While trees have long been seen as nature’s air conditioners for overheating cities, groundbreaking research reveals a more nuanced reality: their cooling effectiveness varies dramatically based on climate, location, and species, sometimes even contributing to nighttime warming.
Published in Communications Earth & Environment | Estimated reading time: 5 minutes
“Unlike droughts—temporary periods of low rainfall—aridity represents a permanent, unrelenting transformation,” explains Ibrahim Thiaw, UNCCD Executive Secretary. “Droughts end. When an area’s climate becomes drier, however, the ability to return to previous conditions is lost. The drier climates now affecting vast lands across the globe will not return to how they were and this change is redefining life on Earth.”
In the most comprehensive analysis of its kind, researchers examined 182 studies across 110 global cities, discovering that while trees can cool pedestrian-level air temperatures by up to 12°C during the day, their effectiveness varies significantly. The study found that trees reduced peak monthly temperatures to below 26°C in 83% of studied cities, meeting critical thermal comfort thresholds.
The research, led by Dr. Ronita Bardhan at the University of Cambridge, revealed striking regional variations. In tropical savanna climates, trees achieved maximum cooling of 12°C during daytime but could warm cities by 0.8°C at night. Arid regions saw cooling effects of up to 9.3°C, while tropical rainforest areas experienced more modest cooling of approximately 2°C due to higher humidity levels.
The Climate Factor
Urban tree effectiveness shows remarkable variation across climate zones. In temperate regions, trees can reduce temperatures by up to 6°C during the day, though they may contribute to warming of 1.5°C at night. The study found that combining different tree species provides optimal cooling in most climates, with mixed use of deciduous and evergreen trees offering 0.5°C more cooling compared to single-species approaches.
Key Terms
- Urban Heat Island (UHI)
- A phenomenon where urban areas experience significantly warmer temperatures than surrounding rural areas due to human activities and infrastructure.
- Sky View Factor (SVF)
- A measure of how much sky is visible from a specific point, affecting the amount of solar radiation reaching the ground and heat dispersion at night.
- Local Climate Zone (LCZ)
- A classification system for urban areas based on surface cover, structure, material, and human activity, influencing local temperature patterns.
Test Your Knowledge
By how many degrees Celsius can trees cool pedestrian-level air temperature at maximum?
Trees can lower pedestrian-level temperatures by up to 12°C through radiation blockage and transpiration.
What percentage of studied cities achieved thermal comfort threshold after tree implementation?
83% of cities reached temperatures below 26°C after implementing trees, meeting the thermal comfort threshold.
How does the combination of deciduous and evergreen trees affect cooling compared to single-species approaches in most climates?
Mixed use of deciduous and evergreen trees generally results in 0.5°C higher cooling compared to using only deciduous or evergreen trees in tropical, temperate, and continental climates.
What factors determine the effectiveness of urban tree cooling in compact urban zones with high background temperatures?
In compact urban zones with high temperatures, factors such as tree canopy cover, street canyon orientation, local climate zones, aspect ratio, and sky view factor must be carefully considered, as excessive tree cover can trap heat at pedestrian level.
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