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The Archbishop of Canterbury has "unequivocally" apologised for comparing politicians who fail to act on climate change to those who "ignored what was happening in Nazi Germany".
Justin Welby made the comments while attending the COP26 climate conference.
But he later apologised, saying it was "never right to make comparisons with the atrocities brought by the Nazis".
He added that he was "trying to emphasise the gravity of the situation facing us".
Asked if climate change would be worse than allowing a genocide to happen, Mr Welby said: "It will allow a genocide on an infinitely greater scale.
"I'm not sure there's grades of genocide, but there's width of genocide, and this will be genocide indirectly, by negligence, recklessness, that will in the end come back to us or to our children and grandchildren."
The COP26 conference in Glasgow is seen as crucial if temperature increases, and changes to the climate, are to be limited.
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Speaking to BBC political editor Laura Kuenssberg, Mr Welby said history would judge current world leaders "probably on this fortnight alone".
"They could have been brilliant in everything else they've done, and they will be cursed if they don't get this right," he said.
"They could have been rubbish at everything else they've done but if they get this right, the children of today will rise up and bless them in 50 years."
Mr Welby said that if they failed to act future generations would speak of them in "far stronger terms than we speak today of… the politicians who ignored what was happening in Nazi Germany because this will kill people all around the world for generations".
However, he later issued an apology, saying: "I unequivocally apologise for the words I used when trying to emphasise the gravity of the situation facing us at COP26.
"It's never right to make comparisons with the atrocities brought by the Nazis and I'm sorry for the offence caused to Jews by these words."