In The World Without Us, Alan Weisman conducts a long thought experiment to examine this very question.
Weisman says that our homes and our cities would start to be overcome by nature almost instantly. In fact, if it weren't for the pumps that keep the water out of the subway tunnels in New York City, they would be underground rivers. So the subway tunnels would fill up with water and New York would start to crumble in 15-20 years.
But that's not all. The chemicals that we started using on our plants to make food are going to stay in the soil for a long time. And aside from nuclear waste, which will never go away, the other big thing that will never really biodegrade is plastic.
Finally, if humans went away tomorrow, how long would it take for the carbon in the atmosphere to return to pre-human levels? According to Weisman:
Well, carbon dioxide has . . . the amount of it in the atmosphere has varied in the past, depending on what was going on geologically. Now, that said, there are three ways that carbon get absorbed, and the ocean is one of the most significant.
The ocean turns over every thousand years, so in the first thousand years, if the ocean couldn't get it all, it would get a good percentage — perhaps up to 80 percent of the carbon that we've overloaded into the atmosphere. And in the first few centuries, a lot of that would go. But to get back to where we were before we started tinkering with the atmosphere, think 100,000 years.
So 1000 years after people leave the planet, will there just be nuclear waste, plastic and cockroaches? Sounds like it....