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The Obama administration will announce a new pollution rule on Monday that limits the carbon dioxide emitted from power plants, the largest source of pollution blamed for global warming. It’s poised to be one of the most significant U.S. environmental regulations in years, though it’s expected to provoke a drawn-out fight with states and electricity companies. “There are no national limits to the amount of carbon pollution that existing plants can pump into the air we breathe. None,” Obama said on Saturday in his weekly address. “We limit the amount of toxic chemicals like mercury, sulfur, and arsenic that power plants put in our air and water. But they can dump unlimited amounts of carbon pollution into the air. It’s not smart, it’s not safe, and it doesn’t make sense.”