Researchers at ETH Zurich used a model of trade between the world's regions to follow plastics through global supply chains and calculate their full carbon and health footprint from 1995 to 2015.
They found plastics were responsible for 2 billion tonnes of greenhouse gases in 2015, measured as the amount of carbon dioxide (CO2) that would have the same warming effect. That was 4.5% of global greenhouse-gas emissions, roughly double the 1995 footprint. Emissions from coal quadrupled over the period, because plastics production shifted to China, India and other coal-reliant economies. Coal burning for plastics also worsened air pollution and its health effects.
The study matters because it shows that where plastic is made, and with what energy, drives its climate impact, and that the shift to Asia has made plastics dirtier per tonne.
Models of this kind share out emissions across supply chains statistically, so figures for individual products are approximate.