Searching for Shade in Buenos Aires

The city of Buenos Aires wasn’t particularly green when I arrived in August 2015—it was the middle of winter, colder than I expected, and most of the trees were deciduous yet decidedly leafless. Earlier that year, I was walking through San Francisco, California with my friend Ken who pointed out that it was all too common to find that any given street in the city was paved with concrete from wall to wall. There were hardly any patches of bare earth or virgin grass outside a designated park in the City by the Bay, and Buenos Aires presented a very similar image. My new neighborhood, Palermo, was split into several smaller areas such as Palermo Soho (charming with the occasional cobblestone streets, featuring trendy Italian-inspired cafes in cozy plazas) or Palermo Hollywood (auto shops, night clubs, and corporate headquarters on less pedestrian-friendly avenues). Trees were everywhere, but I didn’t really begin to appreciate them until the spring bloom sometime in late October.

Around the same time as my stroll through San Francisco, I had started a small project focusing on the urban forest of Buenos Aires. To my delight, the city government offered a tree census in their online data repository. This census was a table documenting the species, genus, common name, height, address, grid coordinates, and other attributes of a few hundred thousand trees in the city. I was able to open the data in QGIS, my computer struggling to load all these thousands of dots stacking up on top of one another within the borders of Capital Federal (the administrative boundary of the Autonomous City of Buenos Aires, or CABA, and capital district of Argentina). QGIS is a powerful open source mapping software, and it would be my primary tool as I moved forward with my study. I changed the size of each dot that represented a tree, and soon was able to see neat lines along the main streets with some parts of the city more densely covered than others.

My first analysis was to look for any connection between the density of trees and demographic data. I found quickly that the poorer, less educated, and more immigrant-dominated southern part of the city was nearly barren of trees in many parts. This area included Nueva Pompeya (an industrial district) and La Boca (“the mouth”, were the old port was built on the mouth of a river and still quite the skid row). Palermo, a haven for expatriates (i.e., non-immigrant foreigners) like me in contrast to less affluent immigrants in the south, was well covered. Palermo’s ritzy but quieter neighbor to the east called Recoleta was also quite green. In the heart of the city—really the northeast corner—Microcentro and San Telmo were also very lacking in urban forest, but featured a wealthier resident population, with high levels of education especially near the University of Buenos Aires and areas dominated by foreign banks and technology headquarters (Google, for example, is located somewhere in the overlap of San Telmo and Microcentro). 

So what to conclude? More trees tended to accompany a better standard of living and higher socio-economic status. In addition, areas with higher proportions of Argentine-born residents are also more densely forested, showing perhaps an attitude of better environmental health and greenery where residents feel more invested in the land as their past, present, and future home. These are my theories, at the least, judging from associations in the data.

On my arrival in Buenos Aires, I began to look at the city under a more environmental lens than a social one. Upon my arrival, I was able to meet with Manuel Swarcz, a middle-aged Argentinean man who was the director of an organization called Arboles sin Fronteras (Trees without Borders). He had overseen tree-planting projects all over the country, including in parts of Buenos Aires. In our first meeting I explained my past research that made use of the tree census. Manuel was quite interested,; I suggested we speak again in another week, and so I retreated back to my Palermo apartment to ponder the next steps over a bottle of Malbec. I created a new layer for the map that showed not the precise location of each tree, but the density within 500 square meter hexagons instead of in kilometer wide squares as before. Now I could see in great detail where the urban forest was concentrated, but I still needed to think deeper about how to make something useful out of this.

The idea came to me slowly, relating to some scientific publications that discussed benefits of urban forest. One was the regulation of urban temperatures, particularly combating what is known as urban heat island (UHI) effect. It appeared that I could help Manuel by suggesting where to target tree-planting efforts in the city in hopes of lowering temperatures and covering the city’s bare spots with green leaves.  I spent several long nights improving my knowledge and skills, eventually being able to proficiently construct two new map layers to accompany the tree census. Both of these layers have their origin in NASA’s Landsat8 satellite, which takes images of every part of the earth several times over the course of each year. I chose imagery from a clear day in December 2014—summer in Buenos Aires—and downloaded these images in 11 different bands of light to include infrared, red, green, and blue.

In my first layer, I depicted was is called Normalized Difference Vegetation Index (NDVI). This is a visualization of vegetation and plant presence, and can even be used to evaluate the health of these plants by people more skilled than me. For my purposes, I was able to create a layer that painted the map green where any form of plant life existed in the city. The premise for this is that plant life, especially in the green variety, reflects a certain level of radiation to the atmosphere that can be detected by the satellite. As a result, I could indeed see the green expanses of many parks, of the ecological reserve near Microcentro, and other widespread locations in the city. Combined with a map of the tree census, this allowed me to really gain insight into which parts of Buenos Aires were truly green—and which were note. Again, the south was lacking, the city center as well, and finally the airport and railways along the northern stretches of the city that made up the right bank of the Rio Plata.

