What’s at stake on Tuesday? The planet.

Analysis by Bill Weir, CNN Chief Climate Correspondent

(CNN) — “Everybody complains about the weather, but no one does anything about it,” Mark Twain used to quip, back when humans had only just begun digging fossil fuel out of the ground and burning it for energy.

It was a borrowed joke — and good thing for Twain, since it hasn’t aged well. Nearly a century and a half after “The Adventures of Tom Sawyer” was published, humanity’s fossil fuel pollution is changing nearly everything about our weather, and with early voting underway, American voters are making a stark choice over what to do about it.

Thousand-year floods are obliterating communities with staggering regularity; hurricanes are getting stronger faster and beating coastlines with more brutal wind and surge; the heat is so extreme that first responders are filling body bags with ice as a last-ditch effort to save people from heat stroke.

At the current rate of ecosystem collapse, scientists predict 1.2 billion people will become climate refugees by 2050, while over a million species of plants and animals are on the brink of extinction.

Even for voters untouched by flood, fire or drought, the crisis is driving up the cost of food, insurance and supply chains for everyone. Their tax dollars are now pouring into billion-dollar efforts to keep the Earth we know from heating beyond salvation, while other ventures race to adapt our built environments to more violent physics. Property values, insurance rates and building codes are changing, and experts are warning that unnatural disasters like Hurricane Helene are just the opening acts of an existential threat.

Looking back through time, it’s hard to imagine a more severe or consequential gap between candidates on a single issue. If Lincoln had lost in 1860, or Reagan dismissed the Soviet threat, the American experiment might have ended, but most life on Earth wouldn’t have noticed.

Now, scientists are warning humanity has run out of time to deal with an existential, global threat. One of the candidates wants to do something about it. The other has said the human-caused climate crisis is “a hoax.”

“The science is clear,” Vice President Kamala Harris said at an event in 2022. “Extreme weather will only get worse, and the climate crisis will only accelerate.”

She cast the tie-breaking Senate vote for the most ambitious climate laws in the nation’s history, designed to supercharge the transition to clean energy and save Americans money on their energy bills, new appliances and at the gas pump. It included credits for appliances that lower your energy costs and a big credit on solar panels so homeowners can start generating their own energy, rather than paying power companies’ increasing prices.

In the three years since the Inflation Reduction Act was passed, American clean tech companies have attracted hundreds of billions in additional private investment, dozens of clean energy projects have broken ground and the number of Americans using federal incentives to buy more efficient cars and appliances has exceeded expectations.

Harris plans to “work to lower household energy costs and create millions of new jobs, while tackling the climate crisis, protecting public lands and public health, and holding polluters accountable to secure clean air and clean water for all,” her campaign website says. She also intends to “safeguard America’s energy security.”

If elected to a second term, former president Donald Trump is vowing to undo it all on day one, methodically rolling back pollution limits on tailpipes, power plants and methane after pulling the US out of the Paris Climate Agreement for the second time. He could also go a step further — as some of his former aides have suggested he should do — and pull the US out of the entire global negotiating framework, ending official US climate diplomacy.

Former Trump administration officials have outlined the speed at which they intend to dismantle the progress the Biden administration made on climate change.

“We lost a fair amount of time early in the Trump administration because we weren’t as prepared as we would be for round two,” Mandy Gunasekara, former chief of staff of the Environmental Protection Agency under Trump, told CNN’s Ella Nilsen this summer. His former Interior Secretary David Bernhardt told Nilsen the climate rollbacks could happen “very quickly” and “in a matter of months, in many instances.”

Gunasekara and Bernhardt are among the authors of Project 2025 — a conservative playbook for a second Trump administration. Trump has publicly distanced himself from the ideological blueprint, but CNN found at least 140 authors worked in the Trump administration.

But given the momentum of the clean energy train, some experts and analysists doubt that Trump would be able to derail it entirely.

“I wish it wasn’t so intolerably uncertain what’s going to happen (in this election),” former Vice President Al Gore said in May. “But if (Trump) were to win, I think it would be difficult for him to roll back the Inflation Reduction Act.”

As the first man to run for President on a climate platform, and win the popular vote doing it, Gore believes one of the Biden Administration’s best decisions was to spend more than 75% of the IRA money in Republican districts.

Bill Gates also has confidence in the direction of America’s climate action, but worries about the pace and the message a second Trump presidency would send to the rest of the world.

“Stop and go, for things that involve 20- or 30-year plant investment, just scares the whole industry away from a country that’s inconsistent,” Gates told CNN. After founding Breakthrough Energy Ventures almost a decade ago, the man who made Microsoft now invests in hundreds of companies devoted to climate solutions at scale, some of which benefit from tax incentives built into Biden’s climate law.

While the green trend in economics now has Republican states like Texas leading the nation in clean energy installations, Germany, Japan and China got there first. “So now, we’re hoping that a lot of those federal incentives survive,” Gates said, “so these new industries aren’t born in some other country.”

A Harris win next week won’t refreeze Greenland or weaken hurricanes, but it would continue Democrats’ climate momentum at a time scientists say is Earth’s most critical moment. Trump’s plan is to undo that progress, throw billions of dollars of clean energy investment out the window and give other countries the economic opportunity, instead.

Voters will decide the path America will take.

“The vote is (like) a pistol,” Mark Twain also wrote. “You may seldom or never draw it, but when your life is in danger you will see that it is a valuable thing to have.”

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