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Without any CO2 in the atmosphere, the Earth would be a frozen planet.

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CO2 molecules can reflect heat.

Image by UCAR | visit website for more information

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This creates a warming blanket around Earth (greenhouse effect).

Now that we know how CO2 levels influence temperature, let's take another look at our graph.

Place the temperatures at the right CO2 level

Source: NASA and this really cool graphic

Our average temperature today is also 14°C. Making it four degrees colder doesn't seem like a big deal.

 

But 4 degrees is the difference between a very pleasant Berlin and a Berlin at the edge of a monstrous glacier that covered a third of Europe.

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This is definitely not completely accurate. The ice is a picture of the Perito Moreno glacier in Argentina. I've done my best to scale it: The Reichstag building is 48 m high, the highest columns of the glacier about 70 m. The real glacier would probably be less steep at its end.

A map of the last ice age from Wikipedia:

A very cool picture by Randall Munroe at https://xkcd.com/

Source of Ice age maps | Photo glacier by Time Travel Turtle

So... how do our human CO2 emissions impact temperatures?