But while they may struggle on a debate stage, many have published detailed climate plans (Vox outlined those here), most notably Washington Gov. Jay Inslee, who has made climate the central priority of his campaign.
How candidates frame the issue of climate change is important, according to Leah Stokes, a political scientist who works on environmental policy at the University of California, Santa Barbara. For one thing, it's a mistake for candidates to frame the issue in terms of “doomsday," she says.
“That's not actually a great way to communicate on climate change and definitely not for a politician running for president,” she said. “You have to create a vision for people, you have to give them hope. You can't just lead with devastation and a question mark.”
Candidates should talk about a “climate crisis” and convey the scope of the problem, rather than using language like "climate change" or "global warming." This is something that some media organizations have started to do.
"'Climate crisis' resonates with people on an emotional level,” she said.
Another challenge for candidates is to bring a measure of hope into their message, and include the possibility of innovation, economic opportunity and job creation. “There are ways that you can talk about climate change that makes the stakes clear for people but don’t leave them in a state of fear,” she said.
So what did the candidates say? Below is how each of the participants who talked about climate framed the issue, and the plans they cited for addressing it.
Not every candidate was asked about climate change, but several, including Sen. Elizabeth Warren, brought it up on their own.
Other candidates, like Sen. Michael Bennet and Andrew Yang, the tech entrepreneur, mentioned climate change briefly in a response to a lightning-round style question about the first issue of their presidency. (The New York Times has charts with how much time each candidate spent talking about each issue in both the first and second debates).
Washington Gov. Jay Inslee
Climate framing: "Our towns are burning, our fields are flooding, Miami is inundated ... this is a climate crisis, an emergency and it is our last chance."
Key argument: "I am the candidate, and the only one to say this has to be the top priority of the United States. The organizing principle to mobilize the United States so that we can do what we've always done, lead the world and invent the future and put 8 million people to work."
Former Rep. Beto O'Rourke
Climate framing: "You have to bring everybody in on the decisions and the solutions to the challenges that we face ... We were in Pacific Junction, a town that had never meaningfully flooded before, just up against the Missouri River in Iowa. Every home in that community had flooded."
Key argument: "We are going to fund resiliency in those communities...that are on the front lines of climate change today. We're going to mobilize $5 trillion in this economy over the next 10 years to free ourselves from a dependence on fossil fuels, and we're to put farmers and ranchers in the driver's seat."
Former HUD Secretary Julián Castro
Climate framing: "My first visit after I announced my candidacy wasn't to Iowa or New Hampshire, it was to San Juan, Puerto Rico." [Devastated by a hurricane in 2017].
"People should know that if I'm elected president everybody will count."
Key Argument: "I'm one of the few candidates in this race with executive experience with a track record of getting things done. When I was mayor of San Antonio, we moved our local public utility. We began to shift it from coal-fired plants to solar and other renewables and also created more than 800 jobs doing that."
Sen. Elizabeth Warren
Warren wasn't asked about climate, but she tied the issue to an argument about the U.S. economy, which she says only works for people at the top.
Key argument: "Who is this economy really working for? ... It's doing great for giant oil companies that want to drill everywhere, just not for the rest of us who are watching climate change bear down upon us."
Rep. Tim Ryan
Climate framing: "If we don’t address that fundamental problem with our connection to workers -- white, black, brown, gay, straight working-class people -- none of this is going to get done."
Key argument: "We are not connecting to the middle-class people. We have got to hinge the center of gravity from being coastal and elitist and Ivy League ... to getting those workers back on our side so we can say: 'we are going to build solar, we are going to build electric vehicles.'"
Former Rep. John Delaney
Delaney wasn't asked about climate change, but he interjected to make a point about using a tax to fight it.
Key argument: "All the economist agree that a carbon pricing mechanism works. You just have to do it right. You can’t put a price on carbon, raise energy prices and not give the money back to the American people. My proposal, which is put a price on carbon, give a dividend back to the American people -- it goes out one pocket, back in the other."
Sen. Kamala Harris
Climate framing: "I don't even call it climate change, it is the climate crisis. It represents an existential threat to us as a species."
Key Argument: "It is a critical issue that is about what we must do to confront what is immediate and before us right now. That is why I support a Green New Deal. It is why I believe on day one, as president I will re-enter us in the Paris agreement."
Mayor Pete Buttigieg
Climate framing: "This is not just happening on the Arctic ice caps. This is happening in the middle of the country, and we've got to be dramatically more aggressive moving forward ... Rural America can be part of the solution instead of being told they're part of the problem."
Key argument: "We need to begin adapting right away. But we also can't skip a beat on preventing climate change from getting even worse ... That's why we need to do a carbon tax and dividend. But I would propose we do it in a way that is rebated out to the American people in a progressive fashion so that most Americans are made more than whole."
Former Gov. John Hickenlooper
Climate framing: "I recognize that we are within 10 or 12 years of actually suffering irreversible damage."
Key argument: "We can't demonize every business, we've got to bring them together to be partners. Ultimately, if we're not able to do that, we will be doomed to failure. We have no way of doing this without bringing everyone together."
Former Vice President Joe Biden
Climate framing: "We make up 15 percent of the problem. Eighty-five percent of the world makes up the rest."
Key argument: "Our administration, we built the largest wind farm in the world, the largest solar energy facility in the world. We drove down the competitive price of both of those renewable sources ... We have to have someone who knows how to corral the rest of the world, bring them together and get something done like we did in our administration."
Sen. Bernie Sanders
Climate framing: "The old ways are no longer relevant. The scientists tell us we have 12 years before [there] is an irreparable damage to this planet. This is a global issue."