Adam Frank, writing for The Atlantic: We're used to imagining extinct civilizations in terms of the sunken statues and subterranean ruins. These kinds of artifacts of previous societies are fine if you're only interested in timescales of a few thousands of years. But once you roll the clock back to tens of millions or hundreds of millions of years, things get more complicated. When it comes to direct Evidence of an industrial civilization -- things like cities, factories, and roads -- the geologic record doesn't go back past what's called the Quaternary period 2.6 million years ago. For example, the oldest large-scale stretch of ancient surface lies in the Negev Desert. It's "just" 1.8 million years old -- older surfaces are mostly visible in cross section via something like a cliff face or rock cuts. Go back much farther than the Quaternary and everything has been turned over and crushed to dust. And, if we're going back this far, we're not talking about human civilizations anymore. Homo sapiens didn't make their appearance on the planet until just 300,000 years or so ago. [...] Given that all direct evidence would be long gone after many millions of years, what kinds of evidence might then still exist? The best way to answer this question is to figure out what evidence we'd leave behind if human civilization collapsed at its current stage of development. Mr. Frank, along with Gavin Schmidt, Director of the NASA Goddard Institute for Space Studies, have published their research on the subject [PDF].
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