Nature-based climate solutions can contribute to climate mitigation, but the vulnerability of land carbon to disturbances means that efforts to slow or reverse land carbon loss could result in only temporary storage. The challenge of accounting for temporary storage is a key barrier to the implementation of nature-based climate mitigation strategies. This paper offers a solution using tonne-year accounting, integrating the amount of carbon over its storage time. It shows that tonne-years of carbon storage are proportional to degree-years of avoided warming, and that a physically based tonne-year accounting metric could effectively quantify and track the climate benefit of temporary carbon storage. Sustaining increasing tonne-years alongside fossil fuel CO2 emissions reductions would lower the global temperature peak.

H. Damon Matthews, Kirsten Zickfeld, Alexander Koch, Amy Luers