I guess I'm confused on the definition of a "tonne" of CO2. Am I to believe that if that cube was completely full of CO2 that volume of CO2 would weigh 1000kg?
Nevermind, just looked it up. It's actually a measure of volume, just 1000 cubic meters, which makes perfect sense.
Edit: it was actually the first one, although a "tonne" as a measure of volume does exist.