The World Without Us

Dioxins, however, were unintended: they’re by-products formed when hydrocarbons combine with chlorine, with tenacious, disastrous results. Besides their role as sex-changing endocrine disruptors, their most infamous application before being banned was in Agent Orange, a defoliant that laid bare entire Vietnamese rain forests so that insurgents would have nowhere to hide. From 1964 to 1971, the United States doused Vietnam with 12 million gallons of Agent Orange. Four decades later, heavily dosed forests still haven’t grown back. In their place is a grass species, cogon, called one of the world’s worst weeds. Burned off constantly, it keeps springing back, overwhelming attempts to supplant it with bamboo, pineapple, bananas, or teak.

Dioxins concentrate in sediments, and thus show up in Rothamsted’s sewage sludge samples. (Municipal sludge, since 1990 deemed too toxic to dump into the North Sea, is instead spread as fertilizer on European farmlands—except in Holland. Since the 1990s, the Netherlands has not only offered incentives that practically equate organic farming with patriotism, but has also struggled to convince its EU partners that everything applied to the land ends up in the sea anyway.)

Will the future visitors who discover Rothamsted’s extraordinary archive wonder if we were trying to kill ourselves? They might find hope in the fact that, beginning in the 1970s, lead deposition in soil waned significantly. But at the same time, the presence of other metals was increasing. Especially in preserved sludge, they would find all the nasty heavies: lead, cadmium, copper, mercury, nickel, cobalt, vanadium, and arsenic, and also lighter ones like zinc and aluminum.

3. The Chemistry

Dr. Steven McGrath hunches over his corner computer, deep-set eyes beneath his gleaming pate crinkling through rectangular reading lenses at a map of Britain and a chart color-coded with things that on an ideal planet— or one that gets the chance to start over—wouldn’t show up in plants that animals like to eat. He points at something yellow.

“This, for instance, is the net accumulation of zinc since 1843. No one else can see these trends because our samples,” he adds, his shirtfront slightly inflating, “are the longest test archive in the world.”

From sealed samples of a winter-wheat field called Broadbalk, one of Rothamsted’s oldest, they know that the original 35 parts per million of zinc present in the soil have nearly doubled. “That’s coming from the atmosphere, because our control plots have nothing added—no fertilizers, no manure or sludge. Yet the concentration is up 25 ppm.”

The test farm plots, however, which also originally had 35 ppm of zinc, now are at 91 ppm. To the 25 ppm from airborne industrial fallout, something is adding another 31 ppm.

“Farmyard manure. Cows and sheep get zinc and copper in their animal feed to keep them healthy. Over 160 years, it’s nearly doubled the zinc in the soil.”

If humans disappeared, so would zinc-laced smoke from factories, and no one would be feeding mineral supplements to livestock. Yet McGrath expects that, even in a world without people, metals we put into the ground will be around a long time. How long before rain leaches them out, returning soils to a preindustrial state, would depend, McGrath says, on their composition.

“Clays will hang onto them up to seven times as long as sandy soils, because they don’t drain as freely.” Peat, also poorly drained, can retain lead, sulfur, and organochloride pollutants like dioxin even longer than clay. McGrath’s maps show hot clusters on peat-covered hilltops on the English and Scottish moors.

Even sandy soils can bind nasty heavy metals when municipal sludge is mixed into them. In sludged earth, leaching of metals drops as chemical bonds form; extraction is mainly via roots. Using archived samples of Rothamsted carrots, beets, potatoes, leeks, and various grains treated since 1942 with West Middlesex municipal sludge, McGrath has calculated how long metals we’ve added to such soil will stay there—assuming crops are still being harvested.

From a file drawer, he produces a table that gives the bad news. “With no leaching, I figure zinc lasts 3,700 years.”

That’s how long it took humans to get from the Bronze Age to today. Compared to the time other metallic pollutants would linger, that turns out to be short. Cadmium, he says, an impurity in artificial fertilizer, will cling twice as long: 7,500 years, or the same amount of time that’s passed since humans began irrigating Mesopotamia and the Nile Valley.

It gets worse. “Heavier metals like lead and chromium tend not to be taken up as easily by crops, and not to be leached. They simply bind.” Lead, the one with which we’ve most recklessly laced our topsoil, will take nearly 10 times as long as zinc to disappear—the next 35,000 years. Thirty-five thousand years ago was a couple of ice ages back.