by Danny Cullenward +Joseph Hamman +Jeremy Freeman
Jun 01 2020
Jun 01 2020
by Danny Cullenward +Joseph Hamman +Jeremy Freeman

Getting soil
carbon right

Press coverageComment letter #1Comment letter #2

There are good reasons to be excited about better soil management practices for carbon removal. Agricultural practices that increase soil carbon storage have the potential to provide benefits both for farmers and ranchers and for the global climate. In turn, working with the agricultural sector could help engage people and organizations that haven’t been the focus of most climate action to date. The complexities of soil carbon quantification, however, pose challenges for designing financial crediting systems.

This post provides context for a comment letter we submitted to the Climate Action Reserve concerning one such effort, its draft Soil Enrichment Protocol. We were proud to work with our collaborators Dr. Grayson Badgley (a member of the expert workgroup for this protocol and an expert in remote sensing) and Dr. Jane Zelikova (Chief Scientist at Carbon180, Research Scientist at the University of Wyoming, and an expert in climate change science), who joined the letter.

Below, we review why quantifying soil carbon is so challenging and summarize our concerns with respect to the technical details of the draft protocol. We also emphasize the conflict of interest presented by the fact that the protocol was funded and drafted by a company, Indigo Ag, that intends to use the protocol to generate carbon credits.

Why soil carbon is challenging

Agricultural practices such as adding cover crops, changing from conventional tillage to no-tillage, or applying soil amendments, have the potential to both improve crop health and remove additional carbon dioxide from the atmosphere. The challenge is that these effects are hard to measure. Accurate quantification relies on physical sampling and soil carbon content measurements, but soil carbon content varies substantially across soil depth, individual plots, spatial locations within a plot (even at the scale of a few meters), and time.11J Sanderman & J A Baldock (2010) Accounting for soil carbon sequestration in national inventories: a soil scientist’s perspective Environmental Research Letters ,22C Poeplau & A Don (2014) Carbon sequestration in agricultural soils via cultivation of cover crops – A meta-analysis Agriculture, Ecosystems and Environment ,33M F Cotrufo et al. (2018) Soil carbon storage informed by particulate and mineral-associated organic matter Nature Geoscience Future changes in land management practices can also reverse gains. Addressing that variability requires careful sampling methods, baseline estimation, meta analysis, and interpretation and projection that is guided, but not replaced, by models.44K Paustian et al. (2017) Field‐ and farm‐scale assessment of soil greenhouse gas mitigation using COMET‐Farm Agronomy Monographs ,55E E Campbell & K Paustian (2015) Current developments in soil organic matter modeling and the expansion of model applications: a review Environmental Research Letters

Compared to forests, soil carbon science hasn’t benefited from decades of extensive public data collection and model verification. In the US, for example, the Forest Service has been operating its national Forest Inventory and Analysis program since 1930, producing a wealth of public information that can directly guide efforts to manage and quantify forest carbon. Several soil monitoring networks around the world are underway to help bridge this gap,66B van Wesemael et al. (2011) How can soil monitoring networks be used to improve predictions of organic carbon pool dynamics and CO2 fluxes in agricultural soils? Plant and Soil ,77A Orgiazzi et al. (2017) LUCAS Soil, the largest expandable soil dataset for Europe: a review European Journal of Soil Science but considerable uncertainty and much future work remains. 88P Smith et al. (2019) How to measure, report and verify soil carbon change to realize the potential of soil carbon sequestration for atmospheric greenhouse gas removal Global Change Biology

Forest carbon quantification has also benefited from advances in remote sensing technology, with air and satellite-based methods providing increasingly accurate insights into above-ground forest carbon stocks. In contrast, while accurately inferring soil carbon from remote measurement could become feasible in the future, physical samples are required for high resolution estimates today. Remote sensing for soil is an active area of research, and will require extensive benchmarking, calibration, and validation with public data.

Any protocol for soil carbon crediting needs to start by appreciating the complexity of the science, and progress will require coordination around public data, sharing of methods, and transparency.

Our comments

Given the enormous challenges in designing a carbon offset protocol to quantify and credit soil carbon storage, we were interested to see the Climate Action Reserve take this problem on. We reviewed the draft protocol and highlight two sets of issues here. See our comment letter for details and check out comments posted by other groups at the protocol website.

Conflict of interest

Success requires independence between the development of the rules and their application to specific projects or companies

Normally, the idea behind third-party carbon offset standards is that an independent body develops technical methods and verification processes. Those methods give buyers confidence that the credited climate benefits meet rigorous standards. (In practice, this approach has always been complicated by the fact that verification is typically paid for by the very projects that seek to generate credits.) Success requires independence between the development of the rules and their application to specific projects or companies.

Unfortunately, that’s not the case here. Indigo Ag paid the Climate Action Reserve to develop the protocol and contributed to drafting its requirements. Not only did Indigo Ag shape the protocol, but the protocol’s design, as we discuss below, allows users like Indigo Ag to pick their own sampling methods and model calculations — subject to the approval of the Climate Action Reserve, which was paid by Indigo Ag to develop the protocol in the first place. The protocol doesn’t even require independent verifiers to collect independent soil samples or reproduce projects’ modeled climate benefits.

As a result, there’s very little that’s independent about the proposed protocol.

