U.S. Treasury Secretary Steven Mnuchin sharply criticized the financial credentials of Swedish climate activist Greta Thunberg on Thursday, saying the 17-year-old should study economics at college before lecturing the U.S. on fossil fuel investments, reports CNBC.

Speaking at a press briefing at the World Economic Forum, Mnuchin was asked whether the world’s largest economy needed to completely and immediately divest from fossil fuels.

“Is she the chief economist or who is she? I’m confused,” Mnuchin said, before adding this was “a joke. That was funny.”

“After she goes and studies economics in college she can come back and explain that to us,” Mnuchin said.

Thunberg, alongside 20 other young climate activists, has called on global leaders attending the forum to stop the “madness” of ongoing investments in fossil fuel exploration and extraction and “completely divest” from fossil fuels.