Pope Francis said Wednesday it was "time to take wise choices" for the future of planet in a message to youth activists ahead of a crunch UN climate summit next month. 

"Solutions can be found to overcome energy poverty and to place the care of the commons at the centre of national and international policies," Francis said in a video message to the Youth4Climate gathering in Milan. 

"It is time to take wise choices to take advantage of the many experiences acquired over the last few years, in order to make possible a culture of care, a culture of responsible sharing."

These included "favouring sustainable production, circular economy, and the sharing of appropriate technologies," the Pontiff said.

The more than 400 young people from 200 nations gathered in Milan are working on a joint declaration to be given to the hosts of the COP26 climate summit, which begins October 31 in Glasgow. 

The Pope congratulated the activists on their "work for the good of humanity."

On Tuesday, youth activists Greta Thunberg and Vanessa Nakate opened the three-day event by blasting world leaders for decades of empty promises over the climate. 

Francis said that "political and technical solutions" were not enough to address the scale of the challenge posed by global warming. 

Efforts needed to be "sustained by the responsibility of each member and by an educational process that favours a cultural model of development and sustainability", he said.