Nature-based climate solutions (NbCS) offer potential for climate mitigation by enhancing carbon sequestration through natural land management. Strategies include avoided deforestation, reforestation, and enhanced soil carbon sequestration. However, NbCS have faced criticism regarding their effectiveness, focus on carbon flows over stocks, limited consideration of non-carbon effects, and prioritization of carbon over other environmental values. Despite this, well-implemented NbCS can generate measurable carbon gains and other benefits. A key challenge is the longevity of stored carbon in ecosystems vulnerable to disturbances, particularly for strategies focused on above-ground biomass. Climate change increases the risk of wildfires, droughts, and insect outbreaks, potentially decreasing forest carbon stock turnover time and increasing carbon loss. Anthropogenic activities, like delayed (rather than permanently avoided) deforestation, can also lead to temporary carbon storage. While at large scales natural disturbances are generally balanced by regrowth, the longevity of individual carbon units is uncertain, especially with increasing disturbance frequency. Corporate and sub-national carbon accounting often includes natural carbon storage, and many corporations are using nature-based carbon removal to achieve net-zero goals. The impermanence of land-based carbon storage is problematic, especially if it's considered an alternative to fossil fuel emission reductions. The indefinite climate effect of fossil fuel CO2 emissions contrasts with temporary land carbon storage, making direct substitution problematic. This leads to increased total emissions and warming over time. The challenge of accounting for temporary storage in relation to fossil fuel emissions has prompted the use of tonne-year accounting.

Tonne-year accounting, defining one tonne-year as one tonne of carbon stored for one year, measures time-integrated carbon storage in temporary stocks. It's also used to estimate atmospheric CO2 response to emissions. To quantify temporary storage value, it calculates a cost-benefit ratio to compare emission costs to the benefit of delayed emission. This leads to proposed equivalency factors representing how many tonne-years of temporary storage equal one tonne of permanent storage. These factors range from 30 to 130 and are used in carbon offsets. However, this approach has been criticized for using subjective economic discount rates, arbitrary time horizons, and weak grounding in the actual climate response. Previous analyses focused on atmospheric CO2 response without quantifying the temperature response or considering short- and long-term differences, leading to proposed equivalency factors with little bearing on the climate consequence of temporary versus permanent storage. As a result, tonne-years haven't been widely adopted as a metric for temporary carbon storage from NbCS.

This study offers a revised approach to tonne-year accounting, proposing its use as a physical metric of carbon storage over time, rather than an economic metric of equivalency, to estimate the climate response to nature-based carbon storage. The University of Victoria Earth System Climate Model (UVic ESCM), version 2.10, an intermediate-complexity global climate model, was used. Simulations represented avoided land-based CO2 emissions/removals as an annual decrease in emissions relative to a baseline scenario. Emissions were decreased beginning in 2022 by 3 GtCO2 per year, reflecting the global cost-effective mitigation potential of actions such as avoided deforestation and reforestation. Permanent storage simulations maintained this decrease until 2050 or the end of the simulation. Temporary storage simulations followed the same initial decrease until 2037 or 2050, after which emissions increased such that all previously stored carbon was re-emitted over 15 or 50 years. Tonne-years of carbon storage were represented as a running total of the amount of stored carbon multiplied by the storage time, calculated as the time-integral of the cumulative emissions difference from the baseline scenario. The climate response to tonne-years of temporary carbon storage was assessed by comparing global temperatures in different scenarios against baseline temperatures under the SSP1-1.9 and SSP1-2.6 mitigation scenarios. The relationship between tonne-years of land carbon storage and the simulated temperature difference was analyzed. The proportionality of cumulative emissions to global temperature change was represented by the Transient Climate Response to cumulative CO2 Emissions (TCRE), applied to carbon removal and storage. The time-integrated temperature difference was defined as "degree-years" (DY) of avoided warming. The relationship between tonne-years and degree-years was analyzed, and the equations were used to understand the relationship between the rate of increase of tonne-years and the temperature benefit. The simulations considered only the CO2 effect of land carbon storage, excluding biophysical effects. The study also compared the actual avoided warming from temporary storage with that inferred using tonne-year equivalency factors from previous studies.

