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Biodiversity and nature‐based carbon offsets are central to strategies addressing biodiversity loss and climate change. Their credibility depends on permanence—the expectation that biodiversity gains or sequestered carbon persist at least as long as the impacts they compensate for, or in perpetuity. Yet ecosystems are dynamic and increasingly exposed to disturbance, making perpetual outcomes difficult to guarantee. Despite this, many offset programs rely on fixed durations and static assumptions ill‐suited to managing long‐term risks, creating a structural misalignment between ecological permanence and the safeguards intended to secure it. To assess this misalignment, we reviewed three decades of literature to identify risks to long‐term durability and strategies for managing them. We developed a typology spanning three domains. Non‐physical risks, such as weak governance and limited data transparency, were most frequently reported, often co‐occurred, and enabled other failures. Physical risks such as fire, storms, or flooding cause material damage and are intensifying with climate change. Methodological risks, including oversimplified metrics and flawed design, expose structural weaknesses in offset systems. Our typology provides a framework for assessing permanence risks and strengthening offset governance. Credible, enduring offsets are achievable, provided robust risk management and adaptive governance are aligned with ecological realities.

Forests have considerable potential to mitigate anthropogenic climate change through carbon sequestration, as well as provide society with substantial co-benefits. However, climate change risks may fundamentally compromise the permanence of forest carbon storage. Here, we conduct a multi-method synthesis of contiguous US forest aboveground carbon storage potential at both regional and species levels through a fusion of historical and future climate projections, extensive forest inventory plots datasets, machine learning/niche models, and mechanistic land surface model ensemble outputs. We find diverging signs and magnitudes of projected future forest aboveground carbon storage potential across contrasting approaches, ranging from an average total gain of 6.7 Pg C to a loss of 0.9 Pg C, in a moderate-emissions scenario. The Great Lakes region and the northeastern United States showed consistent signs of carbon gains across approaches and future scenarios. Substantial risks of carbon losses were found in regions where forest carbon offset projects are currently located. This multi-method assessment highlights the current striking uncertainty in US forest carbon storage potential estimates and provides a critical foundation to guide forest conservation, restoration and nature-based climate solutions.Projections of forest aboveground carbon storage potential in the United States show divergent results across different modelling approaches due to uncertainties in the estimated impact of climate risks, according to a comparison of modelling results.

California operates a large forest carbon offsets program that credits carbon stored in forests across the continental United States and parts of coastal Alaska. These credits can be sold to buyers who wish to justify ongoing emissions, including in California’s cap-and-trade program. Although fossil CO 2 emissions have effectively permanent atmospheric consequences, carbon stored in forests is inherently less durable because forests are subject to significant socioeconomic and physical risks that can cause temporarily stored carbon to be re-released into the atmosphere. To address these risks, California’s program is nominally designed to provide a 100-year guarantee on forest carbon claims based on a self-insurance program known as a buffer pool. Projects contribute credits to the buffer pool based on a suite of project-specific risk factors, with buffer pool credits retired as needed to cover carbon losses from events such as wildfire or drought. So long as the buffer pool remains solvent, the program’s permanence claim remains intact. Here, we perform an actuarial analysis of the performance of California’s buffer pool. We document how wildfires have depleted nearly one-fifth of the total buffer pool in less than a decade, equivalent to at least 95 percent of the program-wide contribution intended to manage all fire risks for 100 years. We also show that potential carbon losses from a single forest disease, sudden oak death, could fully encumber all credits set aside for disease and insect risks. These findings indicate that California’s buffer pool is severely undercapitalized and therefore unlikely to be able to guarantee the environmental integrity of California’s forest offsets program for 100 years.

The costs and technical expertise associated with forest carbon offset projects can be significant, while decades-long time commitments can discourage participation from the outset. Considering these challenges, several new approaches have emerged in the United States under the auspices of both long-standing and recently-established programs, attempting to leverage increased carbon mitigation. What several of these approaches have in common is reduced emphasis on long-term storage, what we refer to as a traditional perspective of permanence. Instead, each considers shorter periods of time—up to and including single year harvest deferrals—as eligible project commitments. Here, we provide a brief discussion of the historical permanence and accounting literature, with an emphasis on contradictory views and how these perspectives have evolved over time. Next, we quantitatively assess the long-term influence of different permanence requirements as envisioned in several new and existing forest carbon programs, estimating net mitigation across a variety of forest types and project configurations. We conclude with a presentation of our quantitative findings in the context of the existing literature, while also highlighting unmet research needs on these so-called new offsets , those emerging novel approaches for forest carbon mitigation that challenge the research and practice status quo .

Forest carbon offset protocols reward measurable carbon stocks to adhere to accepted greenhouse gas (GHG) accounting principles. This focus on measurable stocks threatens permanence and shifts project-level risks from natural disturbances to an offset registry’s buffer pool. This creates bias towards current GHG benefits, where greater but potentially high-risk stocks are incentivized vs. medium-term to long-term benefits of reduced but more stable stocks. We propose a probability-based accounting framework that allows for more complete risk accounting for forest carbon while still adhering to International Organization for Standardization (ISO) GHG accounting principles. We identify structural obstacles to endorsement of probability-based accounting in current carbon offset protocols and demonstrate through a case study how to overcome these obstacles without violating ISO GHG principles. The case study is the use of forest restoration treatments in fire-adapted forests that stabilize forest carbon and potentially avoid future wildfire emissions. Under current carbon offset protocols, these treatments are excluded since carbon stocks are lowered initially. This limitation is not per se required by ISO’s GHG accounting principles. We outline how real, permanent, and verifiable GHG benefits can be accounted for through a probability-based framework that lowers stressors on a registry’s buffer pool.

Reducing deforestation and forest degradation presents a climate-change mitigation opportunity that is critical to meeting the Paris Agreement goals, and to achieving reductions in the atmospheric concentrations of greenhouse gases (GHGs). Reducing Emissions from Deforestation and Forest Degradation (REDD) provides developing countries with results-based financial incentives for reducing deforestation and forest degradation through either non-market payments (payments without generation of carbon credits), or market-based mechanisms (carbon credits). REDD credits have been recently accepted to be used in offsetting programs (e.g., CORSIA) and are being considered under Article 6. However, various publications have questioned whether carbon credits from REDD should be accepted under market-based mechanisms, and have identified issues regarding their environmental integrity and their ability to offset emissions from other sectors. In recent years, REDD implementation has moved from the project level to the national or subnational (jurisdictional) level, and is addressing some of the concerns that have been raised for project-level interventions regarding the robustness of baselines and leakage, for example. In this paper we compare the environmental integrity of credits from REDD programs with that from on-grid renewable energy projects by examining aspects related to permanence, additionality, baselines, uncertainty, and leakage. We show that the environmental integrity of emission reductions sourced from REDD programs has unique strengths, and that those sourced from renewable energy projects have weaknesses of their own. Probably due to a lack of understanding of the respective weaknesses and strengths of these two sources of credits, the emission reductions from REDD programs have been historically questioned and subjected to a level of scrutiny that has not been made with emission reductions from other sectors, such as renewable energy projects. Recognizing the strengths and weaknesses of emission reductions from both types should help decision makers and carbon standards recognize the high quality of emission reductions from REDD programs, and rationalize the current requirements or restrictions imposed.