Need for relevant timescales when crediting temporary carbon storage

Earth faces an urgent need for climate change mitigation, and carbon storage is discussed as an option. Approaches for assessing the benefit of temporary carbon storage in relation to carbon footprinting exist, but many are based on a 100-year accounting period, disregarding impacts after this time. The aim of this paper is to assess the consequences of using such approaches that disregard the long timescale on which complete removal of atmospheric CO2 occurs. Based on these findings, an assessment is made on what are relevant timescales to consider when including the value of temporary carbon storage in carbon footprinting. Implications of using a 100-year accounting period is evaluated via a literature review study of the global carbon cycle, as well as by analysing the crediting approaches that are exemplified by the PAS 2050 scheme for crediting temporary carbon storage. The global carbon cycle shows timescales of thousands of years for the transport of carbon from the atmosphere to pools beyond the near-surface layers of the Earth, from where it will not readily be re-emitted as a response to change in near-surface conditions. Compared to such timescales, the use of the 100-year accounting period appears hard to justify. We illustrate how the use of the 100-year accounting period can cause long-term global warming impacts to be hidden by short-term storage solutions that may not offer real long-term climate change mitigation. Obtaining long-term climatic benefits is considered to require storage of carbon for at least thousand years. However, it has been proposed that there may exist tipping points for the atmospheric CO2 concentration beyond which irreversible climate changes occur. To reduce the risk of passing such tipping points, fast mitigation of the rise in atmospheric greenhouse gas concentration is required and in this perspective, shorter storage times may still provide climatic benefits. Both short- and long-term perspectives should be considered when crediting temporary carbon storage, addressing both acute effects on the climate and the long-term climate change. It is however essential to distinguish between short- and long-term mitigation potential by treating them separately and avoid that short-term mitigation is used to counterbalance long-term climate change impacts from burning of fossil fuels.