The role of carbon sequestration and the tonne-year approach in fulfilling the objective of climate convention

Carbon can be sequestered from the atmosphere to forests in order to lower the atmospheric carbon dioxide concentration. Tonne-years of sequestered carbon have been suggested to be used as a measure of global warming impact for these projects of finite lifetimes. It is illustrated here by simplified example cases that the objective of the stabilisation of the atmospheric greenhouse gas concentrations expressed in the UN Climate convention and the tonne-year approach can be in contradiction. Tonne-years generated by the project can indicate that carbon sequestration helps in the mitigation of climate change even when the impact of the project on the CO₂ concentration is that concentration increases. Hence, the use of the tonne-years might waste resources of fulfilling the objective of the convention. The studied example cases are closely related to the IPCC estimates on global forestation potentials by 2050. It is also illustrated that the use of bioenergy from the reforested areas to replace fossil fuels can in the long term contribute more effectively to the control of carbon dioxide concentrations than permanent sequestration of carbon to forests. However, the estimated benefits depend on the time frame considered, whether we are interested in the decadal scale of controlling of the rate of climate change or in the centennial scale of controlling or halting the climate change.