This historic policy will ultimately save 1.8 billion barrels of oil and reduce greenhouse gas emissions by nearly one billion metric tons — the amount of greenhouse gas emissions from 50 million of today's cars over their lifetime.

The following is a statement by Earthjustice senior legislative representative Sarah Saylor:

"Today the historic clean cars rule announced in April goes into effect. Thanks to the strength of the 40-year-old Clean Air Act and the leadership of the Obama administration, soon the people of America will begin saving money at the gas pump, their cars will more efficient and less polluting, and our economy and environment will be all the better for it.

"The Clean Air Act and the EPA's ability to fight climate change are critical and necessary for both the health of the American public and the growth of the US economy.

"But we must be clear: The clean cars rule is a great start, but alone it is nowhere near enough to protect us from climate change, possibly the greatest challenge of our time.

"In order to confront this challenge, we simply must use all available tools. Both the EPA and the Senate have a role to play. Our leaders in Congress — the Senate especially — must follow the administration's lead and craft and pass strong climate change and energy legislation that complements the actions taken by EPA under the Clean Air Act.

"To be sure, the fight continues. We need to keep fighting to defend and bolster the Clean Air Act protections that are now in place, and we must not take them for granted."