Seven Western states and four Canadian provinces are proposing a comprehensive program to cut greenhouse gas emissions from power plants, manufacturers and vehicles.
The Western Climate Initiative would establish a regional market to trade carbon emissions and is designed to keep costs down for those affected.
The plan is aimed at cutting the region's carbon emissions below 2005 levels by 2020.
The plan affects Arizona, California, Montana, New Mexico, Oregon, Utah, Washington and the four Canadian provinces of British Columbia, Manitoba, Ontario and Quebec.
Each state and province must approve the plan.