Urban vegetation, relief from heat waves

With satellite images, a FAUBA study at CABA found, at the same time, thermal differences of up to 23°C between the north and south of Buenos Aires. One cause is the different amount of vegetation in each area. They stress the need for more squares and parks.

Asphalt burns during heat waves in the city, and parks and squares can be a refuge, as their vegetation mitigates high temperatures. A study by the Faculty of Agronomy of UBA (FAUBA) in CABA recorded, at the same time during the month of February 2020, temperatures as different as 19°C in the north of the city and 42°C in the south , and pointed out that the vegetated surface is the main responsible for this large amplitude. Moreover, he indicated that a large part of the city has rare and small vegetated spaces, and that its distribution between the different neighborhoods is linked to socio-economic factors.

Cities tend to have higher temperatures than their rural surroundings. This phenomenon is known as the “urban heat island effect” and occurs for different reasons. One is the lack of urban vegetation, since, among other benefits, green spaces lower the air temperature. The problem is that instead of green spaces, large impermeable surfaces are placed, such as concrete and asphalt, which retain more heat and release it throughout the day and night.”, explained Paula Galansino of her thesis for the degree in Environmental Sciences (LiCiA) of FAUBA.

city, asphalt, heat, vegetation, parks, squares, temperature, global warming“In satellite images, we often see “holes” caused by clouds, aerosols or atmospheric pollutants. The midday image of February 3, 2020 was the one with the best quality” (P. Galansino)

The heat island effect occurs in different Argentinian cities and, in my thesis, I wanted to observe it in the city of Buenos Aires. On the one hand, I analyzed whether the phenomenon is equally intense in the different districts of the city; on the other hand, the distribution of urban vegetation. Furthermore, I investigated whether there is a relationship between the two properties, that is, whether the vegetation regulates the temperature in the city. With these ideas in mind, I reviewed satellite images of each summer between 2015 and 2020.”, said Galansino, whose study was led by María Semmartin, professor of the chair of ecology at FAUBA.

Galansino selected the midday image of February 3, 2020 and found that the difference between the minimum and maximum temperature within CABA exceeded 20°C. “At the time, the city’s average surface temperature, which is closely related to air temperature, was nearly 36 degrees. What is curious is that if in some places the temperature was 19 degrees, in others it was 42. Basically, the lowest temperatures were recorded north of the city, and the highest South.”. What caused such a difference?

Grey, gray and porteño green

Temperature was lower in areas of the city with the most vegetated area,” Paula noted, adding, “We measure it through the Normalized Vegetation Index —also known as NVI—, which is calculated from satellite information and provides indications of the type of cover a given area may have. If the IVN is 0, it indicates bare ground or with cement or asphalt, while values ​​close to 1 suggest that the ground has vegetation. My results showed that from a certain proportion of vegetated surface – to be exact, an IVN of 0.4 -, if this green surface is increased, the surface temperature decreases”.

As part of his work, Galansino determined that most of CABA has sparse and reduced areas of vegetation, which are unable to mitigate high temperatures. “I divided the city into four units. Units 1 and 2 combine the center and the south, represent 80% of the city and display NVI values ​​below 0.2, have the smallest area occupied by vegetation, respectively 12.5% ​​and 20.1%, and the smallest average vegetated patch area, 900 square meters. 3 and 4 include vast green areas to the north of the city such as the forests of Palermo and also the large parks and reserves to the south. They cover almost 20% of CABA, have more than 45% of its surface vegetated and IVN values ​​greater than 0.4”.

city, asphalt, heat, vegetation, parks, squares, temperature, global warmingGalansino grouped the different areas of the city according to characteristics related to the percentage of the landscape occupied by vegetation, the average area of ​​vegetated plots and their density, among others.

The city of needs

Paula Galansino said that in many cities around the world, access to green space is tied to income level, and this trend is also happening in CABA. “I found that neighborhoods with the highest percentage of households with unmet basic needs, primarily in the south-central part of the city, have the least vegetated area and experience higher temperatures. To obtain these results, I consulted the index of unmet basic needs of the INDEC, which provides information on the characteristics of households, such as their access to basic services such as water, electricity and sanitation. , and on their inhabitants, as the activities they carry out, among others.”.

Heat waves have common consequences for city dwellers, but they do not affect everyone in the same way. “High temperatures can cause health problems. People are increasing their energy use to cool or ventilate their homes, and this has two benefits. First, that the excessive demand for electricity causes power outages, and second, that electrical appliances or bill values ​​are not accessible to everyone“, argued the lawyer.

city, asphalt, heat, vegetation, parks, squares, temperature, global warmingThe southern part of Unit 3 had the highest percentage of households with unmet basic needs, 16.59%, and recorded the highest average surface temperature

On the other hand, it increases the possibility of disease transmission, since the heat favors the development of animals or insects that transmit them. The mosquito that transmits the dengue virus, Aedes aegypti, is a clear example. It is known that there is more dengue fever in the south of the city, where the temperatures are higher“, he warned.

urban ecology

For Galansino, the resources available to management are limited and should be allocated to areas of the city that are exposed to higher temperatures and have less access to parks and squares. “It is necessary to think about the future and add vegetation that attenuates the heat peaks in these places and at the same time urgent measures must be taken to improve the conditions of the people who live there, such as, for example, a help to cool their homes or support medical care during heat waves”.

The question of green spaces is complex. Keep in mind that placing vacant homes in an area also increases house prices and rents, and this change can displace neighbors who live there.Paula thinks. “Overall, populations are increasingly urban. In our country, more than 90% of the population lives in cities. There is a lot to think about for cities to guarantee a good quality of life for all who live there and not just for some”.

Finally, Semmartin, researcher at FAUBA and former director of LiCiA, referred to research in urban ecology in the context of climate change. “Studying the city from the point of view of ecology requires delving into issues such as climate, soil and water. At FAUBA, we know how to solve problems in rural areas and we can perfectly do the same in urban areas. Today, climate change forces us to think and design the best solutions based on the introduction of vegetation and nature into urban systems. Paula’s work leaves open many questions and avenues to investigate.”.

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