“By the end of this century, if emissions keep rising, the average temperature on Earth could go up another four to eight degrees,” Nye said, appearing on a segment of HBO’s “Last Week Tonight with John Oliver” on Sunday.

The famously zany scientist and host of the PBS series “Bill Nye the Science Guy” then aimed a blowtorch at a globe to illustrate his argument: “What I’m saying is: The planet’s on fire,” Nye said, punctuating his point with some R-rated profanity.

Gone was the Nye of the ’90s, the man whose show was a middle school substitute teacher’s secret weapon. This was the Science Guy of 2019, delivering a sermon aimed directly at the legions of Gen Xers and Millennials who were weaned on Nye’s brand of wacky pedagogy.

And he had a message for his erstwhile pupils, especially those who eventually became members of Congress.

“Grow up,” he said, injecting some more language that wouldn’t fly in public broadcasting. “You’re not children anymore. I didn’t mind explaining photosynthesis to you when you were 12. But you’re adults now, and this is an actual crisis; got it?”

Nye took to Oliver’s show to teach his audience about global warming and its possible solutions — namely, the Green New Deal and carbon pricing (in his words, “when something costs more, people buy less of it. Safety glasses off. That’s it.”).

Nye appeared to endorse the trademark legislation proposed by Rep. Alexandria Ocasio-Cortez (D-N.Y.), telling lawmakers they needed to do something and chiding those who have said the Green New Deal is prohibitively expensive. As he spoke into the camera, the globe continued to burn.

“There are a lot of things we could do to put it out,” he said. “Are any of them free? No, of course not. Nothing’s free, you idiots.”

His harsh tone surprised some — one viewer wrote on Twitter that he “just heard Bill Nye swear and it’s blowing my mind.” Another observed, “Global warming is so bad that it now has Bill Nye the Science Guy cursing us out to fix it.”

Even Oliver, as his show ended, gasped, “I think we’ve all broken Bill Nye.”

But for Nye, the man whom The Washington Post once dubbed “half mad professor, half Mr. Rogers,” the role of climate crusader is not a new one. In recent years, he has talked with Sen. Bernie Sanders (I-Vt.) about the dangers of man-made climate change and has debated Fox News’s Tucker Carlson, who questioned whether those dangers really exist.

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In April 2017, he took the stage at the March for Science in Washington and declared that “science is for all,” the best antidote to anti-vaxxers and climate deniers alike.

“Our lawmakers must know that science serves every one of us,” he said then. “Every citizen of every nation in society. Science must shape policy. Science is universal. Science brings out the best in us. With an informed, optimistic view of the future, together we can — dare I say it — save the world!”