World leaders have reaffirmed their commitment to cutting greenhouse gas emissions, after Theresa May urged them to do more on climate change.
However, the United States has not agreed to do so, arguing that meeting the terms of the 2015 Paris climate change agreement would hurt American workers.
Outgoing British prime minister Theresa May called on the G20 countries to set targets for net zero greenhouse gas emissions.
The UK is the first country to enshrine in law a commitment to be a net zero emitter of CO2 by 2050.
At the summit, Mrs May was also one of several world leaders to put pressure on Saudi Arabia over the killing of journalist Jamal Khashoggi last year.
Leading the session on climate change, Mrs May called on other world leaders to match the UK's net zero target, aiming for the summit's joint statement to have "the strongest wording we can deliver" on the issue.
But only 19 of the 20 leaders signed up to the statement, which committed them to the "irreversibility" of the 2015 Paris agreement and pledged the full implementation of its targets to reduce greenhouse gas emissions.
US President Donald Trump declined to sign.
Mrs May said: "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."
The UK's net zero goal was "world-leading", she said as she called on other countries to "raise their ambition and embrace this target".
The UK's target means it will have to eliminate greenhouse gas emissions completely or - in the most difficult cases - offset them by planting trees or absorbing CO2 from the atmosphere.
Under the Paris agreement, reaffirmed by the 19 world leaders, every nation is committed to keeping global temperature rises to less than 2C higher than pre-industrial times.
Scientists say that a 1.5C rise is the threshold for dangerous climate change, and if other countries adopted the UK's net zero target, there was a 50-50 chance of staying below this by 2100.
Human rights concerns
Separately, in a 20-minute meeting with Saudi crown prince Mohammed Bin Salman, she raised the case of murdered journalist Jamal Khashoggi, a British official said.
US intelligence concluded the crown prince directed the killing of the Washington Post columnist at the Saudi consulate in Turkey last year, but Saudi authorities have denied they were acting on his orders.
With 11 unidentified people put on trial behind closed doors, Mrs May told Prince Mohammed that the legal process must be "open and transparent", the official said.
Turkey's President Recep Tayyip Erdogan said it was Saudi Arabia's responsibility to "uncover" the perpetrators, but added: "Nothing is done so far." He said they should be prosecuted in Turkey.
Canadian Prime Minister Justin Trudeau also said he highlighted "continuing concerns" about the investigation, and about human rights in Saudi Arabia.
But the US president praised Prince Mohammed as a "terrific ally" and said "nobody so far has directly pointed a finger at the future king of Saudi Arabia".
Mrs May also called for other countries to follow the UK's pledge of £1.4bn ($NZ2.6bn) to the Global Fund, an international organisation which fights three of the world's deadliest diseases.
The UK will contribute £467m ($NZ881m) a year for three years, providing tuberculosis treatment for more than two million people, 90 million mosquito nets to protect people from malaria, and treatment for more than three million people living with HIV - the virus that causes Aids.
Mrs May said the world needed "urgent international action and a truly collective response" to halt the spread of these illnesses.
The pledge follows an appeal by Sir Elton John and French President Emmanuel Macron for an £11bn ($NZ20bn) cash injection for the Global Fund, which is expected to help save 16 million lives.
Sir Elton, whose Aids foundation works with the fund, said the UK's response set an "extraordinary example for others to follow".
Bill Gates, the billionaire philanthropist who is one of the Global Fund's backers, said the UK pledge was "really fantastic" and urged the country to continue supporting overseas aid.