Accounting for the climate benefit of temporary carbon storage in nature

The study addresses how to rigorously account for the climate benefits of temporary carbon storage achieved through nature-based climate solutions (NbCS), such as avoided deforestation, reforestation, afforestation, and soil carbon enhancement. The core research question is whether and how temporary land carbon storage can be translated into a meaningful climate metric that reflects actual temperature outcomes, given concerns about impermanence due to natural and anthropogenic disturbances. The authors argue that despite concerns, temporary storage can have real climate value if quantified via a physical, rather than economic, framework and if pursued in parallel with rapid fossil fuel CO₂ emissions reductions. The purpose is to reframe tonne-year accounting as a physical tracking metric, linking stored carbon over time to avoided warming and peak temperature outcomes, thereby enabling credible inclusion of temporary storage in mitigation portfolios without assuming permanence upfront.

The paper situates NbCS within a broad literature recognizing both their mitigation potential and criticisms: measurement challenges of sequestration effectiveness, emphasis on fluxes over stocks, neglect of non-carbon biophysical climate effects (albedo, evapotranspiration, clouds), and social-environmental concerns. Prior tonne-year accounting frameworks proposed equivalency factors to compare temporary storage to permanent storage using economic cost–benefit analyses with discount rates and chosen time horizons. These approaches (e.g., 30–130 tonne-years per tonne CO₂) have been critiqued for subjective assumptions and weak connections to physical climate responses, focusing on atmospheric CO₂ burdens without translating to temperature or distinguishing short- vs long-term effects. The authors build on and respond to this literature by grounding tonne-year accounting in the linear relationship between cumulative CO₂ emissions and temperature change (TCRE), proposing tonne-years as a physical tracking metric that maps directly to avoided warming (degree-years), rather than as an offset equivalency for fossil emissions.

The authors use the University of Victoria Earth System Climate Model (UVic ESCM) v2.10, an intermediate-complexity global climate model with dynamic land and ocean carbon cycle processes, a general circulation ocean, a single-layer energy–moisture balance atmosphere, sea-ice models, land vegetation (five plant functional types) including permafrost carbon, and ocean physical/biogeochemical and sedimentary carbon cycles. Spatial resolution is 1.8° latitude by 3.6° longitude. Simulations span 1850–2150 with prescribed forcings: historical fossil fuel CO₂ and non-CO₂ drivers to 2015, then SSP1-1.9 or SSP1-2.6 for fossil fuels and other forcings; land-use CO₂ emissions follow SSP3-7.0 to represent a baseline with ongoing deforestation. This combination defines the baseline scenarios. Against this baseline, they impose NbCS-like interventions by decreasing land-use CO₂ emissions by 3 GtCO₂ per year starting in 2022 (consistent with global cost-effective potential for reforestation/avoided deforestation). Four scenarios are constructed: (1) Permanent storage sustained through end of simulation (emissions reduced relative to baseline throughout; land-use emissions not allowed to become net negative); (2) Permanent storage sustained until 2050, then emissions return to baseline level, with stored carbon maintained; (3) Temporary storage to 2050, then re-emission of all previously stored carbon over 50 years; (4) Temporary storage to 2037 (15 years), then re-emission over 15 years. These represent aggregated global effects of many projects. Tonne-years (TY) are computed as the time integral of the difference in cumulative land-use CO₂ emissions between intervention and baseline, i.e., the time-integrated stored land carbon. The climate impact is mapped via the TCRE: avoided warming over time is proportional to cumulative avoided emissions R(t), ΔT_R(t) = TCRE × R(t). Degree-years (DY) of avoided warming are defined as DY(t) = TCRE × TY(t), making tonne-years directly proportional to time-integrated temperature benefits. The instantaneous temperature benefit relates to the rate of TY accumulation: ΔT(t) = TCRE × TY′(t). They also evaluate two commonly cited tonne-year equivalency factors (31 and 128 tonne-years per tonne permanent storage) by deriving implied permanent storage from modelled TY and converting to temperature using TCRE, comparing these implied trajectories to the actual avoided warming from the simulations. Data and model code availability are provided (UVic ESCM v2.10 official page; source data file).

