Most modern landfills have power stations that burn off these gases for electricity or heat for local consumption. The ones that don't can be GHG emitters, but

Most modern landfills have power stations that burn off these gases for electricity or heat for local consumption. The ones that don't can be GHG emitters, but that just means there are less emissions somewhere else (where trash was collected from), because you just concentrated a bunch of matter in one place. It's not a significant net emitter.