Dec 06 2022

Michael Leitch & Bronson Griscom

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When we launched XPRIZE Carbon Removal, one of the most common questions we were asked was “Can a Natural Climate Solution (NCS) win?”. The question of how such solutions, which typically have shorter sequestration durations than other carbon dioxide removal pathways, fit into the carbon management landscape have been a source of ongoing discussion in the sustainability community. The recently-concluded COP27 meetings were no exception.  In fact, we are firm believers in the power these solutions have in our fight against climate change, and we specifically designed the XPRIZE requirements to such that successful demonstrations of natural climate solutions can win the grand prize. With the milestone round of the competition in the rear-view mirror and as we refocus on the $80M in grand prize money still up for grabs, one of the questions we’ve been asking ourselves is, why hasn’t there been a stronger showing of NCS in the competition to date? 

 The XPRIZE Carbon Removal Guidelines specify that eligible solutions must remove carbon from the atmosphere (on a net basis) and sequester it with a durability of at least 100 years. These technical requirements (in addition to sustainability requirements) necessarily limit the types of NCS projects that may be viable, but they were not written to exclude any specific pathways or project types.  Fundamentally the XPRIZE judges will enthusiastically consider any proposal which makes the case that they meet those functional requirements. In fact, we believe the open competition format presents an opportunity to reward innovations in the NCS space that improve the efficacy of such solutions. The competition is still open to new entries, so it isn’t too late for demonstrations of Natural Climate Solutions to submit (and win)!


The term Natural Climate Solutions (NCS) refers to improved ecosystem stewardship, including protection, restoration, and improved management practices,  that increase carbon storage or avoid greenhouse gas emissions ( Griscom et al. 2017). Roughly half of NCS can be considered Carbon Dioxide Removal (CDR): Those solutions which involve enhanced removals by ecosystems through restoration of native ecosystems and improved management of agricultural and forestry lands. The other half involve avoiding emissions from ecosystems caused by human impacts: For example, projects which protect forests and other ecosystems from logging or degradation ( Griscom et al. 2017, Roe et al. 2021).

The potential of NCS to remove carbon dioxide and mitigate emissions while safeguarding food security and biodiversity are estimated to exceed 20 GT CO2e/year. It is estimated that about half of this potential (over 11 GtCO2e/yr) could be implemented at a cost of less than $100 per tonne CO2e ( Griscom etal 2017, Roe et al. 2021)  Within these estimates, at least 4 GtCO2e/yr could be considered CDR (whether projects within the summary pathways can be considered CDR is highly dependent on project-specific conditions). Further, the latest IPCC report ( IPCC AR6 WG3) identifies the majority of cost-effective removals are forms of NCS. As such, there is strong evidence that NCS could satisfy the majority of needed global CDR, provided they are implemented responsibly (food security, fiber security, and biodiversity conservation, among other factors, are important constraints).


Two critical requirements distinguish carbon dioxide removals from carbon dioxide reduction and avoidance (Tanzer & Ramirez 2019):

• Greenhouse gases must be removed from the atmosphere in a quantity greater than those emitted by the process.
• Removed gasses must be stored out of the atmosphere in a manner intended to be permanent.

The question of what constitutes “permanent storage” is important, both as a philosophical and practical matter. Philosophically, permanence is something of a fallacy, since nothing in nature is truly permanent, and ALL CDR pathways are subject to some risk of re-emission. The practical question is, how long must carbon dioxide need to be removed from the atmosphere to mitigate the worst effects of climate change? And critically, what efforts and costs are associated with maintaining the required sequestration duration? The length of time sequestered carbon dioxide can be expected to remain out of the atmosphere is referred to as ‘durability’, and this concept allows us to recognize and quantify the inevitable re-emission (ie physical leakage) of greenhouse gasses, regardless of CDR pathway (Carbon Direct).

The durability requirement for the XPRIZE Carbon Removal is 100 years (XPRIZE Carbon Removal Guidelines). This was chosen as a practical matter, to ensure that the cost of sequestration among different pathways was calculated using a common baseline, in consideration of ongoing operational issues that may need to be accounted for.

