The ??? ?? ??DNC killed the climate debate. But on Wednesday night 10 Democratic presidential hopefuls will still have the chance to show their climate savvy in a less-heated environment. Each candidate will take the stage, one by one, during CNN's 7-hour climate crisis town hall event.

All 10 participants -- the likes of Elizabeth Warren, Pete Buttigieg, Bernie Sanders, and Kamala Harris -- have now released climate plans detailing how they would decarbonize the U.S. economy to avoid the worsening consequences of a heating planet. Many of these consequences, like increased drought, wildfire, and vanishing ice, are readily apparent.

CNN is hosting the televised event and will live stream the town hall from its website. The first candidate, former Housing and Urban Development Secretary Julián Castro will take the stage at 5 p.m. ET, and the last, Senator Cory Booker, will be interviewed at 11:20 p.m. ET. Each candidate is slotted for 40 minutes.

The Democrats' climate plans are generally ambitious because Earth has an increasingly grim carbon dioxide problem. Atmospheric levels of the potent heat-trapping gas are skyrocketing. What's more, the rate of CO2 increase in the atmosphere is now unprecedented in both the historic and geologicrecord. Human activity is the unquestionable, and only plausible, cause.

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Atmospheric scientists, geologists, and climate researchers globally have determined that the solution is to rapidly slash carbon emissions and curb Earth's warming at 1.5 degrees Celsius (2.7 Fahrenheit) above pre-Industrial Revolution levels. This won't be easy. It might be nearly impossible.

“Limiting warming to 1.5 C is possible within the laws of chemistry and physics but doing so would require unprecedented changes,” Jim Skea, a leading UN Intergovernmental Panel on Climate Change scientist, said last year.

A seven-hour town hall promises to cover a lot of ground among candidates who will have varying grasps of the climate problem, and critically, how to address it. Here's what to watch out for.

Cars and Trucks

If a candidate wants to decarbonize the U.S., they must have a plan to transition vehicles away from gasoline. Why?

The transportation sector is the leading contributor of greenhouse gas emissions in the U.S.

SEE ALSO: How to fix America's worst freeway

This almost certainly means incentivizing the use and purchase of electric vehicles. People will need a plethora of charging stations (which regional climate plans like Los Angeles' New Green Deal call for) as well as financial help so they can afford new electric cars. In 2018, around 2 percent of new vehicle purchases in the U.S. were electric.

Decarbonizing transportation also means ramping up electrified public transportation, including trains that can withstand hotter climes.

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China

The U.S. can't solve Earth's CO2 problem alone -- though it's a giant player (and the nation's carbon emissions ticked up in 2018).

If humanity wants to stay under 1.5 or even 2 C, there must be rapid movement by the four biggest carbon players: the U.S., the EU, India, and China. "The math doesn’t work if you don’t have movement by the major emitters," Kelly Levin, a senior associate at the World Resources Institute, an environmental research organization, told Mashable last year.

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China, with its massive fleet of new coal plants, is a particular problem. How will candidates influence China to rapidly transition to clean energy?

It's a daunting task, as the developing world has a right to and desire for an improved standard of living with conveniences like air-conditioning and electricity that fossil fuels generally provide. "The world’s middle class has been growing at an unprecedented rate, and as you move up the income ladder, your carbon footprint expands," Bill Gates, who is funding futuristic carbon-free technologies, blogged in 2018.

Coal -- the dirtiest burning fossil fuel -- is dominant in China, producing most of the communist nation's energy. Indeed, renewable energy in China is making leaps and bounds, but "non-fossil sources are still minuscule in the Chinese energy system," noted Glen Peters, the research director at the Center for International Climate Research.

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Nuclear

If the U.S. hopes to completely transition away from coal and gas for power generation in the coming decade and beyond, it's almost certain that nuclear power -- which doesn't emit carbon -- will play a critical role.

"I haven’t seen a single U.S. decarbonization study that credibly shows net-zero [carbon emissions] by 2030 without keeping existing nuclear power online," Narayan Subramanian, a decarbonization expert studying climate policy at Columbia University, told Mashable in August.

Nuclear energy generated nearly 20 percent of the nation's energy in 2018. Yet some campaigns, like Bernie Sanders', plan to phase nuclear out.

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The elephant in the room

Decarbonizing the U.S. power sector, while a massive effort, is perhaps the "easiest" part of achieving the nation's carbon-free or carbon-neutral economy. That's because the broad solution -- replacing coal plants with renewable energy (and also natural gas) -- is already happening and is often cheaper than continuing to mine and burn coal.

In 2018, coal use in the U.S. dropped to its lowest levels in 40 years, according to the U.S. Energy Information Administration. In 2019, even lobbying from coal-proponent President Donald Trump failed to keep a coal plant open.

"It’s the market making its voice known," Ahmed Abdulla, an energy expert at UC San Diego's School of Global Policy and Strategy, told Mashable in April.

What's more, a recent report by energy analysts Energy Innovation found that "wind and solar could replace approximately 74 percent of the U.S. coal fleet at an immediate savings to customers."

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But ditching coal won't decarbonize other major sectors of any economy. "The problem is that coal is the low-hanging fruit," Phil MacDonald, an analyst at the climate policy organization Sandbag, told Mashable. MacDonald emphasized the challenges ahead in decarbonizing big industrial sectors like concrete and steel in Europe.

Globally, the cement industry generates a whopping 8 percent of the planet's carbon emissions. How will a candidate propose decarbonizing concrete and industrial sectors in the U.S.?

"That’s the elephant in the room," said Columbia University's Subramanian.

Republicans

If a Democrat is elected president, how will they persuade Republicans -- which presently control the Senate -- to support unprecedented climate policy and investment?

This may be the greatest hurdle of all. Even before President Trump took office, Republicans had been largely suspicious of environmental policy for decades.

The Senate requires 60 votes to pass any bill, so Democrats would need to garner support from Republicans -- some of whom are still stuck on the question of what, exactly, is driving Earth's relentless warming trend.

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One potential path forward is to emphasize the middle-class job creation that comes along with building and maintaining new energy infrastructures. Everyone likes jobs. "That's what is needed to get through this polarization," Max Boykoff, the director of the Center for Science and Technology Policy Research at the University of Colorado Boulder, explained.

It would help, perhaps tremendously, if a Democratic president was successful in getting powerful Republican politicians to heed the observations of scientists.

As Matthew Long, an oceanographer at the National Center for Atmospheric Research, told Mashable in March: "What’s important to recognize is the changes humanity is driving at present are commensurate with the most significant events in the history of life on this planet."

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