A 2025 study warns that 12 months at 1.5°C may signal an earlier breach of the Paris Agreement threshold, urging urgent climate action.
In June 2024, global mean surface temperatures reached 1.5°C above pre-industrial levels for 12 consecutive months, marking the first time in recorded history this threshold was sustained for an entire year.
This study investigates whether this unprecedented temperature streak signals an earlier-than-expected breach of the Paris Agreement's long-term warming limit. By analyzing historical observations and Coupled Model Intercomparison Project Phase 6 (CMIP6) climate model simulations, the research provides critical insights into the implications of temporary temperature spikes and long-term climate targets.
Key Findings: Temporary Spike or Permanent Breach?
Twelve Consecutive Months at 1.5°C
June 2024 marked the twelfth month in a row with global temperatures at least 1.5°C above pre-industrial levels, as recorded by the Copernicus Climate Change Service (C3S) and Berkeley Earth Surface Temperature (BEST) datasets.
This occurrence was driven by the combined influence of anthropogenic warming and a strong El Niño event, peaking with an Oceanic Niño Index (ONI) of 2°C.
Implications for the Paris Agreement
The Paris Agreement aims to limit long-term warming to 1.5°C above pre-industrial levels, defined by a 20-year running mean.
The study finds that 12 consecutive months above 1.5°C typically occur after the long-term 1.5°C threshold is crossed in climate models.
76% of CMIP6 simulations suggest that the Paris target was already breached by the time this streak occurred, indicating a high probability of long-term exceedance.
Timing of Threshold Crossing
In the CMIP6 ensemble, the median time between the first 12-month streak at 1.5°C and the crossing of the long-term threshold is -33 months (i.e., the long-term breach occurs about 2.75 years earlier).
If 1.5°C anomalies continue beyond 18 consecutive months, the study projects a near-certainty of long-term threshold breach under current emission scenarios (SSP 2-4.5 or SSP 5-8.5).
Explaining the Temperature Surge
Internal Variability and El Niño Influence
The 2023–2024 El Niño event was unusually strong, contributing approximately 0.07°C to the temperature anomaly.
97% of climate model members show positive ONI anomalies during the first occurrence of a year above 1.5°C, highlighting the significant role of El Niño–Southern Oscillation (ENSO) variability.
Potential Missing Drivers
The study suggests two possible external forcings that might explain why observed temperatures exceeded model projections:
2022 Hunga Tonga–Hunga Ha’apai Eruption: Injected large amounts of water vapor into the stratosphere, increasing radiative forcing.
2020 Shipping Regulations: Reduced aerosol emissions, decreasing global albedo and amplifying warming.
These factors, not fully accounted for in CMIP6 simulations, could indicate that the Earth system is more sensitive to radiative forcing than previously estimated.
Model Calibration and Reliability
Weighted CMIP6 Ensemble
The study uses a calibrated CMIP6 ensemble, weighting models by their consistency with observed climate sensitivity distributions to reduce biases from overly warm models.
This approach improves predictive reliability, particularly for short-term temperature excursions linked to ENSO events.
CESM2 Large Ensemble Analysis
A 100-member ensemble from the Community Earth System Model v2 (CESM2), run under SSP 5-8.5, confirms that strong El Niño events significantly reduce the time between short-term excursions and long-term threshold exceedance.
Even after accounting for El Niño, the probability of long-term 1.5°C exceedance remains above 56% in all scenarios.
Policy Implications
Redefining Threshold Exceedance
The findings challenge the Paris Agreement’s interpretation of temperature targets, suggesting that temporary spikes are more than just anomalies—they may be early indicators of permanent climate regime shifts.
The study calls for updating global warming metrics to integrate short-term temperature excursions as warning signals for long-term exceedance.
Urgency for Climate Action
If the 1.5°C threshold is indeed breached, achieving the Paris goals would require immediate and deep emission reductions, rapid deployment of carbon dioxide removal (CDR) technologies, and enhanced adaptation strategies.
The research also highlights the need for improved climate models that incorporate emerging external forcings and feedback mechanisms.
Conclusion: A New Reality for Climate Targets
This study provides a critical reassessment of the Paris Agreement's 1.5°C target, indicating that the long-term threshold may have been crossed earlier than expected. By linking short-term temperature anomalies to long-term climate trajectories, the findings underscore the urgency of global climate action. The 2024 temperature surge is a wake-up call: temporary spikes can be harbingers of permanent change, necessitating a paradigm shift in how we define and respond to global warming limits.
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