The Earth has heated up and cooled down repeatedly, according to scientists. And we believe them— but how do they actually know? We’ll tell you how scientists know, today, that an Ice Age occurred millennia ago.

You’re drinking the evidence that scientists use to track climate change. Each molecule of water contains one atom of oxygen. That atom of oxygen can come in one of two different isotopes. The first isotope is oxygen 16, which incorporates eight protons and eight neutrons. Oxygen 18 contains eight protons and ten neutrons.

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Of the two, oxygen 18 is heavier, which means it is harder to accelerate to the point where it stops being liquid water and starts being vapor in the atmosphere. This is true whether it’s in a pan on your stove or in the ocean. Take any body of water, heat it up, and the vapor formed from the heat will contain a higher proportion of oxygen 16 than the liquid. Meanwhile, the liquid will contain more and more heavy oxygen 18. Evaporation essentially enriches the oxygen 18 content of a body of water, and the oxygen 16 content of the steam evaporated from that water.

If the water vapor all ended up right back in the ocean, the ratio of isotopes in the ocean wouldn’t ever really change. Isotope separation would be a temporary state. However, the polar ice caps mean that not everything that evaporates out of the ocean goes back into it—at least not immediately.

Not only does the water with the oxygen 18 lag behind when it comes to evaporating, it is the first to return to the ocean as rain. Moisture tends to move from warmer areas toward the colder poles. On the way, oxygen 18 preferentially rains down out of the sky, especially when temperatures are cool on its trip towards the pole. The water vapor that eventually falls on the ice caps has a very high ratio of oxygen 16. So ocean sediment and fossils filled with oxygen 18 and ice core samples filled with oxygen 16 indicate ice ages.

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As ice caps melt, oxygen 16 comes pouring out into the oceans. The levels of oxygen 16 in a layer of ocean debris and fossils go up as temperatures rise. So a high level of oxygen 16 in sediment from one layer tells scientists that the planet was hot and watery when the sediment was laid down.

Image: Charles Robert Knight