Methane

Methane is the major constituent of natural gas. Methane is a potent green house gas which contributes significantly to global warming. It is produced naturally by decaying vegetation and escapes in substantial quantities from oil and gas production. Vegetation which has been frozen in permafrost may produce methane when it thaws. Herbivores may produce methane during digestion of vegetation as may termites.

Methane leaks
The effectiveness of converting from coal to natural gas depends on the amount of methane which leaks.