• "Today, I graduate from school, which means I'll no longer be able to school strike for the climate," Thunberg said Friday on Twitter.
• The 20-year-old was catapulted to fame for skipping school every Friday to hold a weekly vigil outside the Swedish Parliament in 2018.
• Just as she did in August of that year, Thunberg marked her final school strike by protesting with a handmade sign that read: "School strike for climate."

Swedish climate activist Greta Thunberg on Friday took part in her final school strike, signing off after 251 consecutive weeks of demonstrations with a warning that "the fight has only just begun."

"Today, I graduate from school, which means I'll no longer be able to school strike for the climate," Thunberg said on Twitter. "This is then the last school strike for me, so I guess I have to write something on this day."

The 20-year-old was catapulted to fame for skipping school every Friday to hold a weekly vigil outside the Swedish Parliament in 2018. Just as she did in August of that year, Thunberg marked her final school strike by protesting with a handmade sign that translated to read, "School strike for climate."

Initially a one-person demonstration, Thunberg's act would go onto inspire a global protest for climate action. Millions of children in over 180 countries took part in school strikes for the climate in 2019, before the coronavirus pandemic forced the "Fridays for Future" movement to find new avenues to protest.

Thunberg was named Time magazine's Person of the Year for 2019 — the same year she received criticism from the likes of then-U.S. President Donald Trump and Australian Prime Minister Scott Morrison.

Earlier this year, Thunberg was arrested alongside other campaigners during a protest against the demolition of a small German village to make way for a coalmine.

"Much has changed since we started, and yet we have much further to go," Thunberg said. "We are still moving in the wrong direction, where those in power are allowed to sacrifice marginalised and affected people and the planet in the name of greed, profit and economic growth."

"We're rapidly approaching potential nonlinear ecological and climatic tipping points beyond our control," she continued. "There are probably many of us who graduate who now wonder what kind of future it is that we are stepping into, even though we did not cause this crisis."

"We who can speak up have a duty to do so. In order to change everything, we need everyone. I'll continue to protest on Fridays, even though it's not technically "school striking". We simply have no other option than to do everything we possibly can," Thunberg said. "The fight has only just begun."

Thunberg has previously excoriated the climate inaction of the world's political and business leaders at the annual World Economic Forum in Davos, Switzerland, saying in Jan. 2019, "Our house is on fire."

Major course correction

Earlier this year, the world's leading climate scientists urged government leaders across the global to embark on an urgent course correction to tackle the climate emergency, warning that current plans were insufficient to prevent the worst of the crisis.

The U.N. Intergovernmental Panel on Climate Change said in a landmark report published on March 20 that the unprecedented challenge of keeping global warming to 1.5 degrees Celsius above pre-industrial levels had become even greater in recent years, given the relentless increase in global greenhouse gas emissions.

The threshold of 1.5 degrees Celsius temperature is widely recognized as crucial because so-called tipping points become more likely beyond this level. Tipping points are thresholds at which small changes can lead to dramatic shifts in the entire Earth life support system.