First Mentions

As in November, Sen. Bernie Sanders (I-Vt.) was the first candidate to mention the climate crisis before it officially came up in the debate. That mention came when he was asked if he would vote in favor of a new North American Free Trade Agreement (NAFTA) between the U.S., Canada and Mexico that recently passed the House of Representatives.

Sanders said he would not support the new agreement, partly because it does not address environmental issues.

"And, by the way, the word 'climate change,' to the best of my knowledge, is not discussed in this new NAFTA agreement at all, which is an outrage," Sanders said, according to a debate transcript published by The Washington Post.

Sen. Elizabeth Warren (D-Mass.) was the next candidate to raise the issue independently when answering a question about what she would say to voters who think the economy has been strong under President Donald Trump. Warren echoed other candidates' arguments that many Americans were still struggling and said this was because the government tended to work better for the wealthy than for everyone else. The climate crisis, she argued, was a case in point.

"Works great for giant oil companies that want to drill everywhere, but not for the rest of us who see climate change bearing down upon us," she said.

After these early references, the candidates were then fielded three climate-related questions that led to a robust back-and-forth.

The Question of Sacrifice

The first two climate questions revolved around issues of sacrifice. The first, directed at Sen. Amy Klobuchar (D-Minn.) asked if she would subsidize the relocation of families and businesses away from places vulnerable to wildfires or sea level rise. The second, directed at former Vice President Joe Biden, asked if it was worth it to sacrifice immediate growth in the oil and natural gas industries for the sake of transitioning to a greener economy.

The candidates mostly side-stepped the first question and focused on their climate policies. Klobuchar said she would rejoin the Paris agreement and reinstate Obama-era policies like the Clean Power Plan and higher auto-efficiency standards.

Billionaire Tom Steyer said he would declare a state of emergency on day one of his administration, and challenged South Bend, Indiana Mayor Pete Buttigieg to make the climate crisis a higher priority.

Buttigieg, for his part, promoted his plan to institute a carbon tax and use the dividends to fund renewable energy research.

But the candidates also pushed back on the idea that climate action necessarily meant sacrifice.

"Not only can we clear up the air and water in the black and brown communities where our pollution is concentrated, this is also the opportunity to create literally millions of middle-class union jobs, well-paid, across the United States of America," Steyer said, according to the transcript. "Our biggest crisis is our biggest opportunity."

Biden also argued that he would sacrifice fossil fuel growth — what sacrifice moderator Tim Alberta of Politico said could cost thousands to hundreds of thousands of blue collar jobs — because "the opportunity for those workers to transition to high-paying jobs, as Tom said, is real."

Sanders came out most forcefully against the notion of sacrifice, challenging the framing of the question itself, according to HuffPost.

"It's not an issue of relocating people and towns," Sanders said, to thunderous applause. "The issue now is whether we save the planet for our children and grandchildren."

The Question of Nuclear

Alberta then focused the climate conversation on nuclear energy with a question first directed at Warren.

"Many of our Western allies rely heavily on nuclear energy because it's efficient, affordable, and virtually carbon-free. And many climate experts believe that it's impossible to realize your goal of net zero emissions by the year 2050 without utilizing nuclear energy. So can you have it both ways on this issue?" Alberta asked, according to the transcript.

Warren reiterated her commitment to keeping existing nuclear plants running while transitioning away from fossil fuels, but said she would not build any more reactors.

"We've got to get the carbon out of the air and out of the water. And that means that we need to keep some of our nuclear in place," she said.

On this issue, she differs from Sanders, who has promised to shutter existing nuclear reactors, according to HuffPost.

Businessman Andrew Yang, meanwhile, came out strongly in support of reactors that use thorium, which produces less waste than uranium.

Steyer, however, argued that nuclear was not competitive price-wise in the U.S. and raised the problems of disasters and waste storage.

"We actually have the technology that we need. It's called wind and solar and batteries. So, in fact, what we need to do, we can do," Steyer argued.