Its ambition was clear: The U.N. climate summit was meant to secure a deal to give the world a chance to avert the worst impacts of climate change by capping global warming at 1.5 degrees Celsius above pre-industrial levels.

The accord met that bar, but barely, and its ultimate success will be determined by the future actions of the governments that thrashed it out, according to the summit’s U.K. hosts, participants and observers.

"I think today we can say with credibility that we've kept 1.5 within reach. But its pulse is weak, and we will only survive if we keep our promises," the summit’s president, Alok Sharma, said late on Saturday after the pact was adopted.