Sweden's Greta Thunberg, who pioneered a global climate change campaign as a 15-year-old, turned 18 on Sunday and promised to celebrate by exposing "dark secrets" at her local pub.

"Thank you so much for all the well-wishes on my 18th birthday!," Thunberg said on Twitter.

"Tonight you will find me down at the local pub exposing all the dark secrets behind the climate- and school strike conspiracy and my evil handlers who can no longer control me! I am free at last!!."

Thunberg began a climate change campaign that swelled from a one-person school strike to a worldwide movement, drawing in millions of school children, as well as adults.

She criticised world leaders over climate change in a speech to the United Nations in 2019, has clashed with US President Donald Trump, and last month called for urgent action five years on from the Paris Accord.