[26] On 20 August 2018, Thunberg, who had just started ninth grade, decided not to attend school until the 2018 Swedish general election on 9 September; her protest began after the heat waves and wildfires during Sweden's hottest summer in at least 262 years.

[55] Throughout the autumn of 2018, Thunberg's activism evolved from a solitary protest to taking part in demonstrations throughout Europe, making several high-profile public speeches, and mobilizing her followers on social media platforms.

[73] In July 2019, Time magazine reported Thunberg was taking a "sabbatical year" from school, intending to travel in the Americas while meeting people from the climate movement on her way to attend and address COP25.

[74] In August 2019, Thunberg sailed across the Atlantic Ocean from Plymouth, England, to New York City, in the 60-foot (18 m) racing yacht Malizia II, equipped with solar panels and underwater turbines.

[76] On Thunberg's return voyage aboard the La Vagabonde catamaran, she was quoted that she chose sailing as a way to send a message to the world that there is no real sustainable option to travel across the oceans.

Together, the group announced they had made an official complaint against five nations that were not on track to meet the emission reduction targets they committed to in their Paris Agreement pledges: Argentina, Brazil, France, Germany, and Turkey.

During a press conference before the march, she called for more "concrete action", arguing that the global wave of school strikes over the previous year had "achieved nothing" because greenhouse gas emissions were still rising – by 4% since 2015.

[99] Thunberg's edition of the programme featured interviews on climate change with Sir David Attenborough, Bank of England chief Mark Carney, Massive Attack's Robert Del Naja, and Shell Oil executive Maarten Wetselaar.

"[103][104][105] In February 2020, Thunberg travelled to Oxford University to meet Malala Yousafzai, a Nobel Peace Prize-winning Pakistani activist for female education who had been shot in the head by the Taliban as a schoolgirl.

Thunberg said that the way humans are destroying habitats are the perfect conditions for the spread of diseases and noted zoonotic illnesses such as COVID-19, Zika, Ebola, West Nile fever, SARS, MERS, among others.

"[127] A month later, Thunberg took part in a protest in London, demanding that the financial system stop funding companies and projects that use fossil fuels, such as coal, oil and natural gas.

[11] On 29 June 2023, Thunberg met with Ukrainian President Volodymyr Zelenskyy and other prominent European figures to form a working group to address ecological damage from the 16-month-old Russian invasion.

[149] On 24 July 2023, a trial was held at Malmö District Court where the prosecution presented its case against Thunberg for disobedience to authority after having disrupted traffic and refusing to follow police orders.

[159][160][161] On 23 April 2024, Thunberg was charged with civil disobedience for allegedly ignoring police orders to leave two climate demonstrations which law enforcement claim were blocking Sweden's parliament building on 12 and 14 March.

[166] On 5 December 2023, Thunberg and three researchers/activists affiliated with Fridays for Future Sweden published an opinion piece in The Guardian titled "We won't stop speaking out about Gaza's suffering – there is no climate justice without human rights."

"[177] Thunberg states that humanity is facing an existential crisis because of global warming[178] and holds the baby boomers, and each subsequent generation, responsible for creating and perpetuating detrimental changes to the Earth's climate.

[191][192] Thunberg reiterated her views on political inaction in a November 2020 interview where she stated that "leaders are happy to set targets for decades ahead, but flinch when immediate action is needed.

[27] United Nations Secretary-General António Guterres endorsed the school strikes initiated by Thunberg, admitting: "My generation has failed to respond properly to the dramatic challenge of climate change.

[206]Democratic candidates for the 2020 United States presidential election such as Kamala Harris, Beto O'Rourke, and Bernie Sanders expressed support after her speech at the September 2019 action summit in New York.

Thunberg reacted by changing her Twitter bio to match his description, and stating that she could not "understand why grown-ups would choose to mock children and teenagers for just communicating and acting on the science when they could do something good instead.

Putin criticized her as "poorly informed", adding, "No one has explained to Greta that the modern world is complex and different and people in Africa or in many Asian countries want to live at the same wealth level as in Sweden."

[225] Writing in The Guardian, Aditya Chakrabortty said that columnists including Brendan O'Neill, Toby Young, the blog Guido Fawkes, as well as Helen Dale and Rod Liddle at The Spectator and The Sunday Times, had been making "ugly personal attacks" on Thunberg.

[228] Arron Banks' Twitter post saying that "freak yachting accidents do happen in August" in reference to Thunberg, outraged a number of British MPs (Member of Parliament), celebrities, and academics.

Tanja Bueltmann, founder of EU Citizens' Champion, said Banks had "invoked the drowning of a child" for his own amusement and said that most of those attacking Thunberg "are white middle-aged men from the right of the political spectrum".

[230] In September 2019, Nick Gillespie wrote in Reason that "Greta Thunberg's histrionics are likely heartfelt but neither they nor the deplorable responses they conjure are a guide forward to good environmental policy in a world that is getting richer every day.

"[231] In August 2021, Yasmeen Serhan wrote in The Atlantic that Thunberg had become "the target of a barrage of disinformation and conspiracies" from the far-right and populist right, "including depictions of her as a spoiled child, a leftist pawn, and even a Nazi".

"[17] In February 2019, Thunberg shared a stage with the President of the European Commission, Jean-Claude Juncker, where he outlined: "In the next financial period from 2021 to 2027, every fourth euro spent within the EU budget will go towards action to mitigate climate change.

[236] In June 2019, a YouGov poll in Britain found that public concern about the environment had soared to record levels in the UK since Thunberg and Extinction Rebellion had "pierced the bubble of denial".

[239] Inspired by Thunberg, wealthy philanthropists and investors from the United States have donated about $600,000[240] to support Extinction Rebellion and school strike groups to establish the Climate Emergency Fund.

"[258] Also in December 2019, Thunberg was criticized by the German rail firm Deutsche Bahn (DB), after she tweeted a photo of herself on an overcrowded train in Germany, stating she was "finally on my way home" from the COP25 UN climate conference in Madrid.