Which Came First: Low CO2 or an Ice Age?

This is Scientific American 60-Second Earth. I'm David Biello. Your minute begins now.
What starts an Ice Age?
Clues exist in the remains of coccolithophores, a type of marine algae with a shell.
A study finds that some seven million years ago, the algae had to adapt to low levels of carbon dioxide in the atmosphere.
They pulled CO2 from the surrounding seas for photosynthesis as well as bicarbonate, commonly known as baking soda.
The study is in the journal Nature.
At that same time, sea surface temperatures were dropping,
plants that were more efficient at using CO2 came to predominate on land and vast glaciers began to expand on the continents, an Ice Age was underway.
The low concentrations of greenhouse gases in the atmosphere were thus linked to the era's cool climate.
That situation is now reversed thanks mostly to fossil fuel burning.
And the change is happening at least 30,000 times faster this time.
In May, atmospheric concentrations of CO2 touched 400 parts-per-million for the first time in human existence.
When they touch 500 ppm, the algae might no longer need the bicarbonate trick.
Your minute is up, for Scientific American 60-Second Earth. I'm David Biello.