Bill Nye frolicked in a ball pit to explain how the planet's populations compete for resources. He took a chainsaw to a loaf of bread, comparing it to Earth's crust, and he was nearly blown away in a wind tunnel while shouting "science!"

But he's talking about global warming now - and he's in no mood to mess around.

"By the end of this century, if emissions keep rising, the average temperature on Earth could go up another four to eight degrees," Mr 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," Mr Nye said, punctuating his point with some R-rated profanity.

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

Coral reefs: Trying to combat climate change damage Show all 25 1 /25 Coral reefs: Trying to combat climate change damage Coral reefs: Trying to combat climate change damage Masked Butterflyfish (Chaetodon semilarvatus) swimming over a bommie reef in the Egyptian Red Sea marine reserve of Ras Mohamed, off the southern tip of the Sinai peninsula AFP/Getty Coral reefs: Trying to combat climate change damage Rising sea temperatures cause corals to bleach (go white) and die Getty/iStock Coral reefs: Trying to combat climate change damage A giant clam is seen nestled among coral reefs at the Obhor coast, 30 kms north of the Red Sea city of Jeddah AFP/Gett Coral reefs: Trying to combat climate change damage Coral reef in seychelles that has degraded After the reef has died they break up and become rubble. On this reef there is some regrowth of young corals so there is hope for recovery Getty Coral reefs: Trying to combat climate change damage Coral gardening A rabbitfish in a net H Goehlich Coral reefs: Trying to combat climate change damage A school of fish and a sea can in a healthy coral reef off the coast of Isla Mujeres, Mexico Getty/Lumix Coral reefs: Trying to combat climate change damage Sky views of great barrier reef in Australia Getty/iStock Coral reefs: Trying to combat climate change damage A fish swims among coral reefs at the Obhor coast AFP/Getty Coral reefs: Trying to combat climate change damage Researchers from the Interuniversity Institute for Marine Sciences in the southern Israeli resort city Eilat monitor coral growth while scuba diving in the Red Sea AFP/Getty Coral reefs: Trying to combat climate change damage Coral gardening A rope nursery Nature Seychelles Coral reefs: Trying to combat climate change damage Fish swimming off the coast of Egypt's Red Sea resort of Hurghada. The rebounding tourism sector is worrisome for the fragile marine ecosystem AFP/Getty Coral reefs: Trying to combat climate change damage Coral gardening A parrotfish on the reef C Reveret Coral reefs: Trying to combat climate change damage Gorgonian sea fan on a a coral reef in the Egyptian Red Sea marine reserve of Ras Mohamed AFP/Getty Coral reefs: Trying to combat climate change damage A diver swims during a Great Barrier Reef experience on Lady Elliot Island, Australia Getty/Tourism Queensland Coral reefs: Trying to combat climate change damage Jessica Bellsworthy, a PhD student conducting research on the coral reefs of the Gulf of Eilat, holds a coral in an aquarium at the Interuniversity Institute for Marine Sciences in Eilat AFP/Getty Coral reefs: Trying to combat climate change damage Coral reefs in the water off the Obhor coast, 30 kms north of the Red Sea city of Jeddah in 2008 AFP/Getty Coral reefs: Trying to combat climate change damage A diver photographs golden anthias (Pseudanthias aurulentus) on a coral reef in the Egyptian Red Sea marine reserve of Ras Mohamed AFP/Getty Coral reefs: Trying to combat climate change damage FUNAFUTI, TUVALU - AUGUST 15: From the air the ocean (L) and the logoon (R) and separated by a thin stip of land on August 15, 2018 in Funafuti, Tuvalu. The small South Pacific island nation of Tuvalu is striving to mitigate the effects of climate change. Rising sea levels of 5mm per year since 1993, well above the global average, are damaging vital crops and causing flooding in the low lying nation at high tides. Sea water rises through the coral atoll on the mainland of Funafuti and inundates taro plantations, floods either side of the airport runway and affects peoples homes. The nation of 8 inhabited islands with an average elevation of only 2m above sea level is focusing on projects to help it and its people have a future. Four of the outer islands are 97% solar energy dependent and the Tuvalu Government is working to achieve 100% renewable energy from wind and solar by 2025. Tuvalu's 11,000 inhabitants see the effects of climate change in their daily life. (Photo by Fiona Goodall/Getty Images for Lumix) Fiona Goodall Getty/Lumix Coral reefs: Trying to combat climate change damage A photo taken on April 4, 2019 shows fish swimming off the coast of Egypt's Red Sea resort of Hurghada. - In dazzling turquoise waters off Egypt's Red Sea coast, scuba divers swim among delicate pink jellyfish and admire coral -- but the rebounding tourism sector is worrisome for the fragile marine ecosystem. (Photo by Mohamed el-Shahed / AFP) (Photo credit should read MOHAMED EL-SHAHED/AFP/Getty Images) MOHAMED EL-SHAHED AFP/Getty Coral reefs: Trying to combat climate change damage Coral gardening A damselfish Sarah Frias-Torres Coral reefs: Trying to combat climate change damage Divers swim past a coral reef in the Egyptian Red Sea marine reserve AFP/Getty Coral reefs: Trying to combat climate change damage A puffer fish hovering above coral in the Egyptian Red Sea marine reserve AFP/Getty Coral reefs: Trying to combat climate change damage Researchers from the Interuniversity Institute for Marine Sciences in the southern Israeli resort city Eilat monitor coral growth while scuba diving on June 12, 2017 in the Red Sea off Eilat. Global warming has in recent years caused colourful coral reefs to bleach and die around the world -- but not in the Gulf of Eilat, or Aqaba, part of the northern Red Sea. At the Interuniversity Institute for Marine Sciences in southern Israeli resort city Eilat, dozens of aquariums have been lined up in rows just off the Red Sea shore containing samples of local corals AFP/Getty Coral reefs: Trying to combat climate change damage This photo taken on April 21, 2017 shows an aerial shot of part of mischief reef in the disputed Spratly islands on April 21, 2017. Philippine Defence Secretary Delfin Lorenzana flew to a disputed South China Sea island on April 21, brushing off a challenge by the Chinese military while asserting Manila's territorial claim to the strategic region. / AFP PHOTO / TED ALJIBE (Photo credit should read TED ALJIBE/AFP/Getty Images) TED ALJIBE AFP/Getty Coral reefs: Trying to combat climate change damage fish swimming off the coast of Egypt's Red Sea resort of Hurghada AFP/Getty

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?"

Mr Nye took to Mr 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.").

Mr Nye appeared to endorse representative Alexandria Ocasio-Cortez's trademark legislation, telling lawmakers they needed to do something and chiding those who have said the proposal is too 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."

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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 Mr Oliver, as his show ended, gasped, "I think we've all broken Bill Nye."

But for Mr 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 Senator Bernie Sanders about the dangers of man-made climate change and has debated Fox News' Tucker Carlson, who questioned whether those dangers really exist.

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!"

That month, he also debuted a new TV series with an equally urgent title: "Bill Nye Saves the World." And judging by his turn on Mr Oliver's show, he's willing to go to great lengths to get his message out - even if it takes some fire and fury.