It is not possible to fully understand the functioning of a complex environment like a city by focusing solely on human needs. Paradoxical as it may seem, only a broader perspective can ensure that these same needs are preserved for the future.
Allow me to clarify: studying and planning cities based solely on the immediate needs of their inhabitants is the surest way to ensure that these needs can no longer be met in the near future. On the contrary, understanding the physiology of a city requires considering the entire ecosystem that defines it. Any other method of study is nothing more than a simplification.
Over 90% of cities are coastal and, as such, will be increasingly exposed to frequent and dangerous flooding due to the inevitable rise in sea levels. Atmospheric phenomena, growing in intensity, will cause increasing damage from storms, floods, winds, and droughts. These damages not only directly impact populations but also have significant economic repercussions, disrupting commercial activities and the normal functioning of urban life.
Heatwaves—periods of extreme temperatures well above the average—will become increasingly frequent, with disastrous effects on public health. As temperatures rise, the prevalence of certain life-threatening illnesses increases. A 2017 study estimated that even if we were to limit the rise in global average temperature to just 2°C above pre-industrial levels by mid-century—an increasingly unlikely scenario—the number of deaths in cities caused by heatwaves alone would exceed 350 million.
As if this were not enough, we must also consider that the effects of rising temperatures are magnified in urban environments. The so-called urban heat island effect, for instance, causes city temperatures to be significantly higher than those of surrounding rural areas, making urban areas far more susceptible to temperature increases. Globally, it is estimated that urban heat islands alone contribute to an average temperature increase of 6.4°C in cities. This is a variable figure, depending on the geographical location and the specific characteristics of each urban center. This is a clear indicator of the enormous impact that our methods of construction have on the environment.
The first person to identify this phenomenon was an English chemist and pharmacist, Luke Howard, who is credited not only with the initial observation of the urban heat island effect but also with recognizing that the temperature difference is greater at night than during the day. In 1820, in his treatise The Climate of London—the first work ever dedicated to the climate of a city—Howard documented nine years of temperature data collected in central London and nearby rural areas. He noted that “the night is 3.7°F (equivalent to 2.1°C) warmer in the city compared to the countryside.”
This observation laid the foundation for understanding how urbanization amplifies temperature variations, emphasizing the critical role that urban planning and design play in shaping not only local climates but also global environmental trends.
The reasons behind this overheating are varied and stem from the way our cities are built and function. One of the main factors contributing to the formation of urban heat islands is the artificial nature of urban surfaces. These surfaces, due to their impermeability and lack of vegetation, are unable to cool down through the process of evapotranspiration, unlike rural areas. But that’s not all. In cities, dark surfaces absorb significantly more solar radiation, and materials like asphalt and concrete have thermal properties that differ from those of rural surfaces.
Additionally, a considerable portion of the energy used in cities—whether by vehicles, industry, or for heating and cooling buildings—is lost as residual heat, further increasing the ambient temperature. Then there are other factors: the geometry of buildings, the lack of wind that prevents cooling through convection, higher levels of air pollution, and particulate matter that alters the radiative properties of the atmosphere. All of these elements in cities contribute to raising the overall temperature of the environment.
When we combine the effects of global warming with the typical heat island phenomenon in cities, the results are far from reassuring.
Cities are, therefore, particularly vulnerable to global warming. The good news is that they are also the places where global warming can be most effectively addressed. Since 75% of human-produced CO2 originates in cities, it is here that efforts to reduce it must focus, using trees to remove as much as possible from the atmosphere.
In 2019, a team of researchers from the Zurich Polytechnic published a study claiming that planting one trillion trees globally was by far the best, most efficient, and measurable solution for reabsorbing a significant percentage of the CO2 emitted since the beginning of the Industrial Revolution. Despite the study’s solid scientific foundations, criticisms quickly followed: where would we find the space to plant a trillion trees? What would it cost? Would the results be as significant as estimated? These criticisms were largely unfounded. The necessary space for planting these trees exists, and while the cost would be substantial, it is far lower than any alternative with even a fraction of the potential success of this initiative.
Moreover, if a significant portion of these trees were planted within our cities, the results, I am certain, would be even greater. The efficiency of plants in absorbing CO2 increases significantly when they are closer to the source of emissions. In cities, every surface should be covered with plants—not just the (few) parks, boulevards, flowerbeds, and other conventional spaces, but literally every surface: roofs, facades, streets—every place where a plant can grow should host one.
The notion that cities must be impermeable, mineral environments opposed to nature is merely a habit. Nothing prevents a city from being entirely covered in plants. There are no technical or economic barriers that truly preclude such a choice. And the benefits would be incalculable: not only would massive amounts of CO2 be fixed precisely where it is produced, but people’s lives would improve in virtually every way imaginable. From enhanced physical and mental health to stronger social bonds, from improved focus and attention to reduced crime rates, plants positively influence our lives from every possible perspective.
Why our cities are not already entirely covered with plants, both inside and out, remains a mystery difficult to comprehend, especially considering the thousands of serious studies published on the benefits of urban greenery.
