Argentinian muralist Andres Iglesias, who works under the pseudonym Cobre, is about a week away from completing the enormous artwork on the side of a building in the city’s central Union Square neighbourhood.
Cobre said: “Climate change is real. This girl Greta is awesome and she knows what she’s doing.
“I hope with this mural people will realise we have to take care of the world.”
The environmental not-for-profit group One Atmosphere commissioned Cobre after a building featuring his much-loved mural of late actor Robin Williams was earmarked for demolition.
The artist said he was approached just as he began searching for a suitable building for his next project.
One Atmosphere is understood to be supplying all the paint and materials for the Thunberg project.
It will not be the first time the young Swede has been immortalised in street art.
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Earlier this year, British graffiti artist Jody Thomas created a 50ft portrait of the teenage activist on the face of Bristol’s historic Tobacco Factory.
He said: “She’s very much in the limelight, very current, very contemporary and she’s obviously clearly leading a very, very important issue which affects all of us on the planet,”
At 16, Miss Thunberg is already a Nobel peace prize nominee, leader of a movement to reclaim the planet for future generations, victim of Donald Trump’s mockery and hero among progressives and young people.
“People are suffering. People are dying. Entire ecosystems are collapsing.
“We are in the beginning of a mass extinction, and all you can talk about is money and fairy tales of eternal economic growth. How dare you!
“My message is that we’ll be watching you.”