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The One Basic Fact About History That Time Travelers Always Forget

Annalee Newitz

One of the staples of time travel stories is the idea that our heroes will visit earlier geological periods in Earth's history. Of course — because they'll want to see dinosaurs with ostrich feathers and giant millipedes. But they generally forget something very important about Earth's past.

If you're going back more than 30 million years, you'll probably need to bring an atmosphere suit.

As you've probably heard, the Earth's atmosphere can change a lot in a pretty short period of time. Since the mid-eighteenth century, levels of carbon dioxide have risen substantially in the Earth's atmosphere, from 280 parts per million (ppm) to 395 — the result of industrial processes that have released tons of carbon into the environment. And in previous eras, the fluctuations have been much greater than that.

Considering that the Earth is about 4.5 billion years old, you'd expect there would be a lot of times to visit. The problem? Your body is most comfortable with today's atmospheric concentration of oxygen, which makes up about 21 percent of the gasses in our atmosphere. Unfortunately, for most of those 4.5 billion years the Earth has been around, the air had no oxygen in it at all. In fact, if you wanted to visit Earth any time before the past half-billion years, you'd going to need serious breathing equipment.


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