Across the street from a block of dense office buildings in western Paris, Bernard Sokler was surrounded by trees, weeds and crickets, as he tended to a bush of purple wildflowers in a largely forgotten strip of land.
Mr. Sokler, 60, and his team look after the greenery around a set of disused train tracks that circle Paris, known as the Little Belt, that the city is pushing to revitalize as it aims to mitigate the effects of climate change. With temperatures recently soaring to as high as 95 degrees Fahrenheit, the project is intended to offer some respite for the city’s residents — though it will come at a cost to the flora and fauna that now call the tracks home.
“If you want a true nature reserve, you can’t let humans in,” said Philippe Billot, who oversees Mr. Sokler and other gardeners on part of the Little Belt as part of his work for Espaces, an environmental group that, among other things, helps take care of green spaces in the Paris region. “But,” Mr. Billot added, “Paris will be one of the worst cities in terms of global warming, so we need to open places like these.”
Paris has just half the green cover of Berlin and Madrid, and the dense suburbs surrounding the French capital put the green of the countryside even farther out of reach. Central Paris is typically two or three degrees Celsius (three to five degrees Fahrenheit) hotter than its suburbs, and that difference can stretch to 10 degrees during extreme heat waves as buildings trap the excess heat.
This may explain why, as a Lancet study found, Paris was the European capital with the highest number of excess deaths during heat waves in the first two decades of this century.
“It is hard to leave Paris during a heat wave, whereas cities like Bordeaux or Marseille are surrounded by easily accessible nature,” said Eric Larrey, an engineer who works at a company that helps French cities adapt to climate change.
A longtime pride of Paris, the Little Belt opened at the end of the 19th century, before the city’s subway. The train line shuttled workers to factories, brought cattle to the slaughterhouse and carried raw materials like sugar into the city, before falling into disuse starting in the mid-20th century.
The hope now is that this haven of green can offer crucial breathing space to a city ill adapted to heat. The project, which started in 2006, is scheduled to open 19 more acres to the public in the next three years.
“There is always a little breeze here,” said Mr. Billot of the environmental group, referring to a shaded part of the Little Belt whose silence he enjoys. “It’s magical.”
Wildlife abounds along the rail line, which has a few yards of greenery on each side for most of its 20-mile length. On a recent visit, a bat flew over the tracks in a tunnel, raspberries stained the ground and a baby blackbird took its tentative first steps, yards away from the River Seine.
Yet taking down fences, clearing paths and opening the spaces to the public risks hurting the very biodiversity that those who descend on the Little Belt are no doubt drawn to.
“When people start walking somewhere, a part of the vegetation immediately dies,” Mr. Larrey, the engineer, said.
Already, with about a third of the tracks open, animals are leaving, Mr. Billot noted. “I’m seeing fewer doves, fewer goldfinches, fewer bats and hedgehogs,” he said. When he started working on the Little Belt in 2009, his part of the rail line looked like a very young forest, he recalled. Now, some of the open parts look more like patches of grass with trails through them.
“I call this the highway of joggers,” Mr. Billot said of a part of the tracks in southwestern Paris, where traces of wildlife were scarce as people ran past or walked their dogs.
But some stretches still have an atmosphere of industrial-age relics overtaken by time and covered in grass, and flowers growing in the shade of century-old trees.
“The first trees were planted at the end of the 19th century, when the train line opened, to stabilize the ground,” said Bruno Bretelle, a tech worker who runs a popular website about the Little Belt.
Other trees, including cherry and plum, grew from pits that passengers threw from the trains. Officials turned a blind eye as railroad employees grew small gardens along the tracks to bring extra food home, a practice that aerial pictures show was particularly prevalent during the scarcities of World War II.
Starting in the late 1980s, a local resident, Jean-Jacques Varin, who has described himself as a former mercenary in the Middle East, dedicated decades of his life to growing fruit trees and herbs on a portion of the tracks in the southeast.
There are no current plans to turn the entire Little Belt into a continuous public space like the High Line in New York City or its inspiration, Paris’s own Promenade Plantée, said Christophe Najdovski, an official in charge of the green spaces of Paris.
That’s mostly because some tunnels and bridges on the line, which the city administers along with France’s national railway service, are so damaged that they would cost millions of euros to renovate. There are also concerns for the wildlife.
Meanwhile, the rail service wants trains to be able to use the line with 10 days’ notice. Officials say such a prospect is unlikely, but the tracks remain, just in case.
Mr. Billot said he feared further openings would accelerate the decline of wildlife. Some wildlife is deemed too precious to lose — including Europe’s largest colony of pipistrelle bats, which lives in a Little Belt tunnel in the southwest. Small enough to fit in the palm of a hand, the bats help regulate populations of insects like the tiger mosquito.
“One year we counted 2,000 bats,” Mr. Billot said, shining his phone’s flashlight into the tiny gaps where the bats live among the tunnel’s steel plates. “Now, the bats are down to only 700 — we’re not sure why.”
Mr. Billot said he was grateful to have more freedom in his current role than he did when he worked in more conventional parks, where tidiness was valued above letting nature grow freely.
A particularly lush part of the Little Belt runs through southern Paris, weaving its way underneath Montsouris Park, with long tunnels whose openings are surrounded by ivy-covered stone walls.
Mr. Billot moved through the section of track on a vélorail, a pedal-powered cart that rides the rails, with a flashlight in hand. He said he avoided using motorized vehicles out of respect for the fragile ecosystem. As he rolled through the opening under the park, beams of sunlight threaded through leaves and fell in patches on the train tracks.
“Can you believe I am at work?” he asked.