Good q&a piece by the New York Times. The question: “How do they weigh greenhouse gas emissions?”. The answer: follow the link.
New York Times: While it is possible to weigh a quantity of gas, by comparing the weight of an evacuated container to one filled at a known pressure, climate scientists do not rely on direct measurements. Instead, they use estimates based on the molecular weight of carbon dioxide; the weights of other greenhouse gases are converted to their greenhouse impact as compared with that of a ton of carbon dioxide.