VANCOUVER, British Columbia (Reuters) - A coalition of U.S. states and Canadian provinces that have banded together to cut greenhouse gases will launch their carbon cap and trade system in 2012, according to a draft plan released on Wednesday.

The Western Climate Initiative's system will be phased in starting with industrial process emissions, with emissions from transportation and other fuels added to the system in 2015. It also will include emissions from electricity imported from sources outside of the group.

The WCI launched in 2007 has set a target of cutting greenhouse gas emissions to 15 percent below 2005 levels by 2020.

Cap and trade systems set limits on the emissions of gases such as carbon dioxide and methane linked to global warming, but allow major polluters to comply by purchasing offsetting credits from sellers who have not used their total emission allowances.