For as long as Americans have voted and pundits have bloviated, each presidential election cycle has seemed The Most Important in All History.

Next year, though, may truly – actually, seriously – be different, if climate scientists are right. The next candidate Americans send to the Oval Office, experts say, may also be the very last who can avert catastrophe from climate change.

"It is urgent and the timeframe is critical and it has to be right now," says Vicki Arroyo, executive director of the Georgetown Climate Center at Georgetown Law. "We can't lose another four years, much less eight years."

And it's going to get worse, experts say.

Last year, a U.N. panel of scientists predicted the world had until 2050 to slash emissions by as much as 70 percent to keep temperatures from rising another 1.15 degrees by the end of the century. That's the threshold of an unstoppable cycle of Arctic and Antarctic melting, the release of heat-trapping gases that had been caught in the ice, more warming, more melting, more warming, more melting – until the glaciers and ice caps disappear.

But some researchers – including the man who first presented the facts on climate change to Congress in 1988 – say that that tipping point may come even sooner, perhaps as early as 2036: Humans, in short, are having an even greater impact than expected.

"Sea level projections and upcoming United Nations meetings in Paris are far too sluggish compared with the magnitude and speed of sea level changes," the scientist, Columbia professor James Hansen, wrote Wednesday in a Q&A on the web forum Reddit, discussing a study he published in July.

The needed changes are monumental: Halting climate change and heading off its worst consequences is going to require a wholesale switch from fossil fuels like coal, oil and gas to renewables like wind and solar – potentially upending utilities, energy producers and construction contractors, the sort of change "of the magnitude of the invention of the steam engine or the electrification of society," says Jules Kortenhorst, CEO of the Rocky Mountain Institute, a nonpartisan energy research group.

"How quickly can we transform one of the most complex industrial systems – our energy system – across the globe in order to move toward low carbon?" he asks . "There is absolutely no doubt we have to act now."

This presents an election – and a choice – with no historical analogues.

"This will be a make-or-break presidency as far as our ability to avert a climate change catastrophe," says Michael Mann, meteorology professor and director of the Earth System Science Center at Penn State University, whose "hockey-stick" shaped graph warned of sharply rising emissions and temperatures.

Pick any issue throughout history, he and others argue, none has shared the three qualities that make climate change stand apart: its threat to the entire planet, the short window to respond, and how sharply it has divided the two parties' candidates.

"Republicans and Democrats have argued over issues for years, but I can't think of an example where one party didn't even say that the issue exists," says Katharine Hayhoe, a climate scientist at Texas Tech University who has advised Evangelical and conservative climate action groups, and who has urged policymakers to address warming. ​​

Among the Republicans, eight of the 17 candidates have hedged: Jeb Bush, Carly Fiorina, Lindsey Graham, Jim Gilmore, Bobby Jindal, John Kasich, George Pataki and Rand Paul have acknowledged that humans do contribute to global warming, but have questioned or stopped short of saying how much – a position at odds with the findings of a vast majority of scientists.

"The climate is changing; I don't think anybody can argue it's not. Human activity has contributed to it," Bush said in an email interview with Bloomberg BNA in July – a statement that notably did not mention how much humans were at fault. During a campaign stop in New Hampshire in June, he had previously told listeners, "The climate is changing, whether men are doing it or not," one month after calling it "arrogant" to say climate science is settled.

The rest of the GOP field – including three senators who rejected a January amendment tying human activity to climate change – has dismissed the issue outright. Paul also voted against the amendment.

"As a scientist it's very frustrating to hear politicians basically saying, 'This isn't true,' or, 'They're just making it up to get government money,'" Hayhoe says. "A thermometer is not Democrat or Republican. What observations are telling us is not political – it is what it is."

And there are conservative solutions for warming. Some party members, in fact, see it as an inherently Republican issue: Carbon emissions, for example, distort the free market, forcing others to pay the higher and indirect costs of climate change (storm recovery, disaster relief) plus the health costs associated with air pollution.

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​​​"We allow the coal industry to socialize its costs, and we conservatives don't like allowing people to socialize anything," says former South Carolina Rep. Bob Inglis, who now explores free-market solutions to climate change as head of the Energy and Enterprise Institute at George Mason University.

A revenue-neutral carbon tax, one that does not support other programs and instead goes back to households, could fix that distortion, he and others argue.

"The question is not, 'Is there going to be a tax on carbon?' It's, 'Do you want a tax that you have a voice in and control, or do you want to keep writing checks after disasters that you have no control over?'" says retired Rear Admiral David Titley, who has advised some of the GOP presidential candidates and directs the Center for Solutions to Weather and Climate Risk at Penn State University. "That $60 billion relief bill for Hurricane Sandy that passed very quickly through a Republican-led House, did you get a vote on that tax? Because that's a tax."

Yet Inglis, himself is a living example of what can happen to conservatives who call for climate action. The recipient of the JFK Profile in Courage Award in April, he was unseated in the Republican primary in 2010 after shifting his position on global warming.

"Republicans say, 'Look at what happened to him when he said it was real. Do you want that to happen to you?'" Hayhoe describes.

Oil, gas and coal companies, along with billionaire Libertarian industrialists David and Charles Koch, rank among the biggest campaign donors, and often seem as allergic to new taxes as a bubble boy to fresh pollen. But popular sentiment among voters appears to be changing: Most Republican voters say they support climate action, and last week, Shell did not renew its membership in the Koch-backed American Legislative Exchange Council because of the group's opposition to climate action.

Even the climate statements by the eight Republicans who have hedged on warming, vague as they were, may signify a kind of progress – especially during the primaries, when candidates play to their parties' more extreme bases.

"In the Great Recession in 2010, it was this very atheistic position with regard to climate change: 'We don't believe,'" Inglis says. "Then, in the 2014 cycle, 'I'm not a scientist,' that was an agnostic position. These are data points on a trend line toward a tipping point."

Republicans can exploit a distinct advantage on climate action, too, he adds: Voters tend to support the presidents who buck party stereotypes.

"Nixon goes to China, Bill Clinton signs welfare reform – the country will trust a conservative to touch climate," Inglis argues.

But climate scientists, environmental advocates and Democrats remain deeply skeptical. The most recent Republican president, for one, backpedaled on his 2000 campaign pledge to rein-in carbon emissions. Campaign donations remain hugely influential, and as Republican candidates lambaste the environmental agenda of the Obama administration, stopping climate change will actually require they expand upon Obama initiatives: resist industry pressure to slow the roll-out of tighter fuel standards for cars, push states to reduce emissions from their power sectors and uphold and ratchet-up international commitments to slow carbon emissions.

There's also the Supreme Court: with four Supreme Court justices now over the age of 70, and Ruth Bader Ginsburg pushing 80, the next president will likely have the chance to nominate new jurists to the court – a court that will almost certainly decide challenges to various environmental actions aimed at slowing global warming.

"If we are going to avoid catastrophic, irreversible climate change impacts, we have to be ramping down our carbon emissions dramatically in the years ahead. The current administration has begun that process, but our next president must not only continue but build on that progress," Mann says.

It is on the global stage where perhaps the spotlight – and climate scientists' hopes and expectations – will shine brightest. In December, negotiators from nearly 200 nations will meet in Paris to hammer-out an international climