Environmental activist Greta Thunberg urged young people in Italy on Friday to seize the next decade to fight climate change, and chastised world leaders for acting "as if there is no tomorrow".

The Swedish 16-year-old -- who this week was named Time magazine's 2019 Person of the Year -- reminded young activists at the "Fridays for Future" rally in the northern Italian city of Turin to make 2020 a "year of action".

"In less than three weeks we will move into a new decade and... I cannot say enough how important this decade will be," said Thunberg, bundled up against the cold in a yellow rain slicker.

"This is a decade that will define our future."

Young activists joined a 'Fridays for Future' rally in Turin / © AFP

Thunberg slammed world leaders currently attending the UN Climate Change Conference in Madrid, saying they were "still trying to run away from their responsibilities".

"It is not fair that the old generations are handing over the responsibility to solve this crisis to the young people who have not started this crisis. It is not fair that we have to do all this," she said.

"The adults are behaving as if there is no tomorrow," Thunberg said, cautioning that youth can "no longer take that tomorrow for granted."

The teenager rose to the global stage after launching a solo strike against global warming in mid-2018 which surged into a worldwide movement that has seen her tipped as a Nobel laureate.

After a months-long journey through North America to raise awareness of environmental issues, Thunberg returned to Europe earlier this month after crossing the Atlantic via catamaran. The teenager does not fly due to the heavy carbon footprint of air travel.

In naming her Person of the Year, Time magazine wrote that Thunberg "has offered a moral clarion call to those who are willing to act, and hurled shame on those who are not."