A "vertical forest" will soon be gobbling up carbon dioxide in the pollution-sick Chinese city of Nanjing. Italian architect Stefano Boeri announced plans to build a complex with two towers that will hold 1,100 trees and 2,500 cascading plants and shrubs sprouting from rooftops and balconies in the province capital of about eight million people. The Nanjing towers will be 200 and 108 meters tall, respectively, and hold offices, shops, restaurants, a museum, a green architecture school, a rooftop club, and a Hyatt hotel.
It's perhaps needed in Nanjing more than anywhere else. Industrially rich Eastern China has some of the worst air in the world and Nanjing came in 27th (as in, second-worst) in a Greenpeace ranking of 28 Chinese cities by air quality. According to Boeri's website, the project will provide 25 tons of CO2 absorption each year and produce 60 kilograms of oxygen a day.
Before it can be considered a net positive for Nanjing's air quality, the project will have to make up for much construction pollution, says Lloyd Alter, architect and design editor of the environmental building website TreeHugger.
"[Boeri] tells a big story about how much carbon dioxide these buildings consume, but there is a huge carbon footprint to a new construction like this," Alter says. "One would have to calculate how many decades it would take before the trees make up for it."
Alter adds that trees growing in manmade lots do not sprout as widely and hence do not have as much environmental benefits as those that grow freely in the ground, which means the Nanjing towers might not look like the lush canopy depicted in Boeri's concept drawings.
The buildings could have had another environmental benefit had they contained apartments, Alter says. Residential units would save Nanjing families a commute and lessen the air pollution from cars. Instead, the towers cater to tourists, with the Hyatt as the main draw.
Still, the scale of these buildings makes them much more sensible than some of the garden rooftops that have become trendy in Western cities, Alter says. Those require maintenance and constant water pumping, leading to energy usage that outstrips the benefits of a scant few plants.
Alter also touts the mental health benefits that the presence of trees and plants in the heart of a bustling city can provide. "I think they will look wonderful," he says, "and will improve the outlook of everyone in their vicinity."