Guo et al. (2026) Global urban vegetation exhibits divergent thermal effects: From cooling to warming as aridity increases

Identification

• Journal: Open Access CRIS of the University of Bern
• Year: 2026
• Date: 2026-01-02
• Authors: Zhengfei Guo, Marta Videras Rodríguez, Édouard L. Davin, Heng Huang, Bin Chen, Mohamad Hejazi, Jian Wang, Jin Wu, Yunfeng Ge, Guangqin Song, Yingyi Zhao, Kuishuang Feng, Chen Lin, Peng Gong, Yuyu Zhou
• DOI: 10.48620/93638

Research Groups

• Wyss Academy for Nature, Climate Change Scenarios (CCSN)
• Physics Institute, Climate and Environmental Physics, University of Bern
• Oeschger Centre for Climate Change Research (OCCR)

Short Summary

This study provides the first global assessment of urban vegetation's thermal regulation across 761 megacities, revealing that vegetation can transition from a cooling to a warming agent as aridity increases. In 22% of cities with low annual precipitation, the reduction in albedo and heat storage by vegetation outweighs limited evapotranspiration, leading to net urban warming.

Objective

• To quantify the global temperature regulation effects of different urban vegetation types and identify the biophysical mechanisms that cause vegetation to switch from cooling to warming in arid environments.

Study Configuration

• Spatial Scale: Global; 761 megacities across 105 countries.
• Temporal Scale: Analysis of contemporary high-resolution satellite records and climate data, including periods of extreme heat.

Methodology and Data

• Models used: Analysis of canopy conductance and biophysical energy balance (albedo, heat storage, and evapotranspiration).
• Data sources: High-resolution satellite data and global climate datasets.

Main Results

• Vegetation cooling efficiency weakens significantly in arid environments; in 22% of cities receiving less than 1000 mm of annual precipitation, vegetation (specifically grasslands and croplands) causes net warming.
• The warming effect in water-scarce regions is driven by lower surface albedo and reduced heat storage capacity, which are not compensated for by limited evapotranspiration.
• During extreme heat events, the cooling capacity of vegetation collapses: trees fail to cool 25% of cities, while grasslands and croplands fail in 71% and 82% of cities, respectively.
• Cooling failure during heatwaves is primarily due to high vapor pressure deficits and impeded canopy conductance, which restrict transpirational cooling.

Contributions

• Provides the first comprehensive global quantification of the thermal "warming paradox" of urban vegetation.
• Challenges the universal assumption that urban greening always mitigates heat, identifying specific aridity thresholds where vegetation may exacerbate the urban heat island effect.
• Offers evidence-based guidance for urban planning, suggesting that high-albedo surfaces may be more effective than traditional greening in water-scarce cities.

Funding

• Wyss Academy for Nature
• Oeschger Centre for Climate Change Research (OCCR)

Citation

@article{Guo2026Global,
  author = {Guo, Zhengfei and Rodríguez, Marta Videras and Davin, Édouard L. and Huang, Heng and Chen, Bin and Hejazi, Mohamad and Wang, Jian and Wu, Jin and Ge, Yunfeng and Song, Guangqin and Zhao, Yingyi and Feng, Kuishuang and Lin, Chen and Gong, Peng and Zhou, Yuyu},
  title = {Global urban vegetation exhibits divergent thermal effects: From cooling to warming as aridity increases},
  journal = {Open Access CRIS of the University of Bern},
  year = {2026},
  doi = {10.48620/93638},
  url = {https://doi.org/10.48620/93638}
}

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