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Barack Obama has called for young people to "stay angry" in the fight against climate change, but warned that they "can't be too pure" for politics.
The ex-US president said "the US is back" under Joe Biden, and scolded China and Russia's leaders for not physically attending the COP26 summit.
He criticised Donald Trump for "active hostility" towards climate science which he said had slowed progress.
"Time is running out… We are nowhere near where we need to be", he said.
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Speaking at the COP26 climate summit in Glasgow, Mr Obama received rapturous applause when he took to the stage and a standing ovation at the end of his speech – but there was pushback from activists.
He called out nations for failing to meet the pledges they made in the 2015 Paris Agreement, when he was in the White House. However, activists were quick to point out the pledges that were broken by his own administration, including the failure of developed countries to deliver a promised $100bn a year in climate finance to poorer nations.
Mr @BarackObama, I was 13 when you promised $100B #ClimateFinance. The US has broken that promise, it will cost lives in Africa. Earth's richest country does not contribute enough to life-saving funds. You want to meet #COP26 youth. We want action. Obama & @POTUS #ShowUsTheMoney pic.twitter.com/40HCsqA5s3
— Vanessa Nakate (@vanessa_vash) November 8, 2021
The BBC is not responsible for the content of external sites.View original tweet on Twitter
The former president insisted that despite political divisions in the US and the country's absence from global climate efforts for four years during the Trump administration – it was back on track and committed to change.
He dedicated much of his speech to young activists, who he said were "right to be frustrated".
He honoured the dedication of activists such as Greta Thunberg who are "forming movements across borders", and urged young people to at least get out and vote for politicians who will stand against climate change.
"Vote like your life depends on it, because it does", he said.
'You can't ignore politics'
Mr Obama told young people they "can't ignore politics" and that while protesting raises awareness, they should get involved in politics at some level.
"You don't have to be happy about it, but you can't ignore it. You can't be too pure for [politics]."
Referencing his own daughters' shopping habits, he also called on young people to support businesses that were committed to sustainability, and boycott those that were not.
Mr Obama's catchphrase of the night was telling young activists to "stay angry".
"To all the young people out there – I want you to stay angry. I want you to stay frustrated," he said.
"But channel that anger. Harness that frustration. Keep pushing harder and harder for more and more. Because that's what's required to meet that challenge. Gird yourself for a marathon, not a sprint."
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Mr Obama made a number of comments about his successor, Republican Donald Trump, who pulled out of the Paris climate accord when he took office. The US rejoined it under Joe Biden.
Mr Obama strongly criticised the Republican Party for stalling progress by "sitting on the sidelines" and making it a partisan issue with "four years of active hostility toward climate science."
He said Republicans who take climate change seriously are "a rare breed right now." This is the first year a delegation of Republicans has attended a UN climate summit.
He also blamed the pandemic, rise of US nationalism and a lack of cooperation from the Trump administration for a breakdown in international relations that has affected the climate crisis.
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Taking a swipe at Russia and China, Mr Obama said it was "particularly discouraging" that the leaders of "two of world's largest emitters" had not attended the climate summit in person.
Vladimir Putin and Xi Jinping, who have both avoided travelling internationally since the start of the pandemic, opted to take part remotely instead of travelling to Scotland.
Mr Obama said they were showing a "dangerous lack of urgency".
The 44th US president closed by telling the crowd that fighting climate change will be hard and messy with "every victory incomplete".
"The thing we have going for us is that humanity has done hard things before," he said, adding: "I believe we can do hard things again."