Karimzadeh et al. (2025) Climate change has increased global evaporative demand except in South Asia

Identification

• Journal: Communications Earth & Environment
• Year: 2025
• Date: 2025-11-29
• Authors: Saeed Karimzadeh, Arman Ahmadi, Dennis Baldocchi, Joshua B. Fisher
• DOI: 10.1038/s43247-025-02959-x

Research Groups

• Department of Biological and Agricultural Engineering, University of California, Davis, CA, United States
• Department of Environmental Science, Policy and Management, University of California, Berkeley, CA, United States
• Schmid College of Science and Technology, Chapman University, Orange, CA, United States
• Delft Center for Systems and Control, Delft University of Technology, Delft, The Netherlands
• Department of Built Environment, Aalto University, Espoo, Finland

Short Summary

Climate change has increased global evaporative demand, but this study reveals a significant decline in South Asia due to widespread irrigation, which has increased local moisture, cloud cover, and reduced solar radiation. These contrasting trends highlight how human water use can locally reshape the climate's influence on the water cycle.

Objective

• To provide a consistent global assessment of evaporative demand and quantify trends in its key drivers (air temperature, humidity, solar radiation, wind speed, and cloud cover) over 45 years.
• To unravel the spatial and temporal dynamics of evaporative demand and attribute regional hydrological anomalies, particularly the decline in South Asia, to specific climatic and anthropogenic factors within a hierarchical causal framework.

Study Configuration

• Spatial Scale: Global, with a focus on South Asia (India, Pakistan, Bangladesh, Nepal, Sri Lanka, Bhutan, and Maldives). Data analyzed at 0.25° resolution.
• Temporal Scale: 45 years (1979–2023).

Methodology and Data

• Models used:
  • FAO-56 Penman-Monteith equation for Reference Evapotranspiration (ETo).
  • Priestley-Taylor model for Potential Evapotranspiration (PET).
  • Mann-Kendall (MK) test for trend analysis.
• Data sources:
  • ERA5 Agrometeorological Climate Reanalysis (ECMWF) for time-series data (1979–2023) of air temperatures (maximum, minimum, mean, dew point), surface temperature, solar radiation, net shortwave radiation, net longwave radiation, precipitation, soil moisture, cloud cover, and wind speed at 0.1° spatial resolution.
  • Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) for elevation data.
  • Global historical Area Equipped for Irrigation (AEI) dataset (1900–2015) at 0.0833° spatial resolution.

Main Results

• Global evaporative demand significantly increased from 1979 to 2023, with an average rise of +0.33 mm yr⁻² for PET and +0.80 mm yr⁻² for ETo. Approximately 62% and 52% of terrestrial regions showed significant increasing trends in ETo and PET, respectively.
• South Asia exhibited a contrasting pattern, with a statistically significant decrease in ETo by –2.10 mm yr⁻² (72% of areas) and no significant trend in PET (71% of areas).
• Globally, air temperature increased by +0.016 °C yr⁻¹, and vapor pressure deficit (VPD) showed a significant increasing trend in 60% of global terrestrial areas (excluding South Asia).
• In South Asia, VPD significantly declined in 80% of areas, indicating increasing atmospheric humidity. Wind speed also showed a significant decreasing trend in 71% of the region.
• Net shortwave radiation significantly decreased in 69% of South Asia, corresponding to a steep decline in incoming solar radiation of –0.012 MJ m⁻² yr⁻² (74% of the region). This was primarily attributed to a significant increase in cloud cover (51% of the area).
• Surface temperatures in South Asia remained relatively stable (21.7 °C in 1979 to 21.6 °C in 2023), unlike the global average (excluding South Asia) which increased by +0.015 °C yr⁻² (5.4 °C in 1979 to 6.2 °C in 2023).
• The observed anomalies in South Asia are primarily driven by extensive irrigation expansion, which has led to increased soil moisture (despite no significant change in precipitation) and atmospheric humidity, enhanced cloud formation, and reduced incoming solar radiation and wind speed.

Contributions

• Provides the first consistent global assessment of evaporative demand trends and its drivers over 45 years (1979-2023), moving beyond traditional pan evaporation studies.
• Develops and applies a novel hierarchical causal framework to systematically decompose the energy and atmospheric drivers of evaporative demand, including the interplay between anthropogenic and climatic factors.
• Identifies and explains the unique "South Asia paradox" where evaporative demand has declined, contrasting the global increasing trend, and attributes this anomaly to the cascading effects of massive irrigation expansion.
• Highlights the dual role of irrigation as both a climate adaptation measure and a driver of climatic anomalies, emphasizing the need for multi-variable frameworks in future projections of drought, precipitation extremes, and irrigation demand.

Funding

• NASA’s ECOSTRESS Science and Applications Team (ESAT) (80NSSC23K0309)
• NSF Division of Earth Sciences (2012893) through CUAHSI
• USGS John Wesley Powell Center for Analysis and Synthesis

Citation

@article{Karimzadeh2025Climate,
  author = {Karimzadeh, Saeed and Ahmadi, Arman and Baldocchi, Dennis and Fisher, Joshua B.},
  title = {Climate change has increased global evaporative demand except in South Asia},
  journal = {Communications Earth & Environment},
  year = {2025},
  doi = {10.1038/s43247-025-02959-x},
  url = {https://doi.org/10.1038/s43247-025-02959-x}
}