Efforts to contain the rise in global temperature to within 1.5 degrees have received a strong boost, with the United States declaring it is to halve its carbon emissions by 2030.
The announcement was made in advance of a global summit on climate ambition hosted by President Joe Biden, which was addressed by 40 heads of state.
The move by the world's second-leading emitter after China is an attempt to prompt the adopting of more stringent targets in big carbon-emitting countries in advance of the UN COP26 climate summit in Glasgow.
It coincided with Japan, Canada, the UK and the EU confirming they are to strengthen commitments under the Paris Agreement, though China and India – the third-largest emitter – are sticking with current plans.
Mr Biden said no government could address the climate threat on its own, while collaboration would increase international and food security.
“The signs are unmistakable, the science is undeniable, and the cost of inaction keeps mounting,” he added.
Manish Bapna, chief executive of the World Resources Institute, said the new US target "should make the world sit up and take note".
He added: “This target will serve as the north star for President Biden’s domestic agenda. It will create a more equitable and prosperous society. The message to other major emitters is loud and clear: it’s your move next.”
The US move will take the world closer to the reductions scientists say are necessary to hold global heating within scientifically-advised limits, according to analysis by Climate Action Tracker. To be in line with a 1.5 degree rise, however, the US would need to cut emissions by 57 per cent to 63 per cent below 2005 levels.
US climate envoy John Kerry accepted it was not enough, stressing "it's a beginning heading towards Glasgow".
Speaking at the summit, Pope Francis said the world faced a challenge to care for nature in the post-pandemic era.
“We know that one doesn’t come out of a crisis the same way we entered. We come out either better or worse. We need to ensure the environment is cleaner, more pure. It must be preserved. We need to care for nature, so that nature may care for us.”