Geological Net Zero and the need for disaggregated accounting for carbon sinks

Achieving net zero global emissions of carbon dioxide (CO2), with declining emissions of other greenhouse gases, is widely expected to halt global warming. Warming will continue, however, until all residual CO2 emissions are balanced by active anthropogenic CO2 removals. For practical reasons, many greenhouse gas accounting systems allow some “passive” CO2 uptake, such as enhanced vegetation growth due to CO2 fertilisation, to be included as removals in the definition of net anthropogenic emissions. By including passive CO2 uptake, nominal net zero emissions would not halt global warming and thereby undermine the Paris Agreement. Here we discuss measures that address this problem by ensuring residual fossil fuel use does not cause further global warming: land management categories should be disaggregated in emissions reporting and targets to better separate the role of passive CO2 uptake; where possible, claimed removals should be additional to passive uptake; and targets should acknowledge the need for Geological Net Zero, meaning one tonne of CO2 permanently restored to the solid Earth for every tonne still generated from fossil sources. We also argue that scientific understanding of net zero provides a basis for allocating responsibility for the protection of passive carbon sinks during and after the transition to Geological Net Zero.