Plastic is made from petrochemicals, sourced from fossil fuels, in highly energy and emission intensive processes. Incineration of plastic or decomposition by sunlight or bacteria takes hundreds of years and leads to emissions of carbon from the fossil fuel feedstock which was formerly locked into the plastic products.
In the environment, be it in landfills or by littered plastic all over the world, even more potent greenhouse gases, namely methane and ethylene are released due to solar radiation on plastic material.
Source: Swiss Re