Blackport et al. (2025) Observed warming of cold extremes is not captured with a fixed threshold definition

Identification

• Journal: Communications Earth & Environment
• Year: 2025
• Date: 2025-10-08
• Authors: Russell Blackport, Michael Sigmond
• DOI: 10.1038/s43247-025-02629-y

Research Groups

• Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada, Victoria, BC, Canada

Short Summary

This paper demonstrates that the previously reported lack of warming trends in mid-latitude cold extremes by Cohen et al. (2023) is an artifact of using a fixed threshold definition. By employing a moving threshold, this study reveals clear and statistically significant warming trends in cold extremes, consistent with overall climate warming.

Objective

• To demonstrate that the lack of detectable warming trends in mid-latitude cold extremes reported by Cohen et al. (2023) is an artifact of their fixed threshold definition, and to show that a moving threshold definition reveals significant warming.

Study Configuration

• Spatial Scale: Central and Eastern United States (CEUS; 30° to 50°N, 75° to 110°W), Southern Siberia and Northern China (SSNC; 40° to 60°N, 80° to 120°E), Northern Europe (NEUR; 50° to 65°N, 0° to 45°E), and Southern Europe (SEUR; 35° to 50°N, 0° to 45°E).
• Temporal Scale: 1960–2023, 1990–2023, and 2000–2023.

Methodology and Data

• Models used: Simple statistical model representing daily winter temperature as a Gaussian distribution shifting at a constant rate over time.
• Data sources: ERA5 reanalysis (daily winter temperature data). Synthetic model data generated from the statistical model.

Main Results

• The fixed threshold definition, as used by Cohen et al. (2023), artificially limits the detectable warming of cold extremes, leading to weak and statistically insignificant trends (e.g., 0.02 °C/30 events for CEUS in ERA5).
• A simple statistical model, where the coldest days warm uniformly with the mean, shows that a fixed threshold yields an average trend of 0.03 °C/30 events, with only 17% of samples showing statistical significance, consistent with the ERA5 results using this method.
• When calculated with respect to time, the fixed threshold method yields an average trend of 0.05 °C/decade in the statistical model, nearly eight times weaker than the known underlying warming trend of 0.39 °C/decade.
• Using a moving threshold (5th percentile calculated each winter), the statistical model shows strong warming (0.29 °C/30 events or 0.39 °C/decade) that is nearly always statistically significant and consistent with the overall distribution shift.
• ERA5 reanalysis data for 1960–2023 in the CEUS region, using the moving threshold, shows a strong and highly statistically significant increase in temperature (0.34 °C/30 events or 0.46 °C/decade).
• All mid-latitude regions investigated by Cohen et al. (CEUS, SSNC, NEUR, SEUR) show statistically significant increases in temperature over 1960–2023 when using the moving threshold (e.g., CEUS: 0.34 °C/30 events; SSNC: 0.15 °C/30 events; NEUR: 0.38 °C/30 events; SEUR: 0.17 °C/30 events).
• Even over shorter periods (1990–2023, 2000–2023), the moving threshold metric reveals clearer warming trends, with statistically significant increases in temperature in half of the regions despite the expected dominance of internal variability.

Contributions

• Critically identifies and demonstrates that the choice of a fixed threshold for defining cold extremes leads to an artifactual suppression of warming trends in observational data.
• Provides a robust re-evaluation using a more appropriate "moving threshold" metric, which reveals clear and statistically significant warming of mid-latitude cold extremes, aligning with general climate warming expectations.
• Reconciles previous findings of weak or absent cold extreme trends with the broader scientific understanding of a warming climate, eliminating the need for complex physical explanations for the previously observed lack of trends.
• Emphasizes the crucial importance of methodological choices in climate extreme analysis for accurately detecting long-term changes.

Funding

Not explicitly mentioned in the paper.

Citation

@article{Blackport2025Observed,
  author = {Blackport, Russell and Sigmond, Michael},
  title = {Observed warming of cold extremes is not captured with a fixed threshold definition},
  journal = {Communications Earth & Environment},
  year = {2025},
  doi = {10.1038/s43247-025-02629-y},
  url = {https://doi.org/10.1038/s43247-025-02629-y}
}