- Tonne-years of land carbon storage are linearly proportional to degree-years of avoided warming via the model’s TCRE. The derived slope is approximately −0.45 degree-years of avoided warming per 1000 tonne-years, consistent with a TCRE of ~0.45 °C per 1000 GtCO₂ when expressed as avoided warming. - The rate of accumulation of tonne-years (TY′) determines the contemporaneous temperature benefit: increasing TY′ yields increasing avoided warming; constant TY′ sustains a constant temperature benefit; decreasing TY′ erodes prior benefits; when TY′ reaches zero (no net increase in TY), the avoided warming decays to zero and temperatures return to baseline. - In temporary storage scenarios, maximum global temperature reductions relative to baseline are modest but non-negligible: about 0.02 °C and 0.05 °C at peak benefit, depending on the duration and re-emission timeline. - Lowering peak global temperature requires sustaining a constant or increasing rate of TY accumulation until the time of peak global temperature (i.e., until net-zero fossil fuel CO₂ is achieved); if TY accumulation stops earlier, temporary storage does not reduce peak warming. - The climate response to land carbon storage scenarios is effectively independent of the chosen background mitigation pathway (SSP1-1.9 or SSP1-2.6) in terms of temperature differences induced by the land storage. - Common tonne-year equivalency factors (e.g., 31 and 128 tonne-years per tonne permanent storage) mischaracterize climate impacts: they underestimate near-term avoided warming during storage and incorrectly imply sustained long-term benefits after re-emission, whereas actual avoided warming returns to zero once stored carbon is lost. - Therefore, using temporary storage tonne-years to offset fossil emissions risks a trade-off: near-term cooling at the expense of increased long-term warming if the stored carbon is later re-emitted while the fossil emissions have already occurred.

The findings directly address how to quantify the climate benefit of temporary land carbon storage in a physically meaningful way. By anchoring tonne-year accounting to the TCRE, the study shows that temporary storage yields degree-years of avoided warming and a contemporaneous temperature benefit proportional to the rate of accumulation of total tonne-years. This reframes tonne-years from an offset equivalency tool into a transparent tracking and reporting metric for nature-based removals and avoided emissions. It clarifies when temporary storage contributes to peak temperature outcomes: only if TY accumulation is sustained through the period leading up to the temperature peak driven by fossil CO₂ reductions. The results demonstrate that equivalency-based offsetting is unsuitable for representing climate effects of temporary storage, as it can mislead on both near- and long-term impacts. In contrast, the proposed framework allows organizations and policymakers to pursue and credit temporary storage for its real-time temperature benefits, manage risks of impermanence through portfolio adjustments (adding storage elsewhere if losses occur), and align NbCS with, rather than substitute for, aggressive fossil fuel emissions cuts.

The paper establishes a physically grounded method to account for the climate benefit of temporary land carbon storage using tonne-year accounting linked linearly to avoided degree-years via TCRE. It shows that: (1) temporary storage has quantifiable climate value; (2) the temperature benefit depends on sustaining the rate of TY accumulation; and (3) temporary storage should complement, not offset, fossil fuel emissions reductions. Equivalency factors that convert tonne-years into permanent storage are shown to misrepresent climate impacts and should not be used for offsetting. Instead, a tonne-year tracking framework can enable credible reporting of NbCS contributions without presuming permanence, contributing to limiting peak warming when pursued alongside rapid decarbonization. Future research could refine effective TCRE values that include biophysical land-surface effects, extend analyses across models, regions, and project types (including ocean NbCS), and explore portfolio strategies to maintain TY growth under disturbance risks.

- The simulations isolate CO₂ effects of land carbon storage and exclude associated biophysical impacts (albedo, evapotranspiration, clouds), which can amplify or offset the net climate effect regionally (e.g., afforestation in snowy regions). An effective TCRE that includes CO₂ plus biophysical effects may alter the slope of the tonne-year to degree-year relationship. - Results rely on a single Earth system model (UVic ESCM v2.10) and specific scenario designs (e.g., 3 GtCO₂ yr⁻¹ reduction starting in 2022, particular re-emission durations). Multi-model and scenario robustness was not assessed here. - Aggregated global representation of land-use emissions and storage abstracts from spatial heterogeneity, leakage, and project-level dynamics. - The analysis extends to 2150 and assumes certain SSP pathways; different socioeconomic trajectories or non-CO₂ forcing evolutions could affect timing of peak temperature and interplay with TY accumulation. - Permanent storage scenario constrained land-use emissions to not become negative, simplifying some potential real-world dynamics of large-scale removals.