1.2 cubic kilometres is definitely more intuitively visualized, but I'd prefer it taken even further.
"Enough CO2 to blanket a 35km by 35km area at 1 metre deep."
or
"Enough CO2 that, settled at 1 metre deep, it would blanket the land to nearly 20km away."
I think either really paints a more easily understood picture of what a colossal amount of gas this actually is.
According to the article, you could actually stand up to live if you were far enough from the lake. People lying on the floor, on the other hand, died.
"Enough CO2 to blanket a 35km by 35km area at 1 metre deep." or "Enough CO2 that, settled at 1 metre deep, it would blanket the land to nearly 20km away."
I think either really paints a more easily understood picture of what a colossal amount of gas this actually is.