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This is Scientific American 60-second Science. I'm Julia Rosen.

We're hearing a lot of the arguments against action on reducing CO2 being based on, well, CO2 was higher in the past, so we don't have to worry about it.

Gavin Foster, a geochemist at the University of Southampton.

But Foster says that's a flawed argument.

For starters, just how far back in time do you have to look to find CO2 concentrations like what we expect to see in the

future? And does it even make sense to compare the levels now and then? To answer these questions,

Foster and his colleagues reconstructed the history of atmospheric carbon dioxide for the last 420 million years.

They compiled roughly 1,500 estimates of CO2 concentrations from 112 previous studies.

When the researchers combined these data, they found that atmospheric carbon dioxide went up and down over time,

but that in general it gradually declined from almost 3,000

parts per million, down to less than 300 parts per million before humans started burning fossil fuels.

However, we've already started to reverse that trend.

If we continue on a business-as-usual scenario, by the middle of this century, CO2 could reach levels not seen in 50 million years,

according to Foster's reconstruction. That's long before humans evolved, back when the climate

was much warmer and there were no large ice sheets at the poles. If we continue on that trajectory,

by the year 2250, concentrations could approach what they were in the Triassic, 200 million years ago, when dinosaurs roamed the Earth.

But greenhouse gases aren't the only factor impacting Earth's climate.

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