Updated IPCC emissions scenarios no longer limit warming to 1.5°C

Ongoing failure to reduce anthropogenic greenhouse gas emissions rates has fuelled debate within scientific, policy and public discourses on whether the 1.5°C high-ambition Paris Agreement goal remains within reach. The Working Group III (WG3) contribution of the Sixth Assessment of the Intergovernmental Panel on Climate Change (IPCC) report provided global mean temperature projections from 1202 integrated assessment model derived emissions pathways. Of these, 97 were deemed to be consistent with the 1.5°C Paris goal, interpreted as limiting warming to 1.5°C with no or limited overshoot. Of these 87 temporarily overshoot 1.5°C and 10 scenarios remained below 1.5°C throughout the 21st century. 

However, the IPCC mitigation scenarios are rapidly becoming out of date, as most scenarios depend on rapid greenhouse gas emissions reductions after 2020 which have not occurred in reality. Furthermore, scenario warming outcomes were assessed using simple climate models calibrated in the 2010s, excluding recent observations and advances in understanding. When IPCC emissions scenarios are reharmonized to take into account recent emissions, and simple climate model calibrations are updated to incorporate recent observational constraints, no scenario in the IPCC WG3 database avoids overshooting 1.5°C, and only a handful of scenarios remain consistent with the IPCC definition of a low overshoot. This implies that the window for limiting warming to 1.5°C without overshoot has now closed.