Gačnik et al. (2025) Climate change reflected in 40-year isotopic composition trends of precipitation in Slovenia

Identification

• Journal: Environmental Research
• Year: 2025
• Date: 2025-11-06
• Authors: Jan Gačnik, Klara Žagar, István Gábor Hatvani, Zoltán Kern, Polona Vreča
• DOI: 10.1016/j.envres.2025.123286

Research Groups

• Department of Environmental Sciences, Jožef Stefan Institute, Ljubljana, Slovenia
• Institute for Geological and Geochemical Research, HUN-REN Research Centre for Astronomy and Earth Sciences, Budapest, Hungary
• HUN-REN CSFK, MTA Centre of Excellence, Budapest, Hungary

Short Summary

This study analyzes 40-year stable isotope records of precipitation (δ18O, δ2H, d-excess) from Ljubljana, Slovenia, revealing significant increasing trends consistent with regional warming. The findings indicate an accelerating, exponential-like isotopic response to warming that has propagated into the water cycle, with implications for water resource management.

Objective

• Identify nonlinear and nonstationary processes embedded in isotopic records using time series decomposition methods.
• Test the hypothesis that seasonal and decadal-scale oscillatory components of long-term stable isotope records from Ljubljana are driven by distinct hydroclimatic processes.
• Evaluate the impact of changes in air mass origin, particularly the contribution of Mediterranean-sourced moisture, on isotopic variability.
• Assess the regional relevance of the findings by comparing results with isotope records from a coastal site (Portorož, Slovenia) and a continental site (Vienna, Austria).

Study Configuration

• Spatial Scale: Three stations in Central and Southeastern Europe: Ljubljana (Slovenia), Portorož (Slovenia), and Vienna (Austria).
• Temporal Scale: Multi-decadal records: Ljubljana (May 1981–December 2024, 43 years), Portorož (October 2000–December 2024, 24 years), Vienna (January 1973–December 2023, 51 years).

Methodology and Data

• Models used: Sen’s slope test, Mann-Kendall nonparametric test, Kruskal-Wallis test, pairwise Mann-Whitney U tests, Ensemble Empirical Mode Decomposition (EEMD), Hilbert-Huang Transform (HHT), Fourier transform, major axis regression (MA), reduced major axis regression (RMA), precipitation weighted MA (PWMA), precipitation weighted RMA (PWRMA), Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) analysis.
• Data sources: Monthly composite precipitation stable isotope records (δ18O, δ2H, d-excess) from the Slovenian Network of Isotopes in Precipitation (SLONIP), Jožef Stefan Institute archive, Global Network for Isotopes in Precipitation (GNIP), and the Austrian Network of Isotopes in Precipitation (ANIP). Monthly precipitation and mean monthly air temperature from the Slovenian National Meteorological Network (Slovenian Environmental Agency) and GeoSphere Austria. Meteorological data for HYSPLIT from National Center for Environmental Prediction (NCEP) - National Center for Atmospheric Research (NCAR) reanalysis data (NCEP-NCAR reanalysis I). North Atlantic Oscillation (NAO) index from NOAA Climate Prediction Centre. Mediterranean Oscillation (MO) index based on pressure difference between Algiers and Cairo.

Main Results

• Ljubljana's δ18O, δ2H, and d-excess values show significant increasing trends: +0.02 ‰ per year for δ18O, +0.18 ‰ per year for δ2H, and +0.05 ‰ per year for d-excess.
• The yearly average air temperature in Ljubljana shows a significant increasing trend of +0.069 °C per year.
• Seasonal patterns are evident, with elevated d-excess values during autumn in Ljubljana, Portorož, and Vienna.
• Hilbert-Huang Transform revealed statistically significant annual oscillations in δ18O, δ2H, and d-excess, strongly correlated with the annual temperature oscillation, with a 2–3 month lag identified for d-excess.
• Lower-frequency (multi-year) oscillations of d-excess (2.7-year and 7.5-year) were observed, with increasing significance from coastal (Portorož) to continental (Vienna) stations.
• HYSPLIT back-trajectory analysis confirmed a significant contribution of Mediterranean-sourced air masses to autumn precipitation but did not explain the observed long-term increase in d-excess values.
• The long-term increase in δ18O (1.2 ‰) and δ2H (13.7 ‰) over 43 years (1981–2024) in Ljubljana is substantial, and the trend resembles exponential rather than linear growth, with the strongest increases occurring in the last two decades.
• A weak correlation was found between NAO IMF 4 (2.7-year oscillation) and d-excess IMF 4 (2.6-year oscillation) for both Ljubljana (r = 0.25) and Vienna (r = 0.21).

Contributions

• Presents one of the longest multi-decadal precipitation stable isotope records (1981–2024) in Central and Southeastern Europe, addressing a significant gap in global assessments of long-term isotope trends.
• Applies advanced time series decomposition methods (EEMD, HHT) to precipitation stable isotope records, effectively identifying nonlinear and nonstationary processes and separating short-term fluctuations, seasonal patterns, and long-term trends.
• Provides robust isotope evidence of an accelerating, exponential-like isotopic response to warming that has already propagated into the water cycle, implying changes in precipitation-surface water-groundwater interactions.
• Demonstrates the regional relevance of the findings by comparing results across continental and coastal sites in Slovenia and Austria.
• Identifies a consistent 2–3 month lag between annual temperature and d-excess cycles and attributes the link between precipitation isotopes and the North Atlantic Oscillation (NAO) to a specific 2–3 year oscillatory component.

Funding

• Slovenian Research and Innovation Agency (ARIS; research programs P0-0531, P1-0143, and bilateral research project N1-0054)
• International Atomic Energy Agency (CRP F31002, CRP F33024, CRP F31006, RER7013, RER7017, NC SLO7001)

Citation

@article{Gačnik2025Climate,
  author = {Gačnik, Jan and Žagar, Klara and Hatvani, István Gábor and Kern, Zoltán and Vreča, Polona},
  title = {Climate change reflected in 40-year isotopic composition trends of precipitation in Slovenia},
  journal = {Environmental Research},
  year = {2025},
  doi = {10.1016/j.envres.2025.123286},
  url = {https://doi.org/10.1016/j.envres.2025.123286}
}