The initiative was established by US state governors fed up with what they saw as the Bush administration's inaction on climate change. Last year the initiative set a group-wide greenhouse gas emissions target of 15 percent below 2005 levels by 2020.

"We're proud to be welcomed into this important organization of climate change leaders across Canada and the United States," McGuinty said in a statement. "We all have a part to play in the fight against climate change."

The initiative is expected to announce details in August of a regional "cap and trade" market for emissions of greenhouse gases blamed for global warming.

Cap-and-trade involves setting a cap on industrial emissions, then distributing carbon credits to the participants.

As well as California, US members of the group are Arizona, Montana, New Mexico, Oregon, Utah and Washington.

"Working with our partners, California can achieve nearly double the reductions in global warming pollution than if we go at the problem alone," said California Governor Arnold Schwarzenegger. "Ontario is doing some great things to combat global warming, and I welcome them."

McGuinty, a strong critic of the federal Canadian plan to fight climate change, last month agreed in principle with Quebec to set up a market-based system to curb greenhouse gases.

Under the Kyoto Protocol, Canada agreed to cut emissions by six percent from 1990 levels by 2012. It is well above that target and the federal Conservative government says it cannot meet the Kyoto goal.

Last year Ottawa introduced a plan it said would cut emissions by 20 percent from 2006 levels by 2020, a target critics say is hopelessly inadequate. (Reporting by David Ljunggren in Ottawa and Timothy Gardner in New York; Editing by Peter Galloway)