Success Factors

By 2020, the project had completed 19 Carbon Farm Plans, with 68 parcels waitlisted.[22] The project established ambitious goals:[23]

• By 2030: Engage 60 Marin farms/ranches across 30,000 acres, sequestering 185,839 MTCO2e.
• By 2045: Engage 180 Marin farms/ranches across 90,000 acres, sequestering over 525,000 MTCO2e.

A key factor for success was the project's incorporation into Marin County's Climate Action Plan,[24] which officially recognized agriculture as part of the climate solution, ensuring county support and facilitating fundraising efforts.

The project benefits from diverse funding sources, with pilot funding from the local community foundation playing a crucial role.[25] Ongoing operations rely on a mix of non-profit and government funds, including the California Coastal Conservancy and a local ballot measure that provides roughly $700,000 annually as matching funds.

Santa Clara County—The Reverse Auction Approach

Santa Clara County took a different approach, forgoing local field trials and establishing the Agricultural Resilience Incentive (ARI) Grant Program in 2019 to provide voluntary financial incentives to farmers and ranchers to sequester soil carbon.

Program Design

The ARI program aims to advance climate change goals by improving soil health and sequestering atmospheric carbon, with the additional goal of providing co-benefits like ecosystem services. Unlike Marin, Santa Clara did not first conduct local trials to assess sequestration potential, instead relying on modeling and a list of approved carbon-farming practices.[26]

The Board of Supervisors appropriated $220,000 from the General Fund for the pilot program.[27] The program established simple eligibility requirements:[28]

1. Projects must take place on agricultural lands within Santa Clara County.
2. Projects must implement at least one approved management practice from a list of 27 practices.

The approved practices align with California's existing Healthy Soils Program, and their carbon impact can be quantified using the USDA's COMET Planner online tool.[29] Practices are categorized by land type (grazing lands, annual cropland, and perennial cropland).

Market-Based "Reverse Auction" Approach

Santa Clara's most innovative aspect is its "reverse auction" approach to project selection. Unlike traditional grant programs with fixed payment rates, applicants:[30]

1. Estimate the total cost of their proposed practices.
2. Submit a "bid" amount—the price they are willing to accept to implement the practices.

Projects are ranked based on cost-effectiveness, measured as dollars per metric ton of MTCO2e sequestered. Successful bids are typically below total cost estimates, as applicants are encouraged to consider the long-term value added to their operation and to leverage outside funding sources.[31] To ensure smaller farms can compete, applications are sorted by operation size (above/below 40 acres) and ranked separately.

Program Strengths

Santa Clara's program prioritizes efficiency and cost-effectiveness at every level. By forgoing field trials and relying on USDA resources for carbon estimates, the county significantly reduced both startup costs and implementation timelines. The reverse auction design amplifies these savings by funding projects that sequester the most carbon at the lowest public cost. This competitive approach also encourages applicants to secure additional funding sources, allowing the program to achieve greater impact with its limited budget.

Key Lessons for Local Governments

These case studies offer several lessons for local governments considering carbon farming initiatives:

1. Consider Financial and Capacity Constraints: While Marin County had the funding and expertise to conduct local field trials, Santa Clara's program shows that carbon farming initiatives can succeed with more modest resources and streamlined designs. This involves tradeoffs: skipping field-data collection reduces costs but increases uncertainty in carbon-sequestration estimates.[32]
2. Assess Local Potential: Marin's decision to conduct field trials reflects a crucial first step: determining whether local soils can actually store significant amounts of carbon before launching a program. Not all regions have substantial potential for soil carbon sequestration,[33] making it essential to assess local capacity upfront.
3. Factor in Co-Benefits: Both counties identified co-benefits from carbon farming, such as improved soil health and climate resilience. Programs that account for these broader benefits may find it easier to build support and justify investments.
4. Leverage Existing Partnerships: Marin's success stemmed partly from building on established relationships with agricultural and conservation partners. Leveraging expertise from NRCS offices, agricultural commissioners, and universities can ensure all necessary specializations are represented.[34]
5. Integrate with Broader Strategies: Programs that fit within broader climate action plans (Marin) or existing statewide initiatives (Santa Clara, aligning with the Healthy Soils Program) may garner more political and financial support.
6. Consider Long-Term Planning: Soil carbon is neither inert nor permanent. Successful programs must include mechanisms to ensure that sequestered carbon remains in the soil over time and that short-term efforts do not compromise lasting climate benefits.

