An early warning of the risks of crossing global warming thresholds

Abstract The question of if or when the 1.5°C Paris Agreement threshold is crossed is of great public and scientific interest. A single year above 1.5°C does not necessarily imply that the Paris Agreement threshold is reached but may provide warning of crossing 1.5°C. We find that, under current emissions policies, annual temperatures exceeding 1.5°C in three individual years suggests that it is very likely (90-100% chance) that human-induced global warming has reached 1.5°C. A single year above 1.5°C could occur up to 12 years (very likely range) before the Paris Agreement threshold is reached. A single year above 1.4°C, as occurred in 2023, suggests a crossing of the Paris Agreement threshold within 7-14 years (likely range) under current policies. The observed global temperature change in 2023 is an urgent warning of the need for stringent emissions reductions in this critical decade to keep the Paris Agreement 1.5°C threshold within reach.