While annual emissions is the most commonly used measure of a country’s carbon footprint, the figure omits important factors such as population size and industrial history.
This issue is exemplified when comparing the emissions of China and the U.S. According to a 2017 study, China emits 30% of global greenhouse gasses while the U.S. emits 15%. However, these numbers do not reflect that China's population of 1.4 billion is four times larger than the U.S.'s population of 328 million, rendering China responsible for less than half the emissions per capita of the U.S.
The annual emissions metric also does not reflect countries’ historical emissions. Our World in Data estimated that between 1750-2019, the U.S. has produced nearly twice as many greenhouse gases (405 billion metric tons) as China (210 billion metric tons).