The WCI began in February 2007 when the Governors of Arizona, California, New Mexico, Oregon, and Washington signed an agreement directing their respective states to develop a regional target for reducing greenhouse gas emissions, participate in a multi-state registry to track and manage greenhouse gas emissions in the region, and develop a market-based program to reach the target. 

The WCI built on existing greenhouse gas reduction efforts in the individual states as well as two existing regional efforts. In 2003, California, Oregon and Washington created the West Coast Global Warming Initiative, and in 2006, Arizona and New Mexico launched the Southwest Climate Change Initiative. 

The Premiers of British Columbia, Manitoba, Ontario, and Quebec, and the Governors of Montana and Utah have since joined the original five states in committing to tackle climate change at a regional level.