Nordling et al. (2026) First crossings of global warming levels in CMIP6 in light of observed 1.5 °C exceedance

Identification

• Journal: npj Climate and Atmospheric Science
• Year: 2026
• Date: 2026-04-03
• Authors: Kalle Nordling, Nora L. S. Fahrenbach, Antti Lipponen, Joonas Merikanto
• DOI: 10.1038/s41612-026-01401-z

Research Groups

• Finnish Meteorological Institute, Helsinki, Finland
• Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland

Short Summary

This study quantifies the timing and likelihood of first crossing global warming levels in CMIP6 models, revealing that the observed 2024 exceedance of 1.5 °C occurred 3–7 years earlier than projected. It highlights the difficulty of avoiding a pre-mid-century 2 °C crossing even with stringent mitigation, while emphasizing the critical role of immediate mitigation in avoiding higher long-term warming.

Objective

• To quantify the timing and likelihood of first crossing 1.5 °C and higher global warming levels across CMIP6 models in light of the observed 2024 1.5 °C exceedance.

Study Configuration

• Spatial Scale: Global
• Temporal Scale: Decadal to multi-century projections (analyzing crossings of warming levels from 2024 onwards under various Shared Socioeconomic Pathways).

Methodology and Data

• Models used: Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models.
• Data sources: ERA5 reanalysis data (Copernicus Climate data storage), CMIP6 data (CMIP6 Google Cloud archive).

Main Results

• The observed 2024 global temperature exceedance of 1.5 °C occurred 3–7 years earlier than projected by CMIP6 models.
• Models exhibiting higher present-day warming rates more accurately predicted the timing of the 1.5 °C crossing.
• The 2024 observed timing was likely influenced by internal climate variability superimposed on the anthropogenic warming trend.
• A pre-mid-century 2 °C crossing is difficult to avoid, with 75% of models crossing this threshold even under stringent mitigation (SSP1-2.6).
• Shifting from high (SSP5-8.5) to medium-high (SSP3-7.0) emissions delays a 3 °C crossing by 10 years.
• Low-emission pathways (SSP1-2.6) successfully avoid crossing the 3 °C threshold in 85% of models.
• While near-term 2 °C exceedance poses a significant risk, immediate mitigation remains crucial for enabling adaptation and preventing higher long-term warming.

Contributions

• Provides a timely quantification of the discrepancy between observed global warming and CMIP6 projections, particularly for the 1.5 °C threshold.
• Offers insights into the factors contributing to this discrepancy, including model warming rates and internal climate variability.
• Assesses the likelihood and timing of exceeding higher warming levels (2 °C and 3 °C) under different emission scenarios, reinforcing the urgency and impact of mitigation efforts.

Funding

• Emil Aaltonen foundation (K. Nordling)
• Academy of Finland (grant no. 337552)
• ETH Zurich Research Grant (N.L.S. Fahrenbach)
• Swiss National Science Foundation (Award PCEFP2 203376)

Citation

@article{Nordling2026First,
  author = {Nordling, Kalle and Fahrenbach, Nora L. S. and Lipponen, Antti and Merikanto, Joonas},
  title = {First crossings of global warming levels in CMIP6 in light of observed 1.5 °C exceedance},
  journal = {npj Climate and Atmospheric Science},
  year = {2026},
  doi = {10.1038/s41612-026-01401-z},
  url = {https://doi.org/10.1038/s41612-026-01401-z}
}