By late afternoon, the city settles into its familiar tempo. Sidewalks thrum with foot traffic, delivery bikes weave through cars like water around stones, and behind it all lies the skyline. In this churn of concrete and commerce, it can be easy to miss the quiet shifts taking place: a rooftop garden where there once was tar, bike lanes inching their way across boroughs, a public plaza cooled not by air conditioners but by a canopy of trees.

We are, improbably, in the age of the green city. This wasn’t always a likely story. For most of the 20th century, the city was seen as antithetical to nature—a gleaming, concrete rebellion against it. But as climate change escalates from a scientific forecast to a lived reality, the urban landscape is undergoing a quiet metamorphosis. Our densest, most carbon-consuming environments are being asked to change. And, remarkably, some are rising to the occasion.

Paris is smothering highways with trees. Singapore grows vertical forests from its skyscrapers. Copenhagen has revamped its cycling infrastructure. Meanwhile, in New York, a famous former rail line has become the highline, an elevated linear park and greenway, rain gardens have sprung up in storm-prone neighborhoods, and office buildings are beginning to incorporate solar panels.

The idea isn’t just to plant trees and call it a day. It’s to redesign how we move, eat, breathe, and even rest within the grid. The "15-minute city"—a planning concept where everything one needs is within a quarter-hour walk or ride—is not just a utopian sketch anymore.

Cities, for all their sprawl and smoke, are incredibly efficient. Per capita emissions drop when people live closer together, share transit, and prioritise energy efficiency. The very density that once made them detrimental spaces in the sense of sustainability might yet be their greatest virtue. It’s tempting to think our answers lie in retreating to nature. But perhaps it’s in doubling down on the city, not the one we inherited, but the one we’re learning to build.