References

[1] Katerina Georgiou et al., "Global Stocks and Capacity of Mineral-Associated Soil Organic Carbon," Nature Communications 13, no. 1 (July 1, 2022): 1-12, https://doi.org/10.1038/s41467-022-31540-9.

[2] Damien Beillouin et al., "A Global Meta-Analysis of Soil Organic Carbon in the Anthropocene," Nature Communications 14, no. 1 (2023): 3700, https://doi.org/10.1038/s41467-023-39338-z.

[3] Damien Beillouin et al., "A Global Meta-Analysis of Soil Organic Carbon in the Anthropocene," Nature Communications 14, no. 1 (2023): 3700, https://doi.org/10.1038/s41467-023-39338-z.

[4] Calculated using the estimate of 104 Pg C to a depth of 1 m from Katerina Georgiou et al., "Global Stocks and Capacity of Mineral-Associated Soil Organic Carbon," Nature Communications 13, no. 1 (July 1, 2022): 1-12, https://doi.org/10.1038/s41467-022-31540-9 and given global annual carbon emissions which achieve an annual maximum of 10.1 Pg C according to "CarbonTracker CT2022," NOAA Global Monitoring Laboratory, accessed July 10, 2025, https://gml.noaa.gov/ccgg/carbontracker/.

[5] Claire Chenu et al., "Increasing Organic Stocks in Agricultural Soils: Knowledge Gaps and Potential Innovations," Soil and Tillage Research 188, no. Soil Carbon and Climate Change: the 4 per Mille Initiative (2019): 41-52, https://doi.org/10.1016/j.still.2018.04.011

[6] Yuqing Ma et al., "Global Crop Production Increase by Soil Organic Carbon," Nature Geoscience 16, no. 12 (2023): 1159-65, https://doi.org/10.1038/s41561-023-01302-3.

[7] Johannes Lehmann et al., "The Concept and Future Prospects of Soil Health," Nature Reviews Earth & Environment 1, no. 10 (2020): 544-53, https://doi.org/10.1038/s43017-020-0080-8;

[8] Rattan Lal, "Soil Health and Carbon Management," Food and Energy Security 5, no. 4 (2016): 212-22, https://doi.org/10.1002/fes3.96.

[9] Katerina Georgiou et al., "Global Stocks and Capacity of Mineral-Associated Soil Organic Carbon," Nature Communications 13, no. 1 (July 1, 2022): 1-12, https://doi.org/10.1038/s41467-022-31540-9.

[10] Elisa Bruni et al., "Multi-Modelling Predictions Show High Uncertainty of Required Carbon Input Changes to Reach a 4% Target," European Journal of Soil Science 73, no. 6 (2022): e13330, https://doi.org/10.1111/ejss.13330.

[11] Claire Chenu et al., "Increasing Organic Stocks in Agricultural Soils: Knowledge Gaps and Potential Innovations," Soil and Tillage Research 188, no. Soil Carbon and Climate Change: the 4 per Mille Initiative (2019): 41-52, https://doi.org/10.1016/j.still.2018.04.011.

[12] Claire Chenu et al., "Increasing Organic Stocks in Agricultural Soils: Knowledge Gaps and Potential Innovations," Soil and Tillage Research 188, no. Soil Carbon and Climate Change: the 4 per Mille Initiative (2019): 41-52, https://doi.org/10.1016/j.still.2018.04.011.

[13] Katherine A. Dynarski et al., "Dynamic Stability of Soil Carbon: Reassessing the 'Permanence' of Soil Carbon Sequestration," Frontiers in Environmental Science Volume 8-2020 (2020), https://www.frontiersin.org/journals/environmental-science/articles/10.3389/fenvs.2020.514701.

