In a sheltered corner of one of the greatest megacities on Earth, there is a place where lizards careen around tree trunks, butterflies drink nectar from vermillion flowers and hummingbirds whisk the heavy air with their wings.
Stand in the botanical gardens of the Bosque de Chapultepec (the Chapultepec forest) and listen carefully enough, and something remarkable happens: birdsong begins to pierce the groan of trucks and the screech of taxi horns from the long avenue that bisects the park.
The gardens are home to one of a growing number of azoteas verdes — or green roofs — that are springing up around Mexico City as part of the metropolis’ efforts to purge its air of the pollution that has long been among its least-desired claims to fame.
The azotea verde atop the circular single-story offices of the botanical gardens, is planted with hardy stonecrop, which can withstand the Mexico City summer, but which also produces oxygen and serves as a filter to draw out the carbon dioxide and heavy metal particles in the air. As well as providing the park’s squirrels with an arena in which to practice their parkour, the roof help regulates the temperature of the offices below and soaks up rainwater to keep the building dry.
Last year, the city’s environment secretariat spent almost US$1m on the azoteas verdes project, bringing the total area of green roofs in hospitals, schools and government buildings to 21,949m2. This year, the investment will rise by one-third.
Mexico City’s Minister of the Environment Tanya Muller said: “In a city like ours where urban development puts pressure on the space we have at ground level, we have to take advantage of our rooftops to create a green urban infrastructure.”
The green roofs do far more than simply purify the air: They reduce the “heat island effect,” teach children about nature and speed up the recuperation rates of hospital patients, she added.
A little way across town, not far from the city’s ancient heart, the Zocalo, sits the secretariat’s air-monitoring lab. It too has been given over to greenery and from its neatly planted roof, where dedicated staff congregate for lunchtime exercise classes, the haze that blankets the capital is plain to see. It smudges the outlines of distant towerblocks, as well as the mountains that enclose the city and its 21 million inhabitants. As Muller is keen to point out, fighting air pollution demands rather more technological solutions than sowing seeds on rooftops.
Her glass-and-steel office, which overlooks the Zocalo, feels like a curious hybrid of an Internet startup office and an architectural practice. On the wall by her desk is an enormous screen with a live Twitter feed and electronic maps showing the temperature and ozone levels of Mexico City and the surrounding area. On a wet afternoon, the ozone levels are creeping above the normal levels, but other pollutants are within the usual range.
“I have this dashboard on my smartphone and it’s the same dashboard as the department of air monitoring has and the mayor has,” Muller said. “We know how the air quality is every day and whether we have to take decisions.”
Readings from the 29 air-monitoring stations in the city and the surrounding state of Mexico can trigger a variety of responses. If pollution levels are seriously high and remain so for 48 hours, the environment secretariat’s Hoy No Circula (No driving today) ban kicks in, and those cars with registration plates of a certain color and two-digit code are not allowed on the roads.