When plastic is exposed to sunlight, it releases methane and ethylene. Both gases contribute to global warming. The biggest culprit is a type of plastic called LDPE.

….“It is the type of plastic that is the most produced, consumed, and discarded in the environment. For example, the rings around the beer cans, some forks and knives that are being used in restaurants, some kinds of plastic bags as well.”

…The emissions increase over time as the plastic breaks down in the environment. As it forms smaller and smaller particles with more surface area, more gases are released.

Common plastics emit global warming pollution » Yale Climate Connections

sigh…