Cop26 is the "chance to win a victory for the next century, for life on this planet", US climate envoy John Kerry has said, while warning all big emitters must take action on climate change.
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Speaking at London School of Economics (LSE) before the Cop26 United Nations climate summit kicks off in Glasgow, Mr Kerry said there was "real, meaningful and growable" momentum among countries and businesses to drive a shift to green economies.
But the US special presidential climate envoy warned that there was still a gap between action pledged by countries in the next decade and the levels of emissions cuts needed to avoid dangerous warming.
Glasgow could be a new beginning to drive action on tackling climate change over the next "decisive" decade, he suggested.
Mr Kerry said all countries must work together, with G20 countries, the world's largest economies including China and India as well as the US and UK who account for 80% of emissions, particularly responsible for action.
He criticised those countries such as China who were arguing they deserved "carbon space" because other nations had developed in the past when they had not, and therefore they should be allowed to pollute more.
The former Democratic presidential nominee in the 2004 US general election warned: "Mother Nature only has one measure of how much emissions are in our atmosphere. She doesn't measure whether it's US, China, Russia, Mexico, Brazil, it's just atmospheric. Every ounce matters, every tenth of a degree of temperature matters."
Scientists say the world must cut carbon by at least 45% by 2030 to be on a path to "net zero" emissions by 2050 to limit global rises to 1.5C, but emissions are likely to rise 16% by 2030 on 2010 levels on current efforts.
Mr Kerry said:
"That's what this is about, not ideology, not politics, it's about mathematics and physics," he said, adding scientists had warned of the "devastating consequences" if global temperatures rise more than 1.5C above pre-industrial levels.
The world was already at around 1.2C of warming, and "we're no longer talking about impacts in the future, we no longer need scientists to tell us what will happen because we're seeing it happen now already," he said, pointing to heatwaves, superstorms and floods this year.
He said the world faced destruction of crops and coral reefs, the melting of glaciers that provide water for millions, migration, destabilised regions and collapsing economies beyond 1.5C of warming.