The U.N. COP26 climate summit opened on Sunday, billed as a make-or-break chance to save the planet.
But as one summit began, another ended- the G20 in Rome, which highlighted doubts and anger over whether years of empty pledges would turn into political momentum.
COP26's aim is to keep alive the target of capping global warming at 1.5 degrees above pre-industrial levels - which scientists say is our only hope of averting catastrophe.
As delegates began arriving in Glasgow, Scotland, COP26 President Alok Sharma said an August report by U.N. scientists was a wake-up call.
"It made clear that the lights are flashing red on the climate dashboard. That report, agreed by 195 governments, makes clear that human activity is unequivocally the cause of global warming."
To the fury of protesters in Rome, G20 leaders urged "meaningful and effective" action on Sunday but offered few concrete commitments, even though the bloc of the world's 20 richest nations is responsible for an estimated 80% of global emissions.
A new pledge last week from China, the world's biggest polluter - to reach net zero, but not until 2060 - was labelled a missed opportunity that will cast a shadow over the two-week summit.
The return of the United States, the world's biggest economy, to U.N. climate talks will be a boon after a four-year absence under Donald Trump.
But like many world leaders, President Joe Biden will arrive at COP26 without firm legislation in place, as Congress wrangles over how to finance it.
For any chance of success, COP26 needs to secure far more ambitious emissions pledges and lock in billions in climate finance for poorer countries.
Pope Francis urged world leaders to "listen to the cry of the earth, and the poor".