The importance of vegetation against city warming

Temperature differences between areas with cities and others with vegetation in continental USA (Image NASA's Earth Observatory)
Temperature differences between areas with cities and others with vegetation in continental USA (Image NASA’s Earth Observatory)

An article published in the journal “Environmental Research Letters” describes a NASA research that used data collected by various satellites to assess the impact of American cities on the rise in temperatures. The conclusions are that in large cities the temperature is from 1° to 3° Celsius (1.8 to 5.4° Fahrenheit) higher than that of the neighboring areas rich in vegetation.

In recent years climate changes are becoming more and more discussed. Considering the problems related to the greenhouse effect caused by the emissions of certain gases, in particular carbon dioxide, is absolutely right. However, the impact is being felt increasingly also due to urbanization.

In the northern hemisphere we’re heading toward the end of a summer that in many places has been particularly torrid. It’s known that heat is felt particularly in cities due to what are called impervious surfaces: asphalt, concrete and other materials that contribute to generating a sort of oven effect or urban heat island effect.

The oven effect in cities is generated during the day, when impervious surfaces absorb more solar radiation than vegetation. Plants release water into the atmosphere as a byproduct of photosynthesis, which helps to cool the air in a process called evapotranspiration.

Cities are becoming hotter and hotter not only because of global climate changes but also because in the last century they’ve been developed in a way that is creating growing problems. Urban centers have become concrete jungles with an increasingly wide system of roads but asphalt and concrete help to keep the city hot and in summer that becomes a big problem.

Data collected by the Landsat 7, Terra and Aqua satellites were combined with NASA’s Simple Biosphere model to recreate the interaction between vegetation, urbanization and the atmosphere at a 5 km resolution at half-hour steps in the Continental USA for 2001. This allowed to assess the impact of cities on the temperatures and the importance of vegetation to mitigate it.

The temperature rise measured by this research may seem small but it worsens a situation that in summer is already difficult. A slightly higher temperature also means greater use of air conditioners, which require more electricity with the consequent problems.

There are various studies on the re-introduction of vegetation in the cities as a way to mitigate this problem. On buildings with a flat roof, using it as a garden even with just grass can already bring down the temperature a few degrees on the top stories. Those are solutions that should be explored further to help mitigate this problem.

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