Deep inside landfills, bacteria decompose organic wastes in a low oxygen environment. this anaerobic decomposition results in a mix of gases called landfill gas, which consists of about 50%

Methane

Explanation:

Methane is a greenhouse gas considered more potent than carbon dioxide in creating a greenhouse effect. Landfills also produce carbon dioxide gas. Therefore, measures are usually taken on landfills to prevent global warming. Such measure includes; capturing and collection of the gas and the use of methanotrophs bacteria on the soil over the landfills to convert the produced methane into organic compounds.  

not only is it colder, it also becomes darker, less salty, and more dense.