Swedish activist Greta Thunberg on Thursday said the world needed an economic overhaul to have a chance of beating climate change and that countries should be ready to tear up old deals and contracts to meet green targets.
Ms Thunberg, 17, and other activists sent an open letter to European leaders urging them to take emergency action, saying people in power had practically "given up" on looking for a real solution.
"We need to see it as, above all, an existential crisis," she said from her home in Stockholm.
"And as long as it's not being treated as a crisis, we can have as many of these climate change negotiations and talks, conferences as possible. It won't change a thing."
Thunberg, who blasted world leaders at a UN climate summit last year for believing in "fairy tales" of eternal economic growth, said only fundamental change to the system would bring climate change under control.
She quoted a UN study published in November that suggested planned investments to boost fossil fuel production were likely to push temperature goals from the 2015 Paris Agreement out of reach.
"So that means that if we are to stay below these targets, we have to make it possible to tear up and abandon valid contracts and deals," Ms Thunberg said.
"And that is not possible within today's system.
"So yes, then obviously we need to think differently. And yes, we need to think outside the box."
Demands in the letter, released before Friday's European Council summit, included an immediate halt to all investments in fossil-fuel exploration and extraction, and a rapid end to hydrocarbon subsidies.
The letter also called for binding annual "carbon budgets" to limit how much greenhouse gas countries can emit to maximise the chances of capping the rise in global temperatures at 1.5°C above pre-industrial levels, a goal in the 2015 Paris climate accord.
It urged European governments to back calls for the International Criminal Court in The Hague to adopt a new crime of "ecocide", to prosecute people responsible for large-scale destruction of the natural world.
Ms Thunberg emerged as the face of a growing, youth-led climate movement after a solitary vigil she began outside the Swedish Parliament in 2018 inspired children and youths around the world to stage strikes on Friday afternoons.
"In the beginning, I was very worried," she said, describing the fears over climate change that motivated her to embrace activism.
"But when I started doing something, then there came hope from that, because hope comes from action."
With climate protests mainly online during the coronavirus pandemic, Ms Thunberg joined climate scientists, activists and celebrities including actor Leonardo DiCaprio and author Margaret Atwood in signing the letter.
"The longer we keep pretending that we are on a reliable path to lower emissions and that the actions required to avoid a climate disaster are available within today's system. the more precious time we will lose," the letter read.
It called for climate policies to be designed to protect workers and the most vulnerable, and reduce economic, racial and gender inequalities, along with moves to protect democracy.