Methane is a hydrocarbon, the main component of natural gas and among the six greenhouse gases to be curbed under the Kyoto protocol. As a greenhouse gas it is estimated to have a warming effect about 25 times as great as CO2. (Compared to CO2 its effects are greater but last for a shorter period). As a fuel, it is used in electricity generation and in the form of compressed natural gas it can be used as vehicle fuel. Methane is produced during the decomposition of many materials in landfill, while about 16% of methane emissions are caused by livestock's digestive processes.