WASHINGTON • United States President Barack Obama has described climate change as the toughest and most pressing challenge of our time as he unveiled the first-ever limits on US power plant emissions.

"No challenge poses a greater threat to our future and future generations than a change in climate," he said. He warned: "There is such a thing as being too late."

In an attempt to at least try to slow the process, Mr Obama announced on Monday that power plant owners must cut carbon dioxide emissions by 32 per cent from 2005 levels by 2030.

Electric power plants account for some 40 per cent of US emissions of carbon dioxide, the most common greenhouse gas that contributes to climate change.

Mr Obama described the move as "the single most important step America has ever taken in the fight against global climate change".

His Clean Power Plan, which also mandates a shift to renewable energy from coal-fired electricity, is meant to put the US in a strong position at international talks in Paris later this year on reaching a deal to curb global warming.

The US has pledged to slash its greenhouse gas emissions across the economy by 26 to 28 per cent below 2005 levels by 2025.

Cuts from the high-emitting power sector will be critical to the effort, and the White House hopes this plan will prove Washington's commitment to that goal as it prods other countries to follow suit.

Mr Obama is enacting the plan by executive order, bypassing Con-gress, which rejected legislative attempts to reduce pollution from carbon dioxide, which is blamed by scientists for heating the Earth.

The regulations face certain legal challenges from states and industries, and their long-term fate depends on their ability to withstand such challenges.

"We're the first generation to feel the impact of climate change. We're the last generation that can do something about it," Mr Obama told a sympathetic audience at the White House.

"We only get one home. We only get one planet. There's no plan B."

The Clean Power Plan is intended to be a key part of the US President's legacy on global warming, which he pledged to fight as a candidate for the White House in 2008.

The effort also sets up climate change as a political lightning rod in the 2016 presidential election.

Former secretary of state Hillary Clinton, the front runner for the Democratic nomination, said she would defend the plan, while Mr Jeb Bush, one of a slew of Republicans running for their party's nomination, said it would cost people jobs.

Other leading Republicans also stressed what they said were the costs to the economy from the plan. Senate Majority Leader Mitch McConnell, who represents coal-rich Kentucky, said the new rules would shutter power plants and drive up electricity costs.

"Not only will these massive regulations fail to meaningfully affect the global climate, but also they could actually end up harming the environment by outsourcing energy production to countries with poor environmental records like India and China," he told reporters. "I will do everything I can to stop it."

The Republican Speaker of the House of Representatives, Mr John Boehner, called the plan an "energy tax".

"I believe this final plan is an expensive, arrogant insult to Americans who are struggling to make ends meet," he said.

Mr Obama rejected criticism that his plan would increase energy bills for Americans and hurt the poor, saying: "If you care about low-income, minority communities, start protecting the air that they breathe."

The revamped Clean Power Plan gave a boost to renewable energy, which would account for 28 per cent of the US energy mix by 2030 from just under 10 per cent currently if enacted.

Mr Obama said he would spend much of this month talking about climate change. Later this month, he will become the first US President to visit the Alaskan Arctic.

"Our fellow Americans have already seen their communities devastated by melting ice and rising oceans," he said.

Next month, when the US President hosts Pope Francis, the leader of the Catholic Church, at the White House, they are expected to make an impassioned collective call for action.

REUTERS, AGENCE FRANCE-PRESSE