U.S. Treasury Secretary Steven Mnunchin said Thursday that teen climate change activist Greta Thunberg should get a college degree before she speaks out about fossil fuel divestment. It didn't take long for Thunberg — the 2019 Time magazine Person of the Year — to fire back on social media.

During a press briefing at the World Economic Forum in Davos, Switzerland, Mnuchin questioned Thunberg's credentials after a reporter asked him about her recent remarks. At a Davos panel on Tuesday, Thunberg urged public institutions and private companies to immediately stop investing in fossil fuels.

"Is she the chief economist, or who is she? I'm confused," Mnuchin said. "It's a joke. After she goes and studies economics in college she can come back and explain that to us."

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Thunberg, who's taking a gap year from school to campaign for action on climate change, called him out on Twitter, saying it "doesn't take a college degree in economics" to see that "ongoing fossil fuel subsides and investments don't add up." She paired her tweet with an interactive graph showing the correlation between greenhouse gas emissions and the "carbon budget" — how much room is left before average temperatures rise to a perilous level, another 1.5 degrees Celsius.

My gap year ends in August, but it doesn’t take a college degree in economics to realise that our remaining 1,5° carbon budget and ongoing fossil fuel subsidies and investments don’t add up. 1/3 pic.twitter.com/1virpuOyYG — Greta Thunberg (@GretaThunberg) January 23, 2020

She added in another tweet: "So either you tell us how to achieve this mitigation or explain to future generations and those already affected by the climate emergency why we should abandon our climate commitments."

At her panel in Davos earlier this week, Thunberg accused politicians of all stripes of making promises but failing to deliver any results.

"Our house is still on fire," she said. "Your inaction is fueling the flames by the hour, and we are telling you to act as if you loved your children above all else."

The 17-year-old activist has also been a target of criticism from President Trump, who took a veiled swipe at her remarks when he spoke at Davos on Wednesday. Mr. Trump said it was time to reject the warnings of "prophets of doom and their predictions of the apocalypse."