Over a marathon seven hours on Wednesday, 10 of the Democratic presidential candidates participated in back-to-back CNN town hall events on climate change, fielding questions from audience members and several of the network’s top anchors.

It was something of a consolation prize for what many candidates and activists really wanted: an official Democratic debate devoted to climate change, which the Democratic National Committee refused to hold. But it was also more in-depth than any official debate would have been — because did we mention it lasted seven hours? — and left no doubt that climate change has become one of the most important issues in the 2020 primary.

Here are some takeaways.

Young voters are asking tough questions.

Young people asked some of the toughest, most pointed questions of the night, holding Democrats’ feet to the fire at every turn.

Sila Inanoglu, a high school student, asked Julián Castro, who supported fracking as mayor of San Antonio, “Why should we trust you as president to transition our economy to renewables?”

Ari Papahronis, a Columbia University student, asked Senator Amy Klobuchar of Minnesota whether she would “take on the beef and dairy industries that have so much influence in our government” given the agriculture-rich state she represents.

And Isaac Larkin, a 27-year-old Ph.D. candidate at Northwestern, caused a stir when he asked former Vice President Joseph R. Biden Jr., “How can we trust you to hold these corporations and executives accountable for their crimes against humanity when we know that tomorrow you are holding a high-dollar fund-raiser hosted by Andrew Goldman, a fossil fuel executive?”

Both the moderator, Anderson Cooper, and Mr. Biden pointed out that Mr. Goldman, a co-founder of a natural gas production company, has no current role with the firm. But the question forced Mr. Biden to use much of his 40-minute segment to explain how his link to Mr. Goldman did not violate his pledge to avoid donations from the fossil fuel industry.

Several of the young people who got in questions at Wednesday’s forum were members of the Sunrise Movement, the youth-led environmental group. That’s no accident. The group pushed hard, though ultimately unsuccessfully, for the Democratic National Convention to hold a climate debate, and the CNN forum was in large part a reaction to that demand.

Democratic rivals agreed to agree.

The debate over the best ways to fight climate change has clear parameters. If you are a Democratic candidate for president, you believe climate change is an existential threat not only to the United States but to human civilization. You believe the country needs to reach net-zero carbon emissions by 2050 at the very latest. And there are certain policies you can’t avoid if you want to get there.

Pretty much everyone wants a moratorium on oil and gas leases on public lands. Pretty much everyone wants to create incentives for more sustainable farming practices. And everyone wants to rejoin the Paris climate accord — though the candidates’ proud declarations on that front irritated Senator Cory Booker of New Jersey.

“I’m sorry,” he said. “That is, like, a cost of entry even to run for president or talk about the presidency.”

Most of the candidates also support some form of putting a price on carbon emissions, at least in theory. But their level of specificity varies: Andrew Yang, the tech entrepreneur, knows exactly how much he wants to charge per ton of carbon dioxide, while Ms. Klobuchar wants to see who controls Congress before deciding whether to pursue a tax or a cap-and-trade program or something else.

One thing is certain: All of the candidates want to spend money, and lots of it — money for clean energy research, money to develop carbon-capture technologies, money to expand public transportation, and money to help communities withstand the effects of climate change already underway.

Nuclear energy is deeply divisive.

There were several areas where the candidates differed in how far or fast they wanted to act. But nuclear energy was a major issue where they were fundamentally divided.

One wing, which included Mr. Booker and Mr. Yang, argued that it would be all but impossible to reach net-zero carbon emissions by 2050 without relying to some extent on nuclear energy, even if solar and wind power were preferable in the long term.

Nuclear energy currently accounts for about 20 percent of the country’s power, and people who think it’s possible to get to net-zero carbon without nuclear energy “just aren’t looking at the facts,” Mr. Booker said.

He and Mr. Yang both noted that new technology could make it possible to build nuclear reactors that are not vulnerable to the meltdowns like those at Chernobyl and Fukushima.

Other candidates were much more hesitant. Senator Kamala Harris of California talked about the dangers of nuclear waste before suggesting she would let states decide whether to accept nuclear plants. Ms. Klobuchar similarly emphasized the waste problem and said she would not support new plants “unless we can find safe storage.”

Senator Bernie Sanders of Vermont unequivocally opposed nuclear energy, rejecting a questioner’s premise that it was irresponsible, in the face of such an urgent crisis, to take a carbon-free option off the table.

“It doesn’t make a whole lot of sense to me to add more dangerous waste to this country and the world when we don’t know how to get rid of what we have now,” he said.

Natural gas is the new litmus test.

The battle over eliminating fossil fuels has moved to a new front: natural gas.

Coal, notably, got relatively short shrift during the seven-hour discussion. Despite the Trump administration’s efforts to revive the industry, coal use is rapidly declining. That has helped make encouraging its demise a universally accepted position among Democratic candidates.

But natural gas, and whether to commit to a ban on fracking, is dividing the field.

Mr. Sanders has gone the farthest, urging Democrats to support a “full fracking ban” — a call that Ms. Harris and Senator Elizabeth Warren of Massachusetts also embraced.

Mr. Castro said he has not called for such a ban, but supports states that want to impose one. He also defended his previous support for fracking. “We had been saying that natural gas was a bridge fuel” a decade ago, he said. “We’re coming to the end of the bridge.”

On the other end of the spectrum was Ms. Klobuchar, who said she would not ban fracking because she sees natural gas “as a transitional fuel” that “is better than oil, but it’s not nearly as good as wind and solar.”

“I think I’m being honest about what we need to do to get to where we are,” she said.

Mr. Biden said he opposed new drilling on federal lands but not a nationwide ban.

Republicans took the forum seriously.

President Trump issued a series of mocking tweets as CNN prepared to begin the forum. But other Republicans took the discussion of how to rein in emissions seriously.

That’s not to say they agreed with the Democratic candidates’ views on pricing carbon, banning fracking or spending trillions of dollars to reach net-zero emissions by 2050. Far from it. But as the forum aired on TV, a number of conservatives did strive to show that they also want to address climate change and have their own solutions in mind.

Representative Greg Walden of Oregon, the ranking member of the House Energy and Commerce Committee, called for beefing up nuclear power “which is safe, reliable and emissions free, and which experts agree must be part of our strategy to reduce emissions.”

Representative Dan Crenshaw of Texas pointed to other measures like promoting carbon capture and storage technology that he said “clean up the environment, promote innovation, don’t ruin the economy AND are based in reality!”

Even the United States Chamber of Commerce, which historically has lobbied against laws and regulations that would curb emissions, issued a news release promoting the “pioneering groundbreaking solutions” in the energy sector to curb climate change.

Environmental activists say these kinds of efforts don’t match the urgency of the climate change challenge. But the fact that key Republicans are calling for action at all is noteworthy.