St-Pierre et al. (2026) Emergence time of CO2-forced European summer climate trends

Identification

• Journal: Scientific Reports
• Year: 2026
• Date: 2026-03-23
• Authors: Médéric St-Pierre, Joakim Kjellsson, Wonsun Park, Leonard F. Borchert, Mojib Latif
• DOI: 10.1038/s41598-026-44761-5

Research Groups

• GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
• Christian-Albrechts-University of Kiel, Faculty of Mathematics and Natural Sciences, Kiel, Germany
• Swedish Meteorological and Hydrological Institute, Rossby Centre, SMHI, Sweden
• Center for Climate Physics, Institute for Basic Science, Busan, South Korea
• Department of Integrated Climate System Science, Pusan National University, Busan, South Korea
• Research Unit Sustainability and Climate Risk, Universität Hamburg, Hamburg, Germany

Short Summary

This study quantifies the Time of Emergence (ToE) for European summer climate trends, including near-surface temperature, soil moisture, and the hydrological cycle, using a large ensemble climate model. It reveals rapid emergence for near-surface temperature (20-70 years) but delayed or absent emergence for precipitation, while demonstrating that extreme summer climate distributions are significantly altered even when mean trends do not formally emerge from natural variability.

Objective

• To quantify the Time of Emergence (ToE) for European summer climate trends, including near-surface temperature, soil moisture, and the hydrological cycle, under a 1% annual CO2 increase, with a particular focus on Western and Central Europe.
• To analyze the statistical differences in climate distributions and extreme summers between pre-industrial and quadrupled CO2 conditions over Europe.

Study Configuration

• Spatial Scale: Europe, specifically Northern Europe (NEU), Western and Central Europe (WCE), and the Mediterranean (MED). Global scale for comparison. Atmospheric model resolution of approximately 200 km (T63), ocean model resolution of 0.5°.
• Temporal Scale: 140-year simulations with 1% annual CO2 increase from pre-industrial levels. A 5000-year pre-industrial control (piCTRL) run. Analysis focuses on Northern Hemisphere summer months (June, July, August - JJA) and autumn months (September, October, November - SON) for soil moisture.

Methodology and Data

• Models used: Kiel Climate Model (KCM), which couples the Hamburg atmospheric general circulation model version 5 (ECHAM5), the Nucleus for European Modelling of the Ocean (NEMO) ocean-sea ice general circulation model with the Louvain-la-Neuve Ice Model version 2 (LIM2) sea ice model, and the Ocean Atmosphere Sea Ice Soil version 3 (OASIS3) coupler.
• Data sources: A Single Model Initial Conditions Large Ensemble (SMILE) of 100 simulations performed with the KCM, forced by a 1% per year atmospheric CO2 increase. A 5000-year pre-industrial control (piCTRL) run from the KCM.

Main Results

• Near-surface temperature (T2m) trends emerge relatively quickly across Europe, with ToEs ranging from 20 to 40 years in the Mediterranean (MED) region and 50 to 70 years in Northern Europe (NEU).
• Precipitation and runoff trends generally do not emerge from natural variability within the 140-year simulation period across most of Europe, even when CO2 levels quadruple.
• Evaporation trends emerge over large areas of MED and western Western and Central Europe (WCE) after approximately 70 years (when CO2 doubles).
• Soil moisture trends emerge after approximately 30 years in parts of the MED region, and after about 70 years in western WCE.
• Despite many CO2-forced climate trends not formally emerging, a statistical comparison of 5000 pre-industrial and 3000 4xCO2 summers reveals statistically significant differences (p-value < 0.05) in the distributions of all analyzed variables.
• Distributions in the 4xCO2 climate generally broaden, indicating increased variability and a tendency towards more extreme JJA values. For instance, what is considered an extreme warm summer in pre-industrial WCE and NEU would be an extreme cold summer in the 4xCO2 climate.
• Extreme dry summers in WCE become more intense in a 4xCO2 climate, with anomalies shifting northward and eastward, particularly in the northeastern part of the region.

Contributions

• Provides a comprehensive and unique Time of Emergence (ToE) study for the complete European hydrological cycle, soil moisture, and near-surface temperature using a large ensemble climate model.
• Focuses on Western and Central Europe, a region that has received less attention in ToE studies compared to the Mediterranean.
• Demonstrates that even when mean climate trends do not formally emerge from internal variability (according to the signal-to-noise method), the underlying climate distributions and extreme events are significantly altered in a warmer, high-CO2 climate.
• Highlights the importance of analyzing changes in climate distributions and extreme events, beyond just the ToE of mean trends, for understanding future climate impacts.

Funding

• GEOMAR Helmholtz Centre for Ocean Research Kiel
• ROADMAP project (JPI Oceans/Climate grant no 01LP2002C)
• Institute for Basic Science (IBS), Republic of Korea (IBS-R028-D1)
• Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy—EXC 2037 ’CLICCS—Climate, Climatic Change, and Society’—Project Number: 390683824

Citation

@article{StPierre2026Emergence,
  author = {St-Pierre, Médéric and Kjellsson, Joakim and Park, Wonsun and Borchert, Leonard F. and Latif, Mojib},
  title = {Emergence time of CO2-forced European summer climate trends},
  journal = {Scientific Reports},
  year = {2026},
  doi = {10.1038/s41598-026-44761-5},
  url = {https://doi.org/10.1038/s41598-026-44761-5}
}