Weight of emissions

WE all know by now that gaseous emissions (mainly CO2) are the principal cause of global warming.

The emission of greenhouse gases is referred to in tonnes, kilos or some other measure of mass (weight). How are such gases weighted? May we please have an explanation of the scientific procedure used to measure the ‘weight’ of emission from my car when I drive from A to B and similarly from aircraft engines, factories, chimneys, etc. I suspect not everyone responsible for such emissions has the slightest idea how the quantity of gases is weighed.

PG Martin