Professor Myles Allen CBE FRS

Abstract:

Achieving net-zero global emissions of carbon dioxide (CO₂), with declining emissions of other greenhouse gases, is widely expected to halt global warming. CO₂ emissions will continue to drive warming until fully balanced by active anthropogenic CO₂ removals. For practical reasons, however, many greenhouse gas accounting systems allow some ‘passive’ CO₂ uptake, such as enhanced vegetation growth owing to CO₂ fertilization, 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, undermining the Paris Agreement. Here we discuss measures to address this problem, to ensure 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.