1. When methane is burned, the carbon dioxide emissions are about 30% lower per unit energy than when liquid fuels are burned (50 g/MJ for methane, vs abt 74 g/MJ for liquid fuels) 2. When LNG is produced, an additional ~10% CO2 is generated, via energy consumption in the gas liquefaction plant 3. Methane leakage from production plant and pipelines is estimated at about 3% of total. This adds a greenhouse gas effect of about 63% (since methane is ~21 times more potent than CO2, greenhouse-effect-wise). 4. Pollutants such as sulfur oxides and soot from natural gas combustion are essentially nil. Heavy fuel oil for ships can have a very high sulfur content.

Sooo... it is quite possible that natural gas use has a higher climate change impact, due to leakages. For other pollutants, natural gas produces less.