In an article from Discover magazine.
Titled: Earth Without People
What would happen to our planet if the mighty hand of humanity simply disappeared?
By Alan Weisman
DISCOVER Vol. 26 No. 02 | February 2005 | Environment
The writer suggests that we have some information that can at least suggest what would happen. Specifically he points to Korea, in the 155-mile-long, 2.5-mile-wide mountainous Demilitarized Zone, or DMZ, set up by the armistice ending the Korean War. Aside from rare military patrols or desperate souls fleeing North Korea, humans have barely set foot in the strip since 1953. Before that, for 5,000 years, the area was populated by rice farmers who carved the land into paddies.
Using this as the timelline, he furthers identifies that a "new wilderness would consume cities, much as the jungle of northern Guatemala consumed the Mayan pyramids and megalopolises of overlapping city-states. From A.D. 800 to 900, a combination of drought and internecine warfare over dwindling farmland brought 2,000 years of civilization crashing down. Within 10 centuries, the jungle swallowed all."
But note, that this refers to pretty well built buildings, while much of modern city architecture is pretty ephemeral.
"There’s little soil to absorb it [groundwater] or vegetation to transpire it, and buildings block the sunlight that could evaporate it. With the power off, pumps that keep subways from flooding would be stilled. As water sluiced away soil beneath pavement, streets would crater. Within 20 years, the water-soaked steel columns that support the street above the East Side’s subway tunnels would corrode and buckle, turning Lexington Avenue into a river.
New York’s architecture isn’t as flammable as San Francisco’s clapboard Victorians, but within 200 years tons of leaf litter would overflow gutters as pioneer weeds gave way to colonizing native oaks and maples in city parks. A dry lightning strike, igniting decades of uncut, knee-high Central Park grass, would spread flames through town.
Unless an earthquake strikes New York first, bridges spared yearly applications of road salt would last a few hundred years before their stays and bolts gave way (last to fall would be Hell Gate Arch, built for railroads and easily good for another thousand years). Coyotes would invade Central Park, and deer, bears, and finally wolves would follow. Ruins would echo the love song of frogs breeding in streams stocked with alewives, herring, and mussels dropped by seagulls. Missing, however, would be all fauna that have adapted to humans. The invincible cockroach, an insect that originated in the hot climes of Africa, would succumb in unheated buildings. Without garbage, rats would starve or serve as lunch for peregrine falcons and red-tailed hawks. Pigeons would genetically revert back to the rock doves from which they sprang.
Old stone buildings in Manhattan, such as Grand Central Station or the Metropolitan Museum of Art, would outlast every modern glass box, especially with no more acid rain to pock their marble. Still, at some point thousands of years hence, the last stone walls—perhaps chunks of St. Paul’s Chapel on Wall Street, built in 1766 from Manhattan’s own hard schist—would fall. Three times in the past 100,000 years, glaciers have scraped New York clean, and they’ll do so again. The mature hardwood forest would be mowed down. On Staten Island, Fresh Kills’s four giant mounds of trash would be flattened, their vast accumulation of stubborn PVC plastic and glass ground to powder. After the ice receded, an unnatural concentration of reddish metal—remnants of wiring and plumbing—would remain buried in layers. The next toolmaker to arrive or evolve might discover it and use it, but there would be nothing to indicate who had put it there.
THE WILDS OF NEW YORK
If humans were to vanish from New York, how soon would nature take over? Scientists predict that within . . .
• 10 YEARS Sidewalks crack and weeds invade. Hawks and falcons flourish, as do feral cats and dogs. The rat population, deprived of human garbage, crashes. Cockroaches, which thrive in warm buildings, disappear. Cultivated carrots, cabbages, broccoli, and brussels sprouts revert to their wild ancestors.
• 20 YEARS Water-soaked steel columns supporting subway tunnels corrode and buckle. Bears and wolves invade Central Park.
• 50 YEARS Concrete chunks tumble from buildings, whose steel foundations begin to crumble. Indian Point nuclear reactors leak radioactivity into the Hudson River.
• 100 YEARS Oaks and maples re-cover the land.
• 300 YEARS Most bridges collapse.
• 1,000 YEARS Hell Gate Bridge, built to bring the railroad across the East River, finally falls.
• 10,000 YEARS Indian Point nuclear reactors continue to leak radioactivity into the Hudson River.
• 20,000 YEARS Glaciers move relentlessly across the island of Manhattan and its environs, scraping the landscape clean.
