The Swedish activist hit out at EU governments, saying that they needed to treat the warming up of the planet as an “emergency”.
Arguing for a complete transformation of the entire economic system, the 17-year-old signed an open letter that stated: "We understand and know very well that the world is complicated and that what we are asking for may not be easy.
“The changes necessary to safeguard humanity may seem very unrealistic.
"But it is much more unrealistic to believe that our society would be able to survive the global heating we're heading for, as well as other disastrous ecological consequences of today's business as usual.”
Demands in the letter included an immediate halt to all investments in fossil fuel exploration and extraction, in parallel with a rapid ending of fossil fuel subsidies.
It also called for climate policies to be designed to protect workers and the most vulnerable and reduce economic, racial and gender inequalities, as well as moves to "safeguard and protect" democracy.
The letter was made public the day before a European Council summit where EU countries will try to reach a deal on the bloc's next budget and a recovery package to respond to the economic shock of the coronavirus pandemic.
However, Thunberg argued that conferences “won’t change a thing” unless climate change was treated as an emergency.
She told Reuters TV: "We need to see it as, above all, an existential crisis.
“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.”
She added: ”Above all, we are demanding that we need to treat this crisis as a crisis, because if we don't do that, then we won't be able to do anything.”