Green paradox: planting trees will cool a megacity unless it’s dry, Pg11

Key Highlights:

• Researchers studied the impact of vegetation on urban temperatures in 761 megacities across 105 countries.
• Vegetation generally cools cities, with trees cooling in 98% of cases, but grasslands and croplands can cause warming in drier climates.
• In cities with less than 1,000 mm of annual rainfall, grasslands and croplands often increased temperatures.
• During extremely hot summers, trees reduced temperature increases in 75% of cities, while grasslands and croplands worsened heat in 71% and 82% of cities, respectively.

Detailed Insights:

• The study defined temperature regulation capability as the temperature difference between vegetated and built-up areas, indicating cooling or warming effects.
• Evapotranspiration, the process of water evaporating from soil and transpiring from leaves, is a key mechanism by which vegetation cools surfaces.
• In arid climates, limited water availability restricts evapotranspiration, potentially causing vegetation to absorb more sunlight and increase temperatures.
• During extreme heat, grasses and crops reduce water loss, diminishing the cooling effect of evapotranspiration and exacerbating warming.
• The research highlights that planting trees is not a universally effective solution for urban cooling and can worsen warming in certain environments.
• Misguided greening risks can lead to unintended consequences, emphasizing the need for careful planning and consideration of local climate conditions.

Scientific/Technical Concepts Involved:

• Evapotranspiration: The process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants.
• Temperature Regulation Capability: A measure of the temperature difference between vegetated and built-up areas.