Reimagining tonne-year accounting to capture the climate benefit of temporary carbon storage

Nature-based climate solutions have the potential to contribute to climate mitigation, but the vulnerability of land carbon to disturbances means that any efforts to slow or reverse land carbon losses will likely result in only temporary storage. Tonne-year accounting, which integrates the amount of carbon over the time that it remains stored, has been proposed as a method to measure the value of temporary relative to permanent carbon storage. Here we show that using tonne-year accounting to establish an equivalency of temporary with permanent storage is not grounded in climate science. However, a reimagined approach to tonne-year accounting could be effectively used as a metric to track the contribution of temporary carbon storage towards climate mitigation goals. We show that in a scenario where the total number of tonne-years accumulates at a constant or increasing rate, this results in a sustained or increasing climate benefit. However, as the rate of accumulation of tonne-years slows, this temperature benefit is eroded, and is lost completely when tonne-years stop increasing. This implies that if the world is able to sustain an increasing number of tonne-years alongside achieving net-zero fossil fuel CO2 emissions, then the resulting carbon storage (even if only temporary) would contribute to lowering peak temperature. Tonne-years of temporary carbon storage therefore have considerable climate value, not if treated interchangeably with permanent storage or fossil fuel emissions, but rather if quantified as an independent contribution to lowering the global temperature peak.