Called by Canada, China and the European Union, the summit was taking place 30 years to the day after the signature of the Montreal Protocol on protecting the ozone layer - which Canada's environment minister hailed as a multilateral "success story" by governments, NGOs and ordinary citizens jointly tackling a major global threat.
"We have an opportunity to accomplish even more with the Paris agreement," Catherine McKenna said as she opened the summit, attended by more than half the G20 members as well as some of the nations most vulnerable to climate change -- from the low-lying Marshall Islands and Maldives to impoverished Mali and Ethiopia.
"Changes are real, extreme weather events are more frequent, more powerful and more distressful," she told the gathering, pointing at the devastation wrought by mega-storms such as Harvey and Irma which many climate scientists believe are boosted by global warming.
"We are here together and we need to act together," the Canadian minister said.
Nearly 200 countries agreed in Paris in December 2015 to curb carbon dioxide emissions with the aim of limiting the rise in average global temperatures to 1.5 degrees Celsius by 2050, compared to preindustrial levels.
When Trump decided in June to withdraw the United States from the climate accord, Canada, China and the European Union immediately reaffirmed their respective commitments to the pact, which the Group of 20 declared "irreversible" the following month.
But time is ticking, the EU's top climate official Miguel Arias Canete told AFP, as ministers work to narrow their differences and better understand how to implement the ambitious accord -- with less than two months to go until the next UN Conference on Climate Change (COP23), in Bonn in November.