By the time she spoke to DPA during her long trip home, she had already moved on in her head. In 2020, the greenhouse gas emissions have to drop drastically; 2020 is a key year, she said.
That was January 2019, and a lot has happened since. The way in which the debate on climate change is conducted has changed - in Greta's native Sweden, across Europe and even further afield. And the 16-year-old has played a major role with what started out as a "School Strike for the Climate."
Everyone now recognises the young Swede's face. To millions she is a star; others find her demands crazy.
In this file photo taken on September 13, 2019, Greta speaks at a climate protest outside the White House in Washington, DC.
Greta polarises. She draws praise from people as diverse as former US president Barack Obama, German Chancellor Angela Merkel, the pope, Leonardo DiCaprio and Arnold Schwarzenegger - but not US President Donald Trump, who has repeatedly cast doubt on whether climate change is even real, and, if it is, whether it is caused by humans.
Now, when she joins a demonstration at the UN's Madrid climate summit, she needs security to protect her from the throngs of the curious and well-wishers. Greta is a global phenomenon.
The Fridays for Future demonstrations are now a regular event in many cities worldwide. They came first in their hundreds, and then in their thousands. On September 20, there were 1.4 million on the streets, just in Germany.
Students have now been joined by their parents and grandparents. Merkel has openly acknowledged that the protests have had an effect on a policy to save the climate, laboriously thrashed out by her government in 2019.
While many feel the German measures do not go far enough, few at the start of the year would thought they would have been taken at all.
The climate has moved centre-stage. Ursula von der Leyen, the new president of the European Commission, plans to make it a focus of EU action in the years ahead.
Where President Donald Trump is taking the United States out of the Paris Climate Agreement, many US states, regions, cities and businesses are saying with clarity: "We're still in."
Greta sailed across the Atlantic on a carbon-neutral boat in order to attend climate talks in the US.
It remains true that carbon dioxide emissions from burning fossil fuels has risen during the past year, that new coal-fired power stations are still being built and planned and that economic growth in China and India is leading to higher emissions.
All of this is happening as climatologists warn that business as usual will take the world to an increase of four degrees Celsius by the end the century - and catastrophe.
The aim of the UN's Paris Agreement is "well under two degrees [Celsius]," but for that, emissions will have to fall every year by 7.6 per cent, from next year, according to UN models. Not even the most optimistic believe this will happen.
The year ahead is important, not only because scientists regard a change in the emissions trend as essential, but also because it is the year when the signatories to the Paris Agreement have to put forward their programmes for halting global warming.
Greta celebrates her 17th birthday on January 3, and, on August 20, she will mark two years since her first lonely protest outside the Swedish parliament.
But days earlier, on August 17, the school holidays end in Stockholm. After a year off from her schoolbooks, during which she sailed the Atlantic in both directions, won an Alternative Nobel Prize, launched a global protest movement and was named Time magazine's person of the year, Greta is set to go back to school.