Na et al. (2025) Intensifying hydroclimatic swings under a warming climate: Disentangling anthropogenic climate change and internal variability in North America

Identification

• Journal: Global and Planetary Change
• Year: 2025
• Date: 2025-11-08
• Authors: Wooyoung Na, Andrew Vincent Grgas-Svirac, Mohammad Reza Najafi
• DOI: 10.1016/j.gloplacha.2025.105171

Research Groups

• Department of Civil Engineering, Dong-A University, Busan, Republic of Korea
• Department of Civil and Environmental Engineering, Western University, London, Ontario, Canada

Short Summary

This study investigates the relative contributions of anthropogenic forcing (ACC) and internal climate variability (ICV) to hydroclimatic swing events in North America. It finds that transitions between extreme drought and flood phases will become more abrupt and intense, primarily driven by ACC, with robust trends emerging under +4.0 °C warming and becoming discernible by +2.0 or +3.0 °C warming.

Objective

• To investigate the relative influence of anthropogenic climate change (ACC) and internal climate variability (ICV) on several characteristics of hydroclimatic swing events across North America.
• To comparatively assess how ACC and ICV contribute to projected changes in hydroclimatic events during different warming periods.
• To identify the timeframes and locations where ACC significantly emerges from ICV.
• To analyze patterns in ACC, ACC-influenced trends, ICV, and their ratios concerning hydroclimatic swing characteristics over time.
• To evaluate the consistency of the projected dominance of ACC over ICV across four distinct regional climate models in North America.

Study Configuration

• Spatial Scale: North America, analyzed at a 0.5° spatial resolution, including six climate subregions (NWN, NEN, WNA, CNA, ENA, NCA).
• Temporal Scale: Historical (1950–2005/2014) and future (2006–2100) periods. Analysis focuses on 30-year warming periods corresponding to global mean surface temperature (GMST) increases of +1.5 °C, +2.0 °C, +3.0 °C, and +4.0 °C above pre-industrial levels (1850–1900). A baseline period of 1981–2010 is used for comparison.

Methodology and Data

• Models used:
  • Canadian Regional Climate Model4 Large Ensemble (CanRCM4-LE): 50 members, 0.44° resolution (bilinearly interpolated to 0.5°).
  • Canadian Large Ensembles Adjusted Dataset version 1 (CanLEAD-EWEMBI): 50 members.
  • Canadian Large Ensembles Adjusted Dataset version 1 (CanLEAD-S14FD): 50 members.
  • Seamless system for Prediction and EArth system Research-Geophysical Fluid Dynamics Laboratory (GFDL-SPEAR): 30 members, 0.5° resolution.
  • Total of 180 ensemble runs.
• Data sources: Temperature and precipitation simulations from the four single-model initial-condition large ensembles. CanLEAD models are bias-adjusted using EartH2Observe, WFDEI and ERA-Interim data Merged and Bias-corrected for ISIMIP (EWEMBI) and the S14 global meteorological forcing dataset (S14FD). The Standardized Precipitation Evapotranspiration Index (SPEI) is calculated from these data to define dry and wet spells.

Main Results

• Robust trends in most hydroclimatic swing characteristics emerge from internal climate variability (ICV)-induced fluctuations during +4.0 °C warming, becoming discernible within +2.0 or +3.0 °C warming periods.
• More abrupt and intense hydroclimatic swing events are likely influenced by strengthened anthropogenic climate change (ACC) rather than ICV, irrespective of event type (Dry-to-Wet or Wet-to-Dry), transition types, or models.
• The ICV-induced noise of hydroclimatic swing events is projected to decrease as the climate warms, while most characteristics exhibit increasing mean trends, leading to Trend-to-Noise Ratios (TNRs) greater than 1 or 2 in the +4.0 °C period.
• Transition Abruptness & Intensity (TAI), transition intensity, duration, and intensity of dry spells show the most distinct increases with rising warming levels.
• Spatial analysis identifies ACC hotspots in the Rocky Mountains (from Canada to California) and along Mexico's West and East Coastlines, extending to the southern Central U.S., indicating regions of heightened vulnerability.
• ACC signals from all four large-ensemble models generally dominate over ICV by the +4.0 °C warming period, with detectability as early as +2.0 to +3.0 °C. Model-dependent differences show low-latitude hotspots in CanLEAD projections versus high-latitude hotspots in CanRCM4-LE and GFDL-SPEAR for some characteristics.

Contributions

• First study to comprehensively disentangle the relative contributions of anthropogenic climate change (ACC) and internal climate variability (ICV) to multiple characteristics of hydroclimatic swing events, addressing a significant gap in existing literature.
• Introduces two novel characteristics for hydroclimatic swing events: Transition Abruptness & Intensity (TAI) and the relative Contribution of Preceding and Following spells (CPF), providing a more nuanced understanding of these compound extremes.
• Provides a robust framework utilizing large-ensemble simulations and low-dimensional statistics (Signal-to-Noise Ratio and Trend-to-Noise Ratio) to quantify the relative impacts of ACC and ICV.
• Identifies future spatial hotspots across North America where ACC significantly dominates ICV, highlighting areas requiring more cautious mitigation and adaptation strategies for water resource management.
• Enhances the understanding of how climate change and its variability influence the characteristics and lagged combinations of hydrologic extreme events (droughts and floods).

Funding

• Natural Sciences and Engineering Research Council (NSERC) Alliance grant.

Citation

@article{Na2025Intensifying,
  author = {Na, Wooyoung and Grgas-Svirac, Andrew Vincent and Najafi, Mohammad Reza},
  title = {Intensifying hydroclimatic swings under a warming climate: Disentangling anthropogenic climate change and internal variability in North America},
  journal = {Global and Planetary Change},
  year = {2025},
  doi = {10.1016/j.gloplacha.2025.105171},
  url = {https://doi.org/10.1016/j.gloplacha.2025.105171}
}