A Greta Thunberg lookalike in a circa-1898 photo of three children from the Yukon went viral this week and had some on Twitter jokingly suggesting that the climate activist is a "time traveler."

The face of the girl in the foreground is so similar to Thunberg that fans facetiously insisted it had to be her.

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Thunberg, 16, became a sensation last summer when she sailed across the Atlantic on a racing boat instead of flying to attend the UN Climate Active Summit in New York. Since then, she has been touring around the United States advocating for climate awareness.

“So, ‘Greta Thunberg’ is in a photo from 120 years ago, and it’s my new favourite conspiracy,” one Twitter user wrote. “Greta’s a time traveller, from the future, and she’s here to save us.”

Others joked about other far-out theories.

“This stuff on Greta Thunberg being a time traveller is ridiculous, there’s no such thing as time travel. Quite obviously she’s just immortal,” another wrote.

While Twitter had fun with the photo, some were concerned about the dangers of viral misinformation.

“There was a time not long ago when a "theory" like this would get a chuckle and if someone was seriously pushing it we may pity their instability. Nowadays any of these "theories" can grow legs and become "reality" to 1000's of Americans,” one user cautioned.

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The photo is actually from the University of Washington archives.

Thunberg hitched a ride last week on a 48-foot catamaran for UN-sponsored climate talks in Spain.