Treasury Secretary Steven Mnuchin dismissed criticisms by the climate activist Greta Thunberg on Thursday, saying that she lacked the expertise to call for a complete divestment from the fossil fuel industry.
“After she goes and studies economics in college, she can come back and explain that to us,” he said at a press briefing at the World Economic Forum in Davos, Switzerland, where many world leaders and powerful business figures meet annually.
Ms. Thunberg, a Swedish climate activist who has led demonstrations around the world, spoke at the forum on Tuesday and urged business leaders to stop investing in fossil fuels immediately.
“I wonder, what will you tell your children was the reason to fail and leave them facing the climate chaos you knowingly brought upon them?” Ms. Thunberg, 17, said at the gathering in Davos, a village on the icy reaches of the Swiss Alps. “Our house is still on fire.”
Mr. Mnuchin was asked Thursday about her comments and responded: “Is she the chief economist? Who is she? I’m confused.”
After a brief pause, he said it was only a joke.
A few hours later, Ms. Thunberg shot back on Twitter without mentioning Mr. Mnuchin.
“My gap year ends in August, but it doesn’t take a college degree in economics to realize that our remaining 1.5° carbon budget and ongoing fossil fuel subsidies and investments don’t add up,” she wrote.
She added, “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.”
The exchange came two days after a speech by President Trump at the forum that made little mention of climate change.
His only reference to climate activists was to describe them as “heirs of yesterday’s foolish fortune tellers.”
“They predicted an overpopulation crisis in the 1960s, a mass starvation in the ‘70s, and an end of oil in the 1990s,” he said as Ms. Thunberg listened from the sixth row, where she sat with three other climate activists.
“This is not a time for pessimism,” Mr. Trump said.
An hour later, Ms. Thunberg opened a panel discussion in which she rebuked leaders for ignoring pleas for more action on climate change.
“You say children shouldn’t worry,” Ms. Thunberg said, never mentioning Mr. Trump. “You say, ‘Just leave this to us. We will fix this. We promise we won’t let you down.’”
She then strayed from her prepared remarks to add a new line: “‘Don’t be so pessimistic.’”
She called on the world’s banks, governments and companies to “halt all investments in fossil fuel exploration and extraction.”
“Immediately end all fossil fuel subsidies,” she said. “And immediately and completely divest from fossil fuels.”
Mr. Trump and Ms. Thunberg have clashed before, most prominently on Twitter.
Soon after Time magazine named her Person of the Year for her work around climate change, Mr. Trump commented on social media, “Greta must work on her anger management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!”
She quickly responded.
“A teenager working on her anger management problem,” read a new version of her Twitter biography at the time. “Currently chilling and watching a good old fashioned movie with a friend.”
Ms. Thunberg has criticized American policy on the environment, including Mr. Trump’s decision early in his administration to pull out of the Paris climate accord.
The United States and China are the world’s largest emitters of greenhouse gases. Since taking office, Mr. Trump has rolled back a wide range of emissions regulations, calling them “horrible” examples of federal overreach. Most recently, the Trump administration moved toward finalizing a rule that would strip away environmental protections for streams, wetlands and other water bodies.
Mr. Mnuchin defended the United States’ environmental policies, saying that Mr. Trump “absolutely believes” in a clean environment. He also said that Mr. Trump pulled out of the Paris agreement because he “thought it was an unfair agreement for the United States.”