Green Paradox of Planting Trees in Cities

A new global study reveals that urban greening efforts, while often beneficial, can sometimes backfire particularly in dry climates. 

About the Research

• Researchers analyzed 761 megacities across 105 countries, including India. 
• Temperature Regulation Capability (TRC): TRC measures the temperature difference between a vegetated area (like a park) and a nearby built-up area (like concrete or asphalt).
• A negative TRC means the vegetation is cooler; a positive one means it’s warmer.

The Findings of the Research

• Globally, trees cooled built-up areas in 98% of cases, and grasslands did so in 78%.
• However, in cities with less than 1,000 mm of annual rainfall—trees, grasslands and croplands were actually hotter than built-up surfaces, creating a net warming effect.
• This creates the “green paradox“: while vegetation usually cools, in dry places it can sometimes heat.

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The Science Behind the Green Paradox

• Two main physical processes explain this counterintuitive result:
  • Cooling Effect (Evapotranspiration): Plants release water vapor from soil and leaves, carrying heat away, a natural air-conditioning system.
  • Warming Effect (Albedo & Heat Storage): Some vegetation, especially in dry conditions, absorbs more sunlight (has a lower albedo) than light-colored built surfaces like concrete, which reflect more solar energy. Vegetation can also change how heat is stored and released.
• In water-scarce, arid cities, evapotranspiration is severely limited and the warming effect can increase as reduced cooling is outweighed by increased solar absorption and heat storage changes.

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