President Joe Biden has committed the US to halving its greenhouse gas emissions by 2030.
The pledge came during a virtual global summitat the White House on Thursday joined by world leaders including UK Prime Minister Boris Johnson.
In his address at the start of the event, Mr Biden said: “The science is undeniable.
“The cost of inaction just keeps mounting… we have to step up.”
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The pledge – which amounts to nearly doubling previous commitments – and the summit, represent a concerted US effort to reclaim credibility and leadership on climate change after four years of inaction under Donald Trump.
The target is based on 2005 levels of emissions.
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Mr Biden said: “Scientists tell us this is the decisive decade. This is the decade we must make decisions that will avoid the worst consequences of the climate crisis.”
But he warned that no nation can solve this crisis “on its own” and called for all countries to band together.
“If we do, we will breathe easier, literally and figuratively.
“This is a moral imperative and an economic imperative… but also a moment of extraordinary possibilities.”
In his response, Mr Johnson described the US move as “game changing”.
He said nations needed to work together and be “constantly original and optimistic about new technology and new solutions”.
He added: “It is vital for all of us to show that this is not all about some expensive, politically-correct green act of bunny hugging.
“Let’s use this extraordinary moment and the incredible technology we are working on, to make this decade the moment of decisive change in the fight against climate change – and let’s do it together.”
China has in the past been criticised for dragging its heels on climate ambition. Just last week US climate envoy John Kerry called on the country to “assume responsibility” for its part in global warming – and its president Xi Jinping was called on early to address the summit.
He said he was looking forward to working with the international community on climate change and that the Chinese people “have always value harmony between man and nature”.
And he made a pledge that his country would “strive to peak carbon dioxide emissions before 2030 and achieve carbon neutrality before 2060”.
He added: “Mother nature has nourished us and we must treat nature as our root – respect it, protect it.”
Among US measures to be employed to reduce emissions by 50% to 52% are a “carbon border adjustment” – or carbon tariff – that would tax imports from countries that do not have similar emissions controls.
The White House will also target the biggest emitting sectors – including ensuring zero emissions from power plants by 2035, boosting energy efficiency in homes and businesses, incentives for electric vehicle use and expanding “carbon sinks” like forests and agriculture.
However, US Republicans say the target could damage the economy, with job cuts expected to come from areas such as power plants and the automotive industry, and they could oppose moves to lower the country’s dependence on gas, oil and coal.
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Sector-specific goals will be revealed later in the year, but the administration has already moved to introduce a $2trn (£1.4trn) infrastructure plan, that could help Mr Biden deliver the emissions cuts he needs, including a clean energy standard.
The UK has already announced it will cut its own emissions by 78% by 2035.
It is set to host COP26 climate talks in Glasgow later in the year.
China will also be in attendance at the two-day summit, as will Canada, India and Brazil, with all nations under pressure to pledge more action on climate change, to ensure global warming does not go beyond the 1.5C goal.
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