C.S. Lewis once remarked: “New York is lovely, it will make a nice ruin some day.” This got me thinking . . .
If all human life ceased to exist (insert germ warfare scenario here), and assuming that whatever wiped out the humans spared other mammals, how long would it take before the earth returned to a healed state? i.e. how long before all human-made structures broke down into their trace elements, leaving no trace of human life, save fossilized remains.
Surely all concrete, steel, glass, refined petroleum, uranium etc has a shelf-life? Would there ever be a time when no evidence existed that humans ever walked the earth? And on a related note, in 500 million years, will continental drift have radically changed the surface of the earth?
I remember an interesting article in Smithsonian Magazine about ten years ago with this very topic. They asked what would happen to man’s monuments (roads, buildings, bridges, etc.) if humans suddenly left the Earth. I remember that the article was quite interesting. I believe that buildings such as the World Trade Center would last nearly a century while structures like surface freeways would last nearly forever. Of course, some things would become overgrown with vegatation before the artifact itself was gone.
Sorry I can’t remember the specific date of the magazine, but you might try the library. Surely they have an index or something.
Jan Zalasiewicz, a geologist, also wrote a piece along these lines for New Scientist magazine a couple of years back. You can access his article by signing up for a free 7-day trial at http://www.newscientist.com In a nutshell, he says that what gets preserved (fossilized) or destroyed depends in large part on how quickly things get buried - no different from dinosaurs or anything else we have as fossil remains today. Structures underground are most likely to remain intact longest, although they may suffer compaction and remineralization. In his view, there will be long-lasting remains of human activity, some of which will still be recognizable, but not evenly distributed over the planet’s surface.
He’s of course looking at this issue from a geological time perspective (millions of years in the future). On a shorter time scale, I suppose we can look at the ruins of some of the great civilizations of the past for guidance; again, whether or not something hangs around will depend on the local climate and vegetation. Stone ruins in jungle settings - like Mayan structures or the temple at Angkor Wat - have been abandoned for less than a thousand years, but were completely grown over, a bit crumbly around the edges and largely forgotten until Westerners started mucking about. OTOH, stone ruins in desert settings (e.g., the Great Pyramid) tend to hold up pretty well. How well will glass, concrete, and other modern bits of construction hold up with time if left at the surface? Hopefully a civil engineer will weigh in here; my WAG is that, in general, a few thousand years would be enough to knock much of it down.
On the related note: 500 million years is a lot of time to shuffle the continents around, so yes, the Earth will look mighty different. For one possible way the Earth might appear just 250 million years in the future, have a look at this site.
Since the scientists can currently tell bacteria existed from 3/4 of the earth’s age or longer, they certainly could tell if humans existed for longer than that. Expecially with hundreds of thousands buried in catacombs and mine caveins world wide.