Where did all the carbon dioxide go?

As Figure 5.2.1–5 shows, the three most common gases released by early volcanoes were water vapor (H₂O), carbon dioxide (CO₂), and nitrogen (N₂). Earth’s atmosphere today still contains a lot of nitrogen, and we know that the water vapor condensed to form the oceans. But today’s atmosphere is much less than 1% carbon dioxide (though this small amount is still very important to Earth’s climate, as we’ll discuss in Chapter 7), which means most of the carbon dioxide that once made up the early atmosphere has somehow disappeared. Where did it all go?

The answer is that carbon dioxide can be dissolved in water (as you know from carbonated drinks), and once it got dissolved in the oceans, chemical reactions could gradually transform it into what we call “carbonate rocks,” such as limestone. Earth today has a lot of these rocks. In fact, when scientists add up all the carbon dioxide that is now “locked away” in rocks, they find there is close to 200,000 times as much carbon dioxide trapped in rocks as there is in the atmosphere.

To summarize, Earth’s volcanoes released an enormous amount of carbon dioxide through outgassing in Earth’s early history, but almost all of this carbon dioxide dissolved in the oceans and then got locked away in carbonate rocks. That is why only a small amount of carbon dioxide remains in the atmosphere today. As we’ll discuss in Chapter 7, this is very fortunate; if the carbon dioxide had remained in the atmosphere, our planet would be as hot as Venus.