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It's been two years since the teenager started striking every Friday in a bit to catch the attentions of politicians in her native Sweden and encourage them to act.
Thunberg said: "Effectively, we have lost another two crucial years to political inaction"
Writing in The Guardian alongside fellow climate activists Luisa Neubauer, Anuna de Wever, and Adelaide Charlier, the 17-year-old said: "Looking back [over two years], a lot has happened.
"Many millions have taken to the streets ... and on 28 November 2019, the European parliament declared a climate and environmental emergency.
"But over these last two years, the world has also emitted over 80bn tonnes of CO2. We have seen continuous natural disasters taking place across the globe. Many lives and livelihoods have been lost, and this is only the very beginning."
The article continues: "When it comes to action, we are still in a state of denial.
"The gap between what we need to do and what's actually being done is widening by the minute.
"Effectively, we have lost another two crucial years to political inaction."
Thunberg's protest began in August 2018, in the run-up to Sweden's general election, when she decided to stop going into classes at her school in order to draw attention to the climate crisis.
That particular year saw Sweden hit by heatwaves and wildfires, and was the hottest summer in the Scandinavian country for 250 years.
Her initial protest gained traction, and eventually saw children from around the world taking action in their own countries.
Hundreds of thousands of people worldwide have joined the movement, and Thunberg herself has addressed protests, as well as speaking at the United Nations at their Climate Change Conference, to much international acclaim.
This Thursday, Thunberg and several other climate strikers will meet up with the German Chancellor Angela Merkel, who is currently the sitting president of the European Council.
An open letter calling for more action from politicians worldwide was published by Thunberg in July and has since been signed by 125,000 people including scientists and experts.
That letter will be delivered to Merkel in person tomorrow.
Thunberg added: "The EU and the United Kingdom are accountable for 22 percent of historic accumulative global emissions, second only to the United States.
"It is immoral that the countries that have done the least to cause the problem are suffering first and worst. The EU must act now, as it has signed up to do in the Paris agreement."
She concluded: "We still have the future in our own hands. But time is rapidly slipping through our fingers. We can still avoid the worst consequences.
"But to do that, we have to face the climate emergency and change our ways. And that is the uncomfortable truth we cannot escape."
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