Living Green

Mexico City seeks to change polluted image

MEXICO CITY — Smog routinely hangs thick over this city, shrouding dramatic mountains on the horizon and swirling past skyscrapers like fetid clouds.

But when Tanya Muller looks into the grey expanse, past miles of concrete buildings with flat roofs, she sees something else entirely: possibility.

On those rooftops, Mexico City officials soon hope to see healthy splashes of green.

The city is embarking on an ambitious program to carpet its concrete cityscape with rooftop gardens, or green roofs, that they hope will suck up and filter the city's notorious pollution, cut energy use and eventually lower the city's temperature.

"It's hard to increase green spaces in a city like this because there's really no more space," said Muller, the city's director of urban reforestation. "But almost all of the buildings in this city can support green roofs ... There's an enormous potential."

Mexico City Mayor Marcelo Ebrard wants to install more than 500,000 square feet of green roofs by 2012, starting with government buildings. That would nearly tie Mexico City with Chicago, the hemispheric leader. It's part of Mexico City's $5.5 billion "Green Plan," which Ebrard insists will transform one of the world's most polluted cities.

The plan also seeks to reduce emissions by taking some of the 6 million cars off the city's traffic choked roads. Steps include a new subway line, 10 new bus lanes on major avenues, miles of bike lanes and obligatory school bus transportation for school children currently getting rides from their parents.

The green roofs plan has sparked the most excitement here, but some warn that prohibitively high costs may stymie the city's goals.

At about $10 per square foot, green roof technology is still expensive in Mexico, and throughout the hemisphere. Regular roofs cost less than $2 a square foot.

But experts say the roofs can slash energy bills by decreasing air conditioning needs by 40 percent and increasing property values.

"The only way (green roof technology) will be effective on a citywide scale is if it's cheaper," said Mark Simmons, an ecologist at the Lady Bird Johnson Wildflower Center in Austin, Texas. "That's the big holdup."

Simmons is researching ways to make green roofs less expensive by using recycled elements and waste materials in the unique soil blend needed for green roofs.

The roofs require a lightweight soil layer that allows water to feed the plant roots while excess water eventually seeps into a drainage system. The system also requires a protective layer of material to prevent roots from penetrating the building.

Experts say the roofs store large amounts of rainwater and could help prevent flash flooding, a huge problem in Mexico City, which sits in a bowl-like valley with little natural drainage.

And in large numbers, green roofs reduce the so-called heat island effect, in which black tar and concrete roofs conspire to make cities hotter than surrounding areas. That phenomenon exacerbates Mexico City's exceptionally high ozone levels, which are blamed for residents missing 2.5 million workdays per year because of respiratory illnesses.

"(Green roofs) are like putting little air condition units outside," said Steven Peck, president of the Green Roofs for Healthy Cities, a non-profit industry association based in Toronto. "Get enough of them and you can actually change the climate of the city."

Throughout North America, green roofs grew by 30 percent, or 46,820 square feet, in 2007, according to Peck's group. Chicago leads the hemisphere with more than 517,000 square feet of green roofing, followed by Wilmington, Del. and Baltimore, Md.

Atlanta, which will host the 2009 Green Roofs for Healthy Cities annual conference, has a burgeoning green roof industry, as well as a noted green roof on its city hall.

Austin is home to cutting edge green roof research, and an award-winning green roof on its city hall.

But North America still trails Europe, and especially Germany, which has been building green roofs for decades and requires one on many new buildings.

In downtown Mexico City, a day care for subway workers has one of the city's first green roofs. The children exercise on a carpet of thick green grass and learn about gardening by planting parsley and potatoes.

For some of the youngsters, the green roof is their only regular access to green space.

"There really aren't any spaces in this part of the city for the children to play in," said day care director Nadia Tapia Ortiz. "Up there ... it's like a park. It's a beautiful thing."

Environmental activists, while applauding the city's ambitious green plan, are worried that the high cost of green roofs, and a lack of public outreach, could hinder the effort.

"They need to sensitize and involve the residents of the city," said Hazett Cervantes, the green roof director for CICEANA, a Mexico City environmental group. "They have planted the seed, but what comes next is very important. They need to follow through or it could be a plan just like all the other plans."