[14] Yuqing Ma et al., "Global Crop Production Increase by Soil Organic Carbon," Nature Geoscience 16, no. 12 (2023): 1159-65, https://doi.org/10.1038/s41561-023-01302-3.

[15] Nikki P. Dumbrell et al., "What Carbon Farming Activities Are Farmers Likely to Adopt? A Best-Worst Scaling Survey," Land Use Policy 54 (July 2016): 29-37, https://doi.org/10.1016/j.landusepol.2016.02.002.

[16] From conversation with Nancy Scolari, Executive Director of Marin Resource Conservation District.

[17] From conversation with Nancy Scolari, Executive Director of Marin Resource Conservation District.

[18] "Carbon Farming Overview: Marin RCD's Carbon Farming Program." (2014, January 22). Marin Resource Conservation District. https://www.marinrcd.org/programs/home2/carbon-farming/.

[19] "Carbon Farming Overview: Marin RCD's Carbon Farming Program." (2014, January 22). Marin Resource Conservation District. https://www.marinrcd.org/programs/home2/carbon-farming/.

[20] "Carbon Farming Overview: Marin RCD's Carbon Farming Program." (2014, January 22). Marin Resource Conservation District. https://www.marinrcd.org/programs/home2/carbon-farming/.

[21] From conversation with Nancy Scolari, Executive Director of Marin Resource Conservation District.

[22] "Carbon Farming Overview: Marin RCD's Carbon Farming Program." (2014, January 22). Marin Resource Conservation District. https://www.marinrcd.org/programs/home2/carbon-farming/.

[23] "Marin County Unincorporated Area Climate Action Plan 2030" (County of Marin, December 2020), https://www.google.com/search?q=https://www.marincounty.gov/sites/g/files/fdkgoe241/files/2024-01/cap-2030_12082020final.pdf.

[24] "Marin County Unincorporated Area Climate Action Plan 2030" (County of Marin, December 2020), https://www.google.com/search?q=https://www.marincounty.gov/sites/g/files/fdkgoe241/files/2024-01/cap-2030_12082020final.pdf.

[25] From conversation with Nancy Scolari, Executive Director of Marin Resource Conservation District.

[26] "Agricultural Resilience Incentive (ARI) Grant Program" (County of Santa Clara), accessed May 16, 2025, https://stgenpln.blob.core.windows.net/document/ARI_guide.pdf.

[27] "Agricultural Resilience Incentive (ARI) Grant Program." (County of Santa Clara), accessed May 16, 2025, https://stgenpln.blob.core.windows.net/document/ARI_guide.pdf.

[28] "Agricultural Resilience Incentive (ARI) Grant Program." (County of Santa Clara), accessed May 16, 2025, https://stgenpln.blob.core.windows.net/document/ARI_guide.pdf.

[29] "Agricultural Resilience Incentive (ARI) Grant Program." (County of Santa Clara), accessed May 16, 2025, https://stgenpln.blob.core.windows.net/document/ARI_guide.pdf.

[30] "Agricultural Resilience Incentive (ARI) Grant Program." (County of Santa Clara), accessed May 16, 2025, https://stgenpln.blob.core.windows.net/document/ARI_guide.pdf.

[31] "Agricultural Resilience Incentive (ARI) Grant Program." (County of Santa Clara), accessed May 16, 2025, https://stgenpln.blob.core.windows.net/document/ARI_guide.pdf.

[32] Elisa Bruni et al., "Multi-Modelling Predictions Show High Uncertainty of Required Carbon Input Changes to Reach a 4% Target," European Journal of Soil Science 73, no. 6 (2022): e13330, https://doi.org/10.1111/ejss.13330.

[33] Claire Chenu et al., "Increasing Organic Stocks in Agricultural Soils: Knowledge Gaps and Potential Innovations," Soil and Tillage Research 188, no. Soil Carbon and Climate Change: the 4 per Mille Initiative (2019): 41-52, https://doi.org/10.1016/j.still.2018.04.011.

[34] "Marin County Unincorporated Area Climate Action Plan 2030" (County of Marin, December 2020), https://www.google.com/search?q=https://www.marincounty.gov/sites/g/files/fdkgoe241/files/2024-01/cap-2030_12082020final.pdf.