In @NatureClimate, we show that a year above 1.5 °C signals that Earth is most probably within the 20-year period that will reach the Paris Agreement limit.

The results of both papers depend on how CMIP6 models capture all relevant climate processes over the next decade or so. See Discussion and Work @UFZ_de together with @CarlSchleussner @ZscheischlerJak.

In contrast, while our results are more conservative by indicating we are in the first part of the 20-year period (the goal has not been reached), they rely on the occurrence of a year above 1.5°C, which is supported by the average of multiple observational datasets (1.55°C).

Making their result actionable requires the occurrence of 12 consecutive months above 1.5°C, a condition met in ERA5 and BEST observational datasets, but that has not been met based on the mean of multiple observational datasets (plot for consecutive months > 1.5°C by A. Hoegner)

A note is needed to help link our results with the study by Cannon, published today. In climate models (SSP245), Cannon found a 76% chance that 12 consecutive months above 1.5°C occur after the Paris Agreement goal is reached, that is after the middle of a 20-year window at 1.5°C.

Finally, we highlight that the entry time in the 20-year period at 1.5 °C warming should not be mistaken as the timing of the warming level itself, as the latter is placed at the midpoint of the 20-year period.

Only rapid near-term mitigation can effectively limit peak warming, which is required to hold warming *well below* 2 °C in case of exceedance or overshoot of 1.5 °C. A year above 1.5 °C is not the time for despair but a call to action.

Cutting emissions has never been more important. It can lower the chance of reaching the 1.5°C limit soon after 2024, but this demands stringent mitigation. Halving the chance that 2024 will be within the first 20-yr 1.5°C period demands a fivefold reduction in temperature trends.

The IPCC AR6 flags “High Risks” at 1.5°C for unique systems (such as biodiversity) and extreme events. Floods in Brazil, Spain, and Kenya, the Amazon drought, and heatwaves in 2024 gave us a taste of these risks. We are not prepared for the climate risks at 1.5°C to unfold.

Entering a window at 1.5 °C average warming means entering the window used by scientists to study the impacts of a 1.5 °C world. Thus, our results warn we are most probably in a period where the 1.5 °C impacts are expected to unfold, underscoring the urgency of adaptations.

Our result is due to the ongoing strong anthropogenic multi-decadal warming that, combined with the relatively low variability in the temperature time series, renders it very unlikely for the temperature of a single year to exceed the average temperature over the coming decades.

That is, the first year above 1.5 °C occurs within the first 20-year period with an average warming of 1.5 °C. We found the same behaviour for other recent warming levels already reached in observations starting from the 1980s (0.6 °C to 1.0 °C; Figure 1a).

Via climate models, observations, and idealised experiments, we show that unless stringent mitigation is implemented, the first year above 1.5°C signals that it is highly probable that Earth has already entered the 20-year period that *will* reach the 1.5°C Paris Agreement limit.

However, the implications for the Paris Agreement's 1.5°C goal are unclear because the goal is understood to refer to temperature averaged over a 20-30 year period to account for natural short-term variability.

The year 2024 was announced as the first calendar year to exceed 1.5°C of global warming by several international organizations that independently track the global temperature, with a multi-dataset mean of 1.55°C.

RT @zhangyaonju: Excited to share our last paper of year 2024 published at @NatureComms. We developed a new framework and demonstrated that….

RT @lytarasova: We are looking for a new cohort of PhDs @UFZ_de The #SEESAW will look at the long-lasting #droughts and rapid transitions t….

RT @m_de_Brito: We are hiring 3 PhDs at the UFZ PhD Cohort SEESAW - Societal and environmental impacts of complex extremes in a changing wo….

We are delighted to announce the "Compound weather and climate events" session at #EGU25!. We look forward to receiving your exciting abstracts!. @Compound_Event @anaiscouasnon

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