This decision may be seen as limiting the set of NCS projects that may be eligible for the prize; however, rather than thinking about this requirement as a screening criterion, we consider this requirement to be an invitation to show us what it takes, and what it will cost, to ensure that a quantity of CO2 that is sequestered today, will be sequestered 100 years from now. This may be inclusive of ongoing management or repetition of the process, and there is no requirement for the same CO2 molecule to remain sequestered for a full 100 years. It also represents an opportunity to demonstrate the ways that innovation can extend the durability of pathways traditionally considered to have short duration storage.


Durability is a measure of the expected stability of CO2 sequestered by a project over time. It is important to note that durability is an attribute of projects, not pathways: Durability cannot necessarily be taken for granted, but needs to be evaluated under the specific conditions the project is found. For many forms of NCS and project designs, durability is a function of characteristics of the specific project location and project scale (geology, ecology, agronomy, climate etc.) and both natural and anthropogenic disturbance regimes.  For designs where project durability has not been well established or verified empirically, it is important to quantify an expected rate of re-emission, and inventory all significant risk factors that define the probability of major disturbance event(s) that will lead to additional re-emission.

Some aspects of durability may be difficult and/or not desirable to increase. For example, natural tree mortality and decomposition is an important part of forest ecosystems. Nevertheless, project durability can be managed. For example, increased spatial scale will tend to reduce risks that constrain durability. The probability that an individual tree will die in a forest is higher than the probability that a stand of 1000 trees will all die at once. Some geographies and ecosystem types have higher risk of disturbance than others. The risks of some anthropogenic disturbances can be actively reduced by management actions, and ecosystem resilience to increasingly extreme weather events can be improved with certain restoration interventions (for example, Lagomasino et al. 2021 demonstrate how restoration can improve the resilience of mangroves to hurricanes).   By implementing interventions that target and reduce the effect of specific re-emission pathways, durability may be improved. In other words, the expected levels of re-emission, and hence the deductions applied to estimated project sequestration over a 100 year time period, can be reduced. It is in this area that we believe there is a great deal of space for innovation, especially in natural climate solutions.

One strategy employed by existing NCS standards is the use of buffer pools – a type of insurance policy - to manage the durability of projects by aggregating the risk of re-emission across a large and distributed portfolio of individual projects, effectively reducing the re-emission risk of all participating projects (VERRA). Other strategies to extend or enhance the durability of solutions may include improving the technologies used to measure the rates of re-emission (thereby improving the level of certainty in the durability of stored carbon) or implementing engineering and/or land stewardship controls to suppress specific re-emission pathways (CDR Primer Chapter 3).


The 100 year durability threshold for the XPRIZE competition may be met by quantifying the expected (and probabilistic) rate of re-emission associated with a project, and counting those tonnes as an emission source in their life cycle analysis. How that is done is up to each team to define in the context of their own demonstration, but proponents of natural climate solutions will do well in the XPRIZE competition by considering the following best practices:

Demonstrate a thorough understanding of the CO2 fluxes in and out of the project ecosystem. 

The Prize Guidelines require teams to demonstrate that, on net, 1000 tonnes of removed CO2 can be considered durably sequestered. Being able to give confidence to these primary flows through direct measurement is key, and teams should make an effort to acknowledge both the precision and accuracy of these measurements. CO2 that is expected to re-emit within 100 years should be calculated and considered as an emission when calculating the net removal capacity of the demonstration.

Develop deep expertise in all of the re-emission pathways associated with your demonstration, and show how you are able to measure and respond to re-emission risk. 

Demonstrating expertise in the various modes of re-emission will also lend credibility to your claims about how easily your solution will scale, since many of these risks are dependent on the characterization and selection of project sites.

Show the XPRIZE judges how you actively manage your project to establish durability greater than 100 years. 

Innovation can take the form of novel instrumentation, data collection, analysis, design, and/or active management of a project. Innovation may also involve the configuration of a project, such as manipulating the scale and/or distribution of projects in a way that addresses uncertainty about major reversals of removals. But fundamentally, the durability requirement in this competition requires each team to show a genuine commitment to assuring that the established amount of CO2 sequestered today (established after adjusting for risks and rates of re-emissions), will be sequestered 100 years from now.

Michael Leitch & Bronson Griscom