US President Barack Obama has unveiled what he called "the biggest, most important step we have ever taken" in tackling climate change.
Mr Obama said the world may not be able to reverse global warming if aggressive action to stop it was not taken.
He rejected criticism that his plan would increase energy bills for Americans, hurt the poor, and cost jobs.
"This is the right thing to do," he told an audience in Washington.
"No challenge poses a greater threat to our future and future generations than a change in climate," he said, warning: "There is such a thing as being too late."
"Most of the time, the issues we deal with are ones that are temporally bound and we can anticipate things getting better if we plug away at it, even incrementally," he said in the East Room of the White House.
"But this is one of those rare issues, because of its magnitude, because of its scope, that if we don't get it right, we may not be able to reverse. And we may not be able to adapt sufficiently."
As a step to try to adapt, Obama announced a revised Clean Power Plan under which power plant owners must cut carbon dioxide emissions by 32 percent from 2005 levels by 2030.
The measures will place significant emphasis on wind and solar power and other renewable energy sources.
However, opponents in the energy industry have vowed to fight the plan.
They say Mr Obama has declared "a war on coal". Power plants fired by coal provide more than a third of the US electricity supply.
The President's new plan comes at a crucial time in the run-up to Paris, where representatives from around the world will gather in December to hash out rules designed to limit global temperature increases to 2°C.
The UN is struggling to cut down a negotiating text that still runs past 80 pages, and only 49 countries have so far made public their declarations. The US Clean Power Plan will be a boost to the process and to efforts to get the others to declare their hands.
An environmentalist in New Zealand said the Clean Power Plan was one of the most significant steps ever taken in combating climate change.
Victoria University professor James Renwick said the United States, along with China, was one of the two biggest emitters of greenhouse gases in the world, and this was a necessary step.
"If it's followed through on, then it is a significant step forward," he said,
"It's one of the first times a major emitter has really committed to reducing their emissions in any sector."
Dr Renwick said the plan could very well be defeated should the incumbent Democratic Party lose next year's presidential election.
- BBC / AFP / Reuters / RNZ