Twelve months at 1.5 °C signals earlier than expected breach of Paris Agreement threshold

nature climate change Brief Communication https://doi.org/10.1038/s41558-025-02247-8 Twelve months at 1.5 °C signals earlier than expected breach of Paris Agreement threshold Received: 11 September 2024 Alex J. Cannon Accepted: 14 January 2025 Published online: xx xx xxxx Check for updates June 2024 was the twelfth month in a row with global mean surface temperatures at least 1.5 °C above pre-industrial conditions, but it is not clear if this implies a failure to meet the Paris Agreement goal of limiting long-term warming below this threshold. Here we show that in climate model simulations, the long-term Paris Agreement target is usually crossed well before such a string of unusually warm temperatures occurs. According to the Copernicus Climate Change Service (C3S)1 and the Berkeley Earth temperature update2, June 2024 was the first time in the instrumental record that global mean surface temperatures reached 1.5 °C above the pre-industrial period for 12 consecutive months. The 1.5 °C Paris Agreement threshold was exceeded in multiple data products, including the ERA5 global reanalysis3 and Berkeley Earth Surface Temperature record (BEST)4, before ERA5 data showed temperatures dipping below 1.5 °C in July5 (although they remained above 1.5 °C in BEST6). As noted in the June 2024 Berkeley Earth temperature update, although the parties of the United Nations Framework Convention on Climate Change have “set a goal to limit global warming to no more than 1.5 °C above the pre-industrial, it must be noted that this goal refers to the long-term average temperature. A few months, or a couple years, warmer than 1.5 °C does not automatically mean that the goal has been exceeded”2. When will we know that the world has passed this threshold in the long term? Does one year warmer than 1.5 °C signal that the Paris Agreement target has been crossed? How to best reconcile information about temporary excursions above 1.5 °C and long-term threshold exceedance, typically defined on the basis of a centred 20-year running mean of global surface temperature, is an open problem; the latter, by definition, requires information about the future. One proposal is to combine historical observations of the past decade with climate model predictions for the next decade to estimate the current long-term mean global surface temperature7. The utility of approaches that merge observations and simulations depends on the accuracy of the initial state and external forcings being prescribed in the models. If there are possible exogenous changes in drivers that are not included in the model simulations, there is a danger of biased estimates of time-of-exceedance. At present, predictions differ slightly as a result of choice of historical observations and future forcings scenario, but estimates typically indicate that threshold crossing will occur sometime in the late 2020s or early 2030s. However, recent warming has sparked debate about whether the world might exceed the 1.5 °C Paris Agreement limit earlier than previously estimated8. The fact that the world has experienced 12 consecutive months at or above 1.5 °C provides an important additional data point about the current state of long-term warming. Here, to make use of this information, Coupled Model Intercomparison Project Phase 6 (CMIP6) climate model projections9 are conditioned on the amount of time the 1.5 °C threshold has been reached in observations: the first time global temperatures reach 1.5 °C for 12 consecutive months in a given CMIP6 simulation10 is noted, followed by the time when the 20-year centred mean crosses 1.5 °C. The distribution of differences in these two times in the ensemble of simulations, after statistical calibration to ensure that the results can be interpreted in probabilistic terms, provides an estimate of the likelihood that long-term exceedance has already occurred. As described in the Methods, climate models are calibrated by weighting according to their consistency with the assessed distribution of the climate sensitivity11–14 of the Earth (for example, to reduce biases, primarily due to over-representation of ‘hot models’)15. The relevance of the subset of CMIP6 models assessed here, as noted above, depends on whether the processes associated with 12 consecutive months at or above 1.5 °C in the models are the same as those in the real world. If they are not, then an exceedance in the real world would not necessarily be analogous to an exceedance in the models. However, one could view earlier than expected crossing of the Paris Agreement threshold relative to the timing of short-term exceedance in the models as an indication that missing drivers played a large role in the recent record-breaking warmth. Two potential candidates include changes in radiative forcings due to the Tonga eruption in 2022, which injected water vapour into the stratosphere16, and the introduction of shipping regulations in 2020, which have reduced visible ship tracks Climate Research Division, Environment and Climate Change Canada, Victoria, British Columbia, Canada. Nature Climate Change e-mail: Brief Communication a https://doi.org/10.1038/s41558-025-02247-8 c 2.0 0.8 1.0 0.5 0.015 Density 0 Exceedance probability 1.5 GSAT anomaly (°C) 1.0 1980 2000 SSP 1-2.6 SSP 2-4.5 SSP 5-8.5 0 2040 2020 0.4 0.2 0 –0.5 0.6 0 Year 6 12 18 24 30 Consecutive months b SSP 2-4.5 Density Paris Agreement 12 consecutive months 0.06 0 2010 2020 2030 2040 2050 2060 Year Fig. 1 | Observed and projected crossing of the 1.5 °C global warming threshold. a, ERA5 global temperature anomalies (≥1.5 °C in purple). The histogram shows the distribution of time between first occurrence of 12 months above 1.5 °C and long-term threshold exceedance (CMIP6 SSP 2-4.5; 26 members) superimposed over the observed occurrence date. b, Histograms of year of occurrence of Paris Agreement threshold crossing (grey) and first occurrence of 12 months above 1.5 °C (purple). In a and b vertical dashed (dotted) lines indicate the median (5th and 95th percentiles). c, Probability that the first occurrence of n consecutive months above 1.5 °C coincides with the Paris Agreement threshold already having been breached. Horizontal dotted lines show probabilities for 12 consecutive months. and albedo17. Other changes, for example rapid structural and sectoral changes in emissions following the COVID-19 pandemic, would also not be reflected in the CMIP6 forcings. Figure 1a shows observed monthly global mean surface temperature anomalies from ERA5, highlighting the recent string of ≥1.5 °C values leading up to June 2024. Calibrated projections (Fig. 1b) of the year of Paris Agreement threshold crossing under shared socioeconomic pathway SSP 2-4.5 (median 2031; 5th and 95th percentiles: 2021 and 2047) are roughly consistent with those based on assessed warming in the IPCC Sixth Assessment Report (2031; probable range of 2024 to 2043)15. However, (...truncated)