megatonne

The megatonne earns its keep in climate accounting. National greenhouse gas inventories submitted under the UN climate framework are stated in Mt of CO₂ equivalent per year: Canada reports roughly 700 Mt, and the largest single coal plants emit tens of Mt a year on their own, Bełchatów in Poland having topped European lists at around 30 Mt, which is why individual facilities are visible line items in a national inventory.

The factor buried inside that accounting is 44/12. Emissions are tracked sometimes as megatonnes of carbon and sometimes as megatonnes of CO₂, and the same combustion weighs 3.67 times more as CO₂ than as C, because each carbon atom picks up two oxygen atoms on the way out of the stack. A global figure of 10,000 Mt of carbon and one of 36,700 Mt of CO₂ describe the same year, and which convention a paper uses is the first thing to check and the easiest to miss.

Commodities use the unit at the same scale: world crude steel production runs about 1,900 Mt a year and cement about 4,100 Mt, numbers that anchor most comparisons of what civilisation is actually made of. The mass converters here move Mt to short and long tons for trade statistics, and the caution on this page about the megaton of TNT stands: same two letters, entirely different physical quantity.