was in Buenos Aires when I first saw a city disappear. I was visiting an expatriate brother, and when we had grown tired of the seething capital we walked to a park along the neglected River de la Plata. We arrived without expectation, and had just started down the crest of a dike beneath an arcade of trees when two fork-tailed flycatchers scissored the air in front of us. Small and shaded in white, black, and grey, they would be unremarkable creatures if not for their twin tail feathers, long and trailing and endlessly alive to the breeze. The birds can stall in mid-air and dance fearlessly in place, bound by no natural laws, and for that they call to mind paradise. You stop and you stare. They were everywhere along the green relief of the dikes.

I have known people who can’t see a bridge without also seeing in their mind’s eye the blueprint of its structure, or who pass among strangers with the automatic knowledge of what each looks like naked. In that instant in the park, I had this kind of vision for things concealed. What the fork-tailed flycatcher caused me to see was the presence of an absence. The yellow boil of smog subsided, the rooftops shouldering over the canopy faded, and what remained were the flood plains of the silver river, its reedy oxbows and sloughs, its wooded islands, every inch alive with birds and insects and unseen, bustling beasts. Missing from the streets was all of this. This was the understory of Buenos Aires — the place that lived before the living city, before even the first human footfall.

As epiphanies go, it wasn’t a particularly grand one: the presence of absence is an idea dating back at least to Plato. But questions of scale and character lingered. How large an absence were we talking about, exactly? What was its inventory? The meaning of my newfound awareness seemed to depend upon the details. A story of loss is not always and only a lament; it can also be a measure of possibility. What once was may be again.

I began to wonder about the understory of every place I found myself: Manhattan or Malawi, the banks of the Thames in London or the caribou calving grounds of northern British Columbia. The city had lost much that still existed in the country, and the country much that lived on in the wilderness. Might even the wildest backcountry be a ghost of some older nature?

s it turns out, science has quietly begun to consider this question. The field is an emerging one called historical ecology, and two of its key findings are these: first, the harder we look, the more biologically rich the past seems in comparison to today; and second, human impacts on the natural world were more severe and widespread earlier in our history as a species than anyone had guessed.

Last year in this magazine, I wrote about one outcome of these new understandings: an effort to restore a particular tortoise to its former range. Normally, “former” would refer here to habitat lost to the familiar advance of industry, agriculture, and urban sprawl. In this case, the restoration was prehistoric, a return to territory the animal was hunted out of at the dawn of humanity in the Americas, perhaps as many as 10,000 years ago. Seen from that longer perspective, the tortoise, already a certified endangered species, was truly in extremis: a creature reduced to 1 percent or less of its range.

What, then, of the planet as a whole? I speculated in passing that, when seen through the lens of deep time, ours is a 10 Percent World — a blue-green globe that reflects just one-tenth the natural variety and abundance it once did. Within at least my own small universe of crossed paths and conversations, that idea caused something of a stir. Try it yourself: perfect dinner party fodder. There are those, it’s true, who will shrug — too desensitized by the steady stream of bad environmental news to register any shock, even one that metaphorically strips our living earth to a sudden skeleton. Yet the figure is a challenge to every person who has brought home a slide show of a teeming reef from a tropical holiday, everyone who has felt the call of the wild in Banff or Algonquin or on a hunting trip across the northern tundra, all those whose awe for nature has been shaped by Henry David Thoreau or Annie Dillard or the bbc’s Planet Earth series. In the eyes of the historical ecologist, these are not precious windows into the world unspoiled; even our physical and cultural repositories of the wild reveal only fragments of fragments. A maxim of historical ecology is that the earth is nowhere pristine.

A science that amounts to a catalogue of death — the urge to turn and walk away feels something close to hard wired. Yet I have become a dedicated wanderer through these catacombs, because there’s something more where the reach of science ends: a project of the imagination. Study after study in the discipline reports the usual doom and gloom: jungles emptied by bush meat hunters in Equatorial Guinea, the slow fade of British tree sparrows, the vanishing from sight and memory of the pink Chinese river dolphin known as the baiji. Yet within these same studies I find references to “a very different perception of nature,” calls to “visualize previous states” of local ecosystems and “change the perspective of what is possible.” One researcher will say we need new “mental pictures”; another demands new “sea stories.” At stake is a “pending revolution.”