Problems and recommendations

Our technical comments focused on four issues — additionality, model selection, sampling and verification, and permanence — many of which are especially concerning given the conflict of interest raised above.

ProblemContrary to key standards for compliance grade markets and the Climate Action Reserve’s own requirements for private markets, the protocol does not evaluate project additionality.
RecommendationTo accurately inform prospective credit buyers, the protocol should acknowledge that it is not evaluating the financial viability of credited projects and therefore is not testing for additionality.
Model selection
ProblemProjects can select their own models to calculate the number of credits earned under the protocol, subject to a set of ambiguous quality control criteria.
RecommendationAll model calculations should be based on well-established, scientifically vetted models and either be fully disclosed such that an independent researcher could fully replicate calculations or independently verified by a financially disinterested third party.
Sampling and verification
ProblemThe protocol does not prescribe specific sampling methods to ensure accurate measurement of individual project sites, nor does it require independent measurement of soil carbon in the verification process.

The protocol proposes that fewer than 1% of large project aggregators’ sites be verified, without justifying that rate with respect to known data on soil carbon quantification and sampling.

Numerous loopholes would allow offset projects to avoid all physical site visits in the verification process.
RecommendationProjects should rigorously sample soils at baseline and regular intervals, with sampling designs and randomization schemes that adequately capture underlying variation, in accordance with best practices from the scientific literature.

The rate of sampling of individual project sites for large aggregators should be increased.

Verification sampling must be done on-site and should not be replaced with self-reporting or remote sensing techniques that are not widely accepted in the scientific literature.
ProblemAlthough the protocol offers 100-year guarantees on carbon permanence, it only requires physical soil carbon monitoring for 30 years and allows projects to select shorter compliance timeframes in private contracts with the Climate Action Reserve.

The protocol does not support its choice of buffer pool parameters. The buffer pool rules also include a series of loopholes that do not protect against the possibility that participating projects might default on their contracts.
RecommendationThe permanence of any credits issued under the protocol should be the lesser of (1) the period of time over which projects must physically monitor soil carbon and (2) any shorter time period the carbon offset project elects in its private contract with the Climate Action Reserve.

The protocol should justify the choice of parameters used to calculate the contribution of its buffer pool and eliminate loopholes that allow private parties to avoid contributing to the buffer pool to cover their default risks.

Why do we care?

Protocols and standards must represent the most accurate approaches to calculating climate benefits

Scientific integrity is at the core of our mission. With growing interest in public and private markets for climate solutions, protocols and standards must represent the most accurate approaches to calculating climate benefits — especially in areas like soil carbon that will likely require public policy support in the years ahead.

Private companies are of course welcome to market proprietary solutions, and we appreciate that some exciting areas of carbon removal are developing under confidential terms. We hope that, when ready, they bring these solutions into the public to demonstrate reliability and rigor.

Meanwhile, it’s one thing for a company to sell proprietary technology to private clients, but an entirely different matter for a company to hold out a proprietary technology or approach as having third-party approval or broad scientific validation. When organizations make public claims about the performance of their projects or adequacy of their protocols, we have an interest in making sure those claims match the facts — and that those projects and standards contribute to a better public understanding of the field of climate solutions.

Alternative approaches

Carbon offsets are particularly challenging because they require effectively perfect calculations — even one ton of over-crediting causes a buyer’s claim of carbon neutrality to fall short. 99B Haya et al. (2019) Managing Uncertainty in Carbon Offsets: Insights from California’s Standardized Approach Climate Policy (in press)

Perfection is a tall order in the face of conceptually complicated and empirically fraught problems like leakage and additionality. Achieving these standards is difficult even when the quantification science is mature and readily applied to carbon offset projects. In the case of soil carbon, where comprehensive measurement is challenging and there are few long-term public data sets from which to extrapolate trends, that task is even harder.

But carbon credits aren’t the only way to support soil carbon. An alternative might be to think about soil carbon as a public investment, where funds are used to support beneficial changes in land management and agricultural practices but not used to claim precise carbon benefits. In contrast to an offsets-oriented paradigm, perfection isn’t required for public investment — especially not if soil carbon is a co-benefit that helps justify financial support.

Meanwhile, any efforts built around carbon offset credits should be transparent, subject to third-party verification, and capable of thorough validation by financially independent organizations.

Update — Aug 25 2020

After reviewing an updated version of the draft protocol, we submitted a second comment letter on remaining concerns and additional issues.


Danny led the effort to write the comment letters and drafted the first version of this article. All authors contributed to writing the comment letters and the article. Collaborators Jane Zelikova and Grayson Badgley contributed to the comment letters, but were not involved in writing this article.

Please cite as:

D Cullenward, J Hamman, J Freeman (2020) “Getting soil carbon right” CarbonPlan https://carbonplan.org/research/soil-carbon-comment


CarbonPlan received no financial support for this work and has no financial conflicts of interest with the Climate Action Reserve or Indigo Ag.

Article text and figures made available under a CC-BY 4.0 International license.


Questions? Interested in collaborating on these problems?
email us
Why the science of soil carbon quantification is complicated, and why getting the details right matters for soil carbon protocols
CarbonPlan is a registered nonprofit public benefit corporation in California with 501(c)(3) status.
SCROLL: 0.00