My second layer showed Land Surface Temperature (LST)—an indication of the temperature in degrees Celsius not in the air, but on the actual ground. This comes from a similar process as calculating the NDVI. Uncovered rock and dirt, impervious surfaces such as roads, and other man-made structures like rooftops and runways reflect their own rather unique level of radiation back to the satellite. This is first detected through something call at-satellite brightness, which is converted again to degrees Kelvin and then Celsius to be more informative for the common viewer. Adding a color gradient to the result, I was able to find the maximum and minimum LST during that December day, with the coolest being shown in blue, moderate temperatures in yellow and finally the highest temperatures in orange and red.

What I expected was to find that it was visually obvious that more trees and greenery resulted in lower temperatures. Indeed, the airport and railway areas in the north, as well as another railway station in the central part of the city near the Floresta neighborhood, were the color of a hot iron as expected. Major streets were also red, in addition to other rail lines, the sea port near La Boca, and much of the industrial south such as Nueva Pompeya. Surprisingly, though, there were some ultra-cool spots. The Rio Plata was blue, indicating of course that the water was a lower temperature (it’s certainly not inviting to swim in, neither due to temperature nor its brown color). Some other ponds and small lakes in the city were also blue. These made sense—but then in the Floresta neighborhood, near the red hot train station, there was a little oasis of dark blue that was unmatched anywhere else in the city.

I went to Manuel with my latest map. The NDVI layer was interesting to see, as it expanded the conclusion on which parts of the city could be called green or not. The LST, however, was the true point of conversation. Manuel put on his reading glasses and pored over my screen, impressed to see the variation in temperatures as indicated by the rich colors. He nodded as he looked at the airport and railways. Near the Plaza Italia underground station in Palermo, the base layer labeled the Sociedad Rural (Rural Society) beside what was also a dark red despite being surrounded by one of the city’s largest, grassiest, and most forested parks.

“What could this be?” he asked. I wasn’t sure. Then his eyes drifted to Floresta and we both furled our brows at the dark blue. “And this?” he asked again, putting the eraser of his pencil against the screen. I wasn’t sure about that one either, but it held some value—in the realm of UHI, there was a more desirable effected that could be called an urban cool island (UCI). I had hoped a density of trees and greenery might cause this effect, but neither the tree census nor the NDVI indicated anything special about Floresta. I left Manuel’s office again, determined to investigate the scene of the UCI and find the cause.

The next week, my friend Aditya and I went strolling through the center of the cool spot. It was about four by four city blocks—or manzanas (apples), as the locals called city blocks—and faded still over several blocks in every direction. I brought Aditya as an extra set of eyes, hoping he would notice something I might miss. The area was dominated by sidewalk flea markets, two- to four-story buildings, clothing shops with broad windows, and wide streets with an unremarkable number of trees. The sun seemed to cast its light on the streets throughout the whole day, so it wasn’t particularly shady on any of the streets. After an hour of walking, Aditya and I were stumped. “Maybe there was a cloud over this area when the satellite took the image,” he suggested. I sighed, nodding and thinking that it must have been a stupid mistake on my part.

Back behind my computer screen, I chose another clear day that same December to convert to LST—and the cool spot was still there, in just the same shape. This was certainly no cloud, but a persisting phenomenon. While this news was exciting, my next meeting with Manuel left us both disappointed at the same time. The mystery would take more time to solve, but I was scheduled the fly back to the US in a week’s time with no plan to return anytime soon. I mused with Manuel over how to get better insight—maybe the elevation was significantly different in Floresta than in neighboring parts, causing some effect on the wind. But the city was overall quite flat, we both agreed, shaking our heads. Could there be something underground causing the temperature to cool—perhaps a sort of cavern? There didn’t seem to be any effect like this in the areas with underground stations, so perhaps it wasn’t the case. Maybe an underground river, or the rooftops were comprised of some peculiar material. Maybe the height and shape of the buildings did something to reflect heat differently? It was all a mystery. The real question, however, was still useful—how could we recreate this effect in the hotter parts of the city, and make them more habitable and temperate?

The mystery still remains. UHI is an important issue being studied around the world today, as heat waves kill more humans annually than any other type of natural disaster or extreme weather event. Urbanization is on the rise, with more people flocking to city centers for economic and social reasons. Global average temperatures are also on the rise, which makes concrete jungles even more hellish and in need of greenery. Trees and vegetation are certainly crucial to lowering urban temperatures, especially when placed along waterways. But what happened in the Floresta neighborhood on those December days, and perhaps every day, may hold the key to some other advancement—a way to go beyond just augmenting urban greenery and to more drastically lower urban temperatures on sweltering summer days. It may be a secret that neither I nor anyone else ever unlocks, but nonetheless it remains as a reminder of how complex our world is despite all the technological lenses we use to study it’s changing relationships.