The Bronx River will never be the way it used to be, but it sure looks a lot better today than it did 20 years ago.

New York City’s Bronx River used to be an open sewer, more useful for carrying industrial waste than for hosting fish. Today, thanks to the efforts of environmental groups and the communities that live along this 37-kilometre stretch of water, the river is steadily making its way back to health.

This is called restoration ecology. And from the northern reaches of New York City, as elsewhere, this 80-year-old philosophy is slowly making its way into the political mainstream, now taking climate change and modern living into account.

Success stories aside, there is a long-standing debate about the value of restoring natural environments.

Opponents say that we are not really able to return degraded landscapes to their previous states. And that claiming to have done so risks more destruction because it generates the expectation that things can always be put back together. This problem is known as moral hazard.

If restoration is feasible, then what’s to stop mining companies from blowing mountains up and then just “repairing” them?

On the opposite side of the debate are pragmatists, who believe restoration efforts to more good than harm. They’re not unconcerned about moral hazard, nor do they assert that humans are able to recover landscapes to exactly as they once were.

But, they say, if we can make terrible situations better for people and nature alike, why not try?

Aldo Leopold is a towering figure in this camp. His 1949 Sand County Almanac, an account of the now famous “land ethic” that urges people to reconnect with nature, is one of the cornerstones of the environmental movement.

Leopold’s trips to the Rio Gavilan region of the northern Sierra Madre in 1936 and 1937 helped to shape his thinking about land health.

In the 1930s, he led the world’s first restoration project, the University of Wisconsin Arboretum, which established the basis of modern restoration ecology: returning degraded environments to their pre-disturbance states.

The Wisconsin project aims to recreate the pre-colonial environment once present south of lakes Mendota and Wingra, restoring prairie, savanna, forest and wetlands.

Though the aim of turning back the clock remains, environmentalists think about restoration in other ways, too. Given the rapid advance of climate change, it might not be possible to make landscapes as good as…