Theresa May has called on other countries to "raise their ambition" and copy the UK by aiming for net zero carbon emissions by 2050.
The prime minister said she wanted other world leaders to "embrace this target" as she held a news conference at her last G20 summit.
It came as Donald Trump confirmed he would not be changing his mind on climate change action, saying US factories don't work on wind power and he doesn't want to subsidise green energy.
His stance means the US is the only country in the G20 that has not reaffirmed its commitment to the full implementation of the 2015 Paris climate accord.
Mrs May told reporters: "Over the past two days, leaders have discussed some of the most pressing challenges facing our nations.
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"In recent months we have heard hundreds of thousands of young people urge us – their leaders – to act on climate change before it's too late.
"I am proud that the UK has now enshrined in law our world-leading net zero commitment to reduce emissions. And I have called on other countries to raise their ambition and embrace this target."
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A joint declaration aiming to get countries in the G20 – the group of the world's 20 richest nations – to tackle the climate crisis was only agreed after the US inserted a clause exempting it from commitments signed up to by the other 19.
The communique said: "[All G20 nations] will strive to foster inclusive finance for sustainable development, including public and private financing mobilisation and alignment between them, as well as innovation in a wide range of areas for low emissions and resilient development.
"Climate actions at all levels with broad participation, including by non-state actors, will be the key to realising such a paradigm shift."
Because of Mr Trump's unwillingness to follow other nations in cutting greenhouse gas emissions, a clause was added that allowed the US not to fully join in.
It said: "The United States reiterates its decision to withdraw from the Paris Agreement because it disadvantages American workers and taxpayers.
"The US's balanced approach to energy and environment allows for the delivery of affordable, reliable, and secure energy to all its citizens while utilising all energy sources and technologies, including clean and advanced fossil fuels and technologies, renewables, and civil nuclear power, while also reducing emissions and promoting economic growth."
When asked why he had refused to join with the other nations in committing to tackle the climate crisis, Mr Trump told a news conference the US had the cleanest water and air "ever", but was not prepared to sacrifice US companies.
He said the other nations "understand" where the US stands and added other countries were "losing the power" with factories.
"The plants don't work with a windmill, when the wind goes off," he said, adding that solar power was not enough.
France had threatened to veto any final communique if the US watered down the language and it appeared as though that may have happened when the final declaration was late appearing online.
But Germany's Chancellor Angela Merkel welcomed the fact that the leaders managed to hold the line on the issue of climate change, with 19 countries committing themselves to the Paris climate accord.
The Paris accords commit all signed up nations to cut CO2 and other emissions to a point that will limit a rise in worldwide temperatures to 1.5C – half a degree above the current rise.
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