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Trees Make Cities Cooler


With the return of El Nio, this summer could be even worse than the hottest on record for Europe in 2022, which resulted in over 20,000 additional deaths throughout western Europe.


GS Paper-3: Biodiversity and Environment

Mains Question

What is Urban Heat? How trees could improve our mental and physical health, cool cities, and reduce pollution? (250 words)

What are your knowledge of El Nio?

  • El Nio is a term used to describe a particular type of climate pattern that causes unusually warm surface waters in the eastern tropical Pacific Ocean.
    • El Nio is a larger phenomenon known as the El Nio-Southern Oscillation, and it is its “warm phase” (ENSO).
  • El Nio events sporadically happen every two to seven years.
  • Trade winds that blow towards the west along the equator deteriorate during an El Nio event.

Key Points:

  • Our cities, which are typically 1.5° Celsius warmer than the surrounding areas due to so-called urban heat islands, may be where the effects are most noticeable (UHIs).
  • At that point, machinery (cars, trucks, air conditioners) and building materials (concrete, asphalt, metal) absorb and produce heat, converting city blocks into baking ovens.
  • Cities can be unhealthy places to live for a variety of reasons; during the summer, UHIs are responsible for 4.3% of premature deaths in urban areas.
  • Over 4 million people per year are killed by outdoor air pollution. In Europe, noise is responsible for the loss of more than 1.8 million healthy life years.
  • According to studies, city dwellers have smaller carbon footprints than those who reside in the suburbs and the countryside.
  • In comparison to their counterparts in suburbs and rural areas, city dwellers use public transportation more frequently, walk and cycle more frequently, and live in smaller homes with less clutter. In the race to achieve net zero carbon emissions, encouraging denser living may be essential.
  • But if we want to persuade people to live there, we must first improve the quality of life in cities.

Island of Urban Heat

  • The urban heat island effect is a phenomenon where air temperatures in cities are higher than those in the nearby countryside. This result may be quite observable.
  • On average, daytime temperatures in cities are 1–7°F warmer.
  • Cities can still be up to 5°F warmer than the surrounding areas during the night because of this difference.
  • Urban heat islands are the name given by scientists to places that experience these higher temperatures.

Why do urban heat islands occur?

  • As cities expand, natural features like trees, ponds, and soil are replaced by new construction like roads and buildings.
    • As a result of this change in the local environment, the local climate also changes.
    • This is due to the fact that these organic surfaces help to regulate air temperatures.
    • By offering shade, plants and trees, for example, can reduce air temperatures.
    • Through “evaporative cooling,” a natural process whereby evaporating water absorbs heat, this vegetation, along with soil and water, also contributes to the cooling of nearby air (much like sweat cools the human body).
    • However, the artificial surfaces that take their place frequently don’t cool.
    • Instead, they have a propensity to absorb and emit more heat, making their surroundings warmer as a result.
    • Although a large portion of that heat originates from the sunlight that those surfaces receive, human activities such as the creation of electricity and the use of automobiles and air conditioners are also sources of heat.
    • The slender spaces between tall buildings, known as urban canyons, can block wind and trap heat, which is another way that cities’ geometry contributes to the development of heat islands.

How could trees cool cities, cut down on pollution, and enhance our mental and physical well-being?

  • According to a recent study published in The Lancet, increasing the amount of tree cover in a city to 30% could reduce the number of summertime deaths in cities by about 40%. This is because more trees provide shade, evaporate more water, and block the sun’s heat from surfaces like concrete and asphalt.
  • In the 93 cities included in the study, satellite data has shown that a target of 30% tree coverage is achievable, but the average in Europe is still only 14.9%.
  • According to a 2011 study by Soares, annual tree maintenance expenses amount to $1.9 million. The cost of the benefits was $8.4 million.
  • Along with improving aesthetics, lining streets with trees has been shown to have positive effects on mental health.
  • Additionally, having access to urban green spaces is linked to lower rates of diabetes, heart disease, healthier births, and better cognitive performance.
  • Additionally, trees serve as useful carbon sinks and can aid in reducing the risk of flash flooding, another climate risk.

Obstacles in urban forestry include the following:

  • Cities historically prioritised parking for cars.
  • There is also the cost. Wide roads and parking lots could be narrowed to make room for foliage, but as conflicts over cycle lanes in various cities have demonstrated, there is little space left for trees to be planted.
  • New trees need upkeep after purchase and planting, and they can struggle to survive in cities just like people do.

Way Forward

  • The key going forward is to make sure that trees are distributed fairly throughout the city.
    • Many streets lined with trees will have a greater positive impact than one enormous forest park.
    • Ensuring that everyone has equal access to nature also has a social justice component because richer areas already tend to be greener than poorer ones.
    • The 3-30-300 rule was proposed by Cecil Konijnendijk, professor of urban forestry at the University of British Columbia: Everyone should live in a neighbourhood with 30% tree cover, be able to see 3 trees from their window, and be 300 metres from a green area.


Improving the sustainability and livability of our cities will necessitate making many difficult choices that concern more than just the environment. But given that trees can take decades to grow, we need to start planting right away.

December 2023