Presidential candidates talked up their vision for fighting climate change Thursday, with Vermont Sen. Bernie Sanders and others promising "bold" action on an issue that has become a defining one for Democrats ahead of the 2020 election.

In total, 12 presidential candidates — 11 vying for the Democratic nomination and one Republican mounting a primary challenge President Donald Trump — are pitching their environmental plans during the two-day, town-hall style event taking place at Georgetown University's Institute of Politics and Public Service in Washington. The forum, which is hosted by MSNBC's Chris Hayes and Ali Velshi, comes amid a week-long series of climate coverage from NBC News, MSNBC, Telemundo and NBC News digital.

Activists, lawmakers and presidential candidates such as Washington Gov. Jay Inslee, who ended his campaign last month, have pushed the Democratic National Committee to hold a climate-specific primary debate. So far, the DNC has not agreed to do so as some pressure builds for candidates to zero-in on the issue. CNN hosted a similar climate event earlier this month.

Hayes and Velshi interviewed the participating candidates and took questions from the audience. Colorado Sen. Michael Bennet spoke first, followed by entrepreneur Andrew Yang, author Marianne Williamson, Sen. Bernie Sanders of Vermont, former Rep. John Delaney, D-Md., Rep. Tim Ryan, D-Ohio, and former Housing and Urban Development Secretary Juliàn Castro.

Juliàn Castro

Castro promoted his sweeping $10 trillion climate plan that he said would result in the country having net-zero emissions in the next 30 years.

His plan calls for a public-private partnership, and he said it would result in 10 million new jobs.

"I believe we can reach our goals if this is all hands on deck all across the federal government" and private sector, Castro said. "We have to begin immediately."

A key component of his plan is undoing the damage the Trump administration is doing by loosening environment regulations, Castro said.

"The next president is going to have spend the next two and half years rolling back the bad things this administration has done," he said, adding that he’d have people "who believe in science" leading the fight against climate change.

The former San Antonio, Texas mayor would also create a new category of refugee — climate refugees, for people in other countries who are being displaced by global changes. He cited a report that found over 200 million people could be displaced worldwide by 2050.

The former housing secretary was optimistic that the country would turn its back on the Trump era. "Texas is going to go blue in 2020. So is Arizona," he said.

Tim Ryan

Ryan focused on how an aggressive climate policy would help bring back manufacturing jobs to struggling industrial and rural areas.

"Let's get away from the left-right conversation and get into the new and better conversation,” the Ohio congressman said.

He said he would tie financial incentives with environmental incentives in a bid to spur investment, and invest federal funding into infrastructure to provide jobs for displaced coal and fossil fuel workers.

“People can start making money from this stuff,” Ryan said, touting the success he’s already seen around the country with regenerative agriculture, which captures carbon from the air, enriches the soil and reduces flood risks. Increased reliance on solar and wind power would lead to more manufacturing, as would building more electric cars and charging stations, he said. "There’s a new economy that’s ready to bust out and is ready to go," Ryan said.

Unlike some of the other candidates, he said he would not look to immediately ban fracking, noting it’s brought a number of jobs to his home state.

“We’re not going to be able to power the country in two years on renewables,” he said. “I think natural gas has got to be the bridge.”

John Delaney

The former congressman from Maryland took aim at Trump, criticizing the current administration’s “economic isolationism” and calling for a return to “the old-fashioned U.S. business of having alliances.”

"I'll sign us back into Paris before I sit down in the chair in the Oval Office," he promised, referring to the landmark Paris Agreement that set goals for a coalition of nations to curb greenhouse gas emissions and limit global warming. Trump withdrew the U.S. from the agreement in 2017.

Delaney outlined a three-point climate change plan that would aim to achieve net zero carbon emissions by 2050, promote the global development of clean technologies and remove 20 percent of the atmosphere’s carbon dioxide through a Midwest pipeline, which he referred to as the “CO2 Thruway.”

The “cornerstone” of his plan, according to Delaney, is a carbon fee and dividend that would place a 3 trillion dollar tax on carbon dioxide emissions and redistribute the money back to American citizens. He likened this model to the Alaska Permanent Fund, which pays an annual dividend to Alaskan citizens from investment earnings on mineral royalties.

Delaney also envisioned the U.S. leading a “Paris 2.0,” a “global consortium” in which countries would “contribute intellectual capital” and “build the energy solutions that are needed for the developing world.”

Bernie Sanders

The Vermont senator said that he would be the enemy of the fossil fuel industry because "unlike Trump, I do believe in science."

He said that one of his first acts a president would include signing an executive order that would end fossil fuel extraction on public land, which he said would swiftly and severely disrupt the industry.

Sanders also made the case that, as president, he would visit states in which fossil fuels are dominant, such as coal in West Virginia, and mobilize its residents around the issue.

"I intend to be commander-in-chief of the military but I will also be the organizer in chief," he said. "We’re going to help rebuild states like West Virginia and states all over this country."