Simulations showed that regardless of whether carbon storage was temporary or permanent, the total number of tonne-years initially increased with time. During the period that land carbon emissions were lower than the baseline simulation, tonne-years accumulated at an increasing rate. For the permanent storage scenarios, tonne-years continued to accumulate at a constant rate. For the temporary storage scenarios, when stored carbon was re-emitted, tonne-year accumulation slowed. When all stored carbon was re-emitted, tonne-years stopped accumulating. Decreased land CO2 emissions lowered global temperatures proportionately to the cumulative emissions difference over time. For temporary storage scenarios, temperature decrease reached a maximum and then returned to baseline levels following re-emission. For permanent storage, temperatures continued to decrease as long as emissions remained below baseline. A clear relationship was identified between tonne-years of land carbon storage and the simulated temperature difference. A constant rate of tonne-year accumulation resulted in a stable temperature difference, while a decreasing rate resulted in a decreasing difference. The potential for temporary land carbon storage to lower peak warming was tracked as a function of the rate of accumulation of total tonne-years. A linear or faster-than-linear increase in tonne-years until net-zero fossil fuel CO2 emissions lowered the peak temperature. A slower rate, decreasing to zero before net-zero emissions, had no effect on peak warming. Tonne-years were found to be proportional to degree-years of avoided warming, with the proportionality constant equal to the TCRE. Using equivalency factors to infer the climate benefit of temporary carbon storage produced a time series of presumed temperature benefit that bore almost no resemblance to the actual avoided warming. Using equivalency factors resulted in an underestimation of near-term climate benefit and an incorrect suggestion of increasing and sustained climate benefit after carbon re-release.

The findings demonstrate that tonne-years of temporary land carbon storage have a well-defined climate effect, stemming from the proportionality between avoided cumulative emissions and avoided temperature increase. The climate effect can be quantified using the TCRE as an equivalent number of avoided degree-years. The amount of avoided warming is related to the rate of increase of tonne-years. A constant or increasing rate sustains or increases the temperature difference, while a decreasing rate erodes it. Temporary tonne-years affect peak temperature only if their accumulation rate is sustained until temperatures peak and decline due to fossil fuel CO2 emission reductions. Using a single equivalency factor to relate temporary to permanent storage inadequately captures the climate response. Offsetting fossil fuel emissions with tonne-years of temporary storage slows near-term warming but could lead to increased long-term warming if the stored carbon is lost. Tonne-year equivalency factors also underestimate near-term climate benefits. The study suggests that temporary carbon storage has climate value, distinct from that of permanent storage or avoided fossil fuel CO2 emissions.

Tonne-year accounting can effectively track total carbon storage over time and infer its near-term climate benefit. The climate benefit can be quantified without prior knowledge of carbon longevity. This addresses a key barrier in establishing the climate value of nature-based carbon storage. The framework could be applied to any NbCS aimed at increasing carbon storage in non-permanent reservoirs. A sustained global increase in tonne-years would result in a sustained climate benefit, complementing fossil fuel emission reductions and affecting peak temperature change. Losses of stored carbon can be compensated for by adding additional stored carbon elsewhere, as long as the total number of tonne-years continues to increase. At the corporate level, investment in carbon storage would result in a sustained temperature benefit, maintainable by further investments even if carbon is lost at one location. Reimagined as proposed, tonne-year accounting provides a critical contribution to climate mitigation, not dependent on carbon permanence. Accumulating tonne-years of temporary carbon storage would represent a real contribution that requires no subjective time-horizon choices or permanence guarantees. Accumulated degree-years would contribute to reducing slow-responding impacts like sea-level rise. A sustained global increase in tonne-years would result in a sustained temperature benefit.

The simulations only represented the CO2 effect of land carbon storage, excluding biophysical effects on surface albedo, evapotranspiration, or cloud cover. While previous analyses show that these effects scale proportionately with land-use cumulative CO2 emissions, including them would alter the slope of the tonne-year:degree-year relationship. An "effective TCRE" reflecting the net CO2 + biophysical effect on climate could be used for more accurate estimations, especially for afforestation projects in areas with seasonal snow cover.