A year above 1.5 °C signals Earth is most probably within the 20-year period that reaches the Paris Agreement limit

The temperature goals of the Paris Agreement are measured as 20-year averages exceeding a pre-industrial baseline. A first single calendar year above 1.5 °C relative to pre-industrial levels is imminent and may have already occurred in 2024, but the implications for the corresponding temperature goal are unclear. Here, we show that, without very stringent climate mitigation, the first year above 1.5 °C occurs within the first 20-year period with an average warming of 1.5 °C. This is due to the ongoing strong anthropogenic multi-decadal warming trend that renders it very unlikely for the temperature of a single year to exceed the average temperature over the coming decades. The results provide an early warning that signals the onset of a period where the climate impacts of a 1.5 °C warmer world will start to emerge, underscoring the urgency of adaptation action. Yet, our findings also indicate that, by rapidly slowing down the warming rate, very stringent near-term mitigation may substantially reduce risks of exceeding the 1.5 °C global warming level soon after the first single year above 1.5 °C has occurred.