Sanders said that one of his plans include job training for people working in industries that contribute to climate change and transition many to green jobs through job training. He said his plan would also include providing health care and other services to ease the shift.

"We are paying a lot of attention to those workers but we also understand that we are going to create some 20 million good-paying (green) jobs," he said.

Sanders also pointed out his plan would also combat environmental pollution in the air and water, particularly in economically struggling areas of the country where many minorities live.

"I happen to believe what the scientist are telling us and that means if we are going to save the planet we have to be extremely bold and extremely aggressive," he said.

Sanders made the case that there needs a World War II type of mobilization in America to fight climate change across the country and the globe. He said that he would welcome more refugees into the country who have been displaced because of the effects of climate change.

"These type of extreme weather disturbances hitting us more and more frequently with greater intensity. Think about people all over the world driven from their homes because they can't grow crops anymore, they can't find drinking water, they are going to go elsewhere," he said. "We have to welcome people all over the world, I mean we are talking about God knows how many of millions and millions and millions of people who are going to be dispossessed as a result of climate change. It's a huge issue."

He also said that as president he would sanction countries that do not take steps to reduce emissions that contribute to climate change.

Marianne Williamson

Williamson told the audience it's time for some "radical truth-telling" in America, including on the issue of climate change. The best-selling self-help author made the case for a revolution to push back against corporations ⁠— and fossil fuel-friendly lawmakers ⁠— to combat the issue.

"I want to tell the American people it's as bad as you fear and we are going to get through this because were American and we're going to have a season of repair," she said. "We are going to mobilize and it's not during WWII it wasn't Democrats versus Republicans, left versus right, we are all Americans and we are going to mobilize."

But, she also said a Williamson administration would work with oil, gas and fossil-fuel industries to combat the warming of the planet.

"There is more money to be made in green energy, there is more money to be made in green jobs," she said.

Invoking 16-year-old Swedish climate activist Greta Thunberg, Williamson also made the case for a one-year mandatory national service for young Americans, ages 18 to 26, to work on climate issues.

"I wanna go to Washington and be a grown-up Greta Thunberg," she said, adding that she wants to be a president with "moral influence" and "the highest ideals of humanitarian concern."

She said that it takes American voters to stand up against both parties so that Washington can take the issue seriously. She called President Donald Trump is an "opportunistic infection" and he won the presidency because American voters had become disengaged from the political process.

She also pushed back against the assertion that she is anti-science because of a now-deleted tweet in which she suggested prayer could deter Hurricane Dorian.

"I'm Jewish, I go to the doctor. There's nothing anti-doctor about me, there's nothing anti-science about me," she said. "Mock and make fun of everybody who believes that God is powerful. I am a woman who believes in God - it doesn't mean I don't believe in science. it doesn't mean I lost some brain cells, doesn't mean I'm less intelligent."

Andrew Yang

It's already too late to curb "some of the warming" that will affect the earth, Yang said, adding that decades ago would have been the right time to deal with a changing climate. The world will continue to warm, more areas will become uninhabitable and there is nothing humans can do to stop it, he said.

Still, Yang said he wants to address the environment with "big moves" and called it problem "1A" that the U.S. needs to address.

"We should've been doing this work 20 years ago, but the second best time is now," he said.

Yang has proposed taxing carbon production by corporations, subsidizing and investing in nuclear power alternatives and passing a constitutional amendment to "safeguard the environment."

Yang on Thursday called climate change the "clearest existential threat to our way of life and our entire species," adding that climate change and automation are closely tied together for him. Yang has proposed a sort of universal basic income he calls a "Freedom Dividend," which would amount to a $1,000 monthly payment to every American adult aimed at offsetting some of the disruption caused by a changing economy.

Michael Bennet

Bennet said climate change should be treated as the biggest problem facing society at the moment, not just one problem among many. Still, the Colorado senator repeatedly stressed the need to shape climate policy in a way that will appeal in swing districts and states.

Democrats can offer "no compromise on the science, but we have to produce a set of policy choices for the American people that can allow us to win these races and produce an enduring coalition" to pass climate legislation, Bennet told Hayes.

He noted the importance, particularly in 2020, of not losing the economic debate on climate change and said too often Democrats are painted as "anti-jobs and anti-infrastructure" with their environmental efforts.

"We can't lose an economic debate to a climate denier," he said, pointing to Trump. "He should never have won that economic argument, and he did."

If elected president, Bennet said he would convene all "people we needed to convene to begin to draft the legislative plan" on climate change. If after nine months, the Senate was unable to or refused to pass such legislation, Bennet said he would use the authority within his power to begin addressing climate change through executive action.

The two biggest challenges with climate policy is transitioning the economy, which he said he is "completely confident" will be able to happen, and whether "our democracy is up to this task."

Bennet was much less optimistic on the latter point.