LONDON, Nov 1 (Reuters) - Archbishop of Canterbury Justin Welby apologised on Monday after saying world leaders who fail to act on climate change could be making a bigger mistake than their predecessors who ignored warnings about the rise of the Nazis. Welby, the spiritual leader of the worldwide Anglican Communion of about 85 million Christians, speaking at the start of the climate change summit in Scotland said he was sorry for the offence caused to Jews by his comments. "I unequivocally apologise for the words I used when trying to emphasise the gravity of the situation facing us at COP26," Welby said on Twitter. "It"s never right to make comparisons with the atrocities brought by the Nazis." The U.N. summit critical to averting the most disastrous effects of climate change opened on Monday, with world leaders, environmental experts and activists pleading for decisive action to halt global warming. read more Welby earlier told the BBC world leaders will be "cursed" if they fail to reach an agreement on climate change. "People will speak of them in far stronger terms than we speak ... of the politicians who ignored what was happening in Nazi Germany because this will kill people all around the world for generations," he 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."