📊 Warming was the main driver, but precipitation, humidity, and wind changes either enhanced or counteracted warming effects in many regions.

📄 Paper in npj Climate and Atmospheric Science www.nature.com/articles/s41... or rdcu.be/eiYcp

@natureportfolio.nature.com @compoundnet.bsky.social

Hi Glen, adding to Carl, more text related to this is in paragraphs 2-3 of our brief communication. For those interested in expanding, see also Betts' discussion: www.nature.com/articles/d41.... The same windows are also used to document impacts/effects emerging at a given warming level.

The results in both papers depend on how CMIP6 models capture relevant climate processes over the next decade or so. See Discussion and bsky.app/profile/clim...

Work @ufz.de together with @carlschleussner.bsky.social and @zscheischlerjak.bsky.social

In contrast, while our results are more conservative by indicating we are in the first part of the 20-year period and not that the goal has been breached, they rely on the occurrence of a calendar 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 strictly > 1.5°C by @hoegner.bsky.social).

An extra note is needed to help link our results with another study published by Cannon 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 midpoint 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. www.nature.com/articles/s43...

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 very stringent mitigation. For example, halving the chance that 2024 will be within the first 20-yr 1.5°C period requires 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, mega-drought in the Amazon, tropical storms, 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 same window used by scientists to project the impacts of a 1.5 °C warmer world. Thus, our results warn we are most probably in a period where the impacts of a 1.5 °C world are expected to unfold, underscoring the urgency of adaptations.

Our result is due to the ongoing strong anthropogenic multi-decadal warming trend 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 (Figure 2c).

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 in the paper).

Through climate models, observations, and idealised experiments, we show that unless stringent climate mitigation is implemented, the first year above 1.5°C in 2024 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 organisations that independently track the global temperature, with a multi-dataset mean of 1.55 °C.