The way you see the natural world determines much about the world you are willing to live in. Among the tangled roots of historical ecology, oddly enough, is a 1995 study in child development and psychology. In it, Peter H. Kahn Jr. and Batya Friedman of Colby College in Maine present the results of interviews on environmental views and values with children from an “economically impoverished inner-city Black community” in Houston, one of America’s more polluted cities. The children clearly understood the idea of pollution in general (one describes a bayou as a place that is “big and long and green and it stinks”), but only one-third reported that environmental issues affected them directly. In an attempt to explain this unexpected outcome, the authors write:

One possible answer is that to understand the idea of pollution one needs to compare existing polluted states to those that are less polluted. In other words, if one’s only experience is with a certain amount of pollution, then that amount becomes not pollution, but the norm against which more polluted states are measured… Indeed, what we perceive in the children we interviewed might well be the same sort of psychological phenomenon that affects us all from generation to generation. People may take the natural environment they encounter during childhood as the norm against which to measure pollution later in their life. The crux here is that with each generation, the amount of environmental degradation increases, but each generation takes that amount as the norm — as the nonpolluted condition. Researching such “generational amnesia” may help provide a psychological account of how it is that our world has moved toward an environmentally precarious state.

You already knew this at some level, assuming you, like everyone, everywhere, have had some sacred play space from your childhood erased to make way for a parkade or a freeway or an American military installation. If you doubt that inner-city kids in Houston can reflect a global state of consciousness, however, look no further than the story of the BP Deepwater Horizon blowout in the Gulf of Mexico. Forgotten amid the images of oil-soaked pelicans and poisoned mangrove forests is the fact that the coast was already in a catastrophic state. How will we measure the restoration of the Gulf shore? On June 15, showing insight lacking in most media reports, President Barack Obama declared that the cleanup would go beyond the “crisis of the moment.” Even before the offshore gusher, he said, the region had suffered “decades of environmental degradation.” He promised a long-term recovery plan that would return the coast and its waters to “normal.”

In ecology, “normal” is a dangerous word. What it means to the Gulf Coast depends on how deeply into generational amnesia you care to probe. Count back through enough decades, let alone centuries, and the potential restoration plan begins to resemble the Labours of Hercules. A number of groundbreaking studies in historical marine ecology have been carried out in the Gulf of Mexico and the adjoining Caribbean Sea. The most visually arresting of them compared big game fishermen’s trophy photos from the 1950s through to ones from the modern day. In the old black and whites, the fish are mainly large groupers and other impressive species, with an average length of nearly a metre; by 2007, the catch is mainly small snappers that measure just a little longer than a grade school ruler. The fishermen look equally pleased with themselves through the years, yet many of the “small” fish of past decades — often literally heaped beneath the hanging trophies — are larger than the “big game” of today.

No surprise, then, that a separate study found that even the healthiest Caribbean reefs are likely home to at least two tonnes less fish per hectare than they once were. To see the reefs in their glory, we’d need to reach back to the seventeenth century, when the waters may have been home to 300,000 Caribbean monk seals (now extinct; the last sighting was in 1952); or centuries earlier still, when as many as 91 million green sea turtles churned the waves (they number fewer than 300,000 today). Yet perhaps the most remarkable research involves two humble varieties of sea sponge, once so significant a part of the aquatic environment that in the first years of the twentieth century some 20,000 tonnes of the living animals were annually hauled ashore in the northern Caribbean alone. In 1939, the wild sponges, decimated for uses ranging from household scrubbing to contraception, succumbed to epidemic disease. Their populations have never recovered. Sponges have a mind-blowing capacity to remove microbes from water; in a single day, a sponge the size of a soccer ball can sieve 90 percent of the bacteria from more water than you will drink in your lifetime. Obviously enough, the loss of the sponges damaged water quality throughout the region, which in turn was linked to a crash in the number of lobsters, and of an economy that sustained thousands of sponge and lobster fishers.