Context:
A large-scale global study across 761 megacities in 105 countries reveals a counter-intuitive outcome of urban greening: while vegetation generally cools cities, in dry and arid climates it can increase surface temperatures, challenging conventional urban climate mitigation strategies.
Key Highlights:
Overall Findings on Vegetation and Temperature
• Trees provided cooling benefits in about 98% of cities, making them the most reliable form of urban vegetation.
• Grasslands and croplands, however, often increased temperatures in cities receiving less than 1,000 mm of annual rainfall.
Performance During Extreme Heat Events
• During extremely hot summers:
– Trees reduced temperature rise in 75% of cities.
– Grasslands worsened heat in 71% and croplands in 82% of cities, particularly in dry regions.
Scientific Explanation
Temperature Regulation Capability
• Defined as the temperature difference between vegetated and built-up areas, indicating whether vegetation cools or warms urban surfaces.
Role of Evapotranspiration
• Evapotranspiration—evaporation from soil and transpiration from plant leaves—is the primary cooling mechanism of vegetation.
• In arid and semi-arid climates, limited water availability reduces evapotranspiration, causing vegetation to absorb more solar radiation, leading to warming.
• During extreme heat, grasses and crops restrict water loss, further diminishing cooling effects and amplifying heat stress.
Policy and Planning Concerns
• One-size-fits-all urban greening strategies risk unintended consequences.
• Misguided plantation drives without considering local climate, water availability, and vegetation type can exacerbate urban heat islands.
Relevant Prelims Points:
• Evapotranspiration: Combined process of evaporation and plant transpiration transferring moisture to the atmosphere.
• Urban Heat Island Effect: Higher temperatures in urban areas compared to surrounding rural regions due to built-up surfaces.
• Megacity: Urban agglomeration with a population exceeding 10 million.
Relevant Mains Points:
• Urban climate adaptation requires region-specific ecological planning, not symbolic greening.
• Water-intensive vegetation in dry cities can strain urban water security.
• Findings highlight the importance of science-based urban governance and climate-sensitive infrastructure planning.
• Aligns with debates on nature-based solutions and their limits under climate change.
Way Forward:
• Prioritize tree-based greening suited to local climatic and hydrological conditions.
• Integrate urban planning with water management strategies.
• Promote climate-resilient native species and mixed land-use designs.
• Use data-driven assessments before large-scale greening interventions.
UPSC Relevance:
GS 3 – Environment & Ecology, Climate Change
GS 1 – Physical Geography
Prelims – Climate processes, Urban geography