Adaptive emission reduction approach to reach the Paris Agreement temperature targets

The parties of the Paris Agreement agreed to keep global warming well below 2°C and assess the necessary greenhouse gas emissions reductions every five years during the global stocktake. Globally, the necessary reductions in greenhouse gases are often derived using the remaining emissions budget concept. However, estimations of this budget vary by a factor of two to three and may hamper efforts to establish ambitions emissions reductions. Here, we propose an adaptive approach that side-step these uncertainties to quantify these global emissions reductions during the successive global stocktake solely based on regularly updated observations of past temperatures, radiative forcing, and emissions statistics. The approach consists of three main steps repeated every five years: (1) determining the anthropogenic warming to date and hence the remaining warming allowed, (2) estimating the remaining CO₂ forcing equivalent (CO₂-fe) emission budget, and (3) proposing a CO₂-fe or CO₂ emission trajectory for the next 5 years. We test this approach using the Bern3D-LPX Earth System Model of Intermediate Complexity and demonstrate that the temperature targets 1.5°C and 2°C can be reached following a smooth emissions pathway. The adaptive nature makes the approach robust against inherent uncertainties in the observational records, climate sensitivity to emissions, and effectiveness of emissions reduction implementations. The approach thus allows developing an emissions trajectory that would iteratively adapt to ultimately meet the agreed temperature goal. The approach also provides a strong alternative to the often-used pre-defined emissions or concentration pathways (such as SSPs), which can result in very different end-of-century temperatures for the same emission or concentration trajectories. Some of these pathways are developed to be consistent with a given warming level (e.g., SSP1-1.9 for 1.5°C), not knowing the actual response of the Earth system to emissions. As opposed to these simulations, simulations from different models using the adaptive approach we propose here would be directly comparable in terms of warming and broader climate impacts but would differ in terms of required emissions. Our approach would hence guide a valuable and highly policy-relevant complementary set of simulations for the next generation of CMIP models resulting in a range of future emission trajectories compatible with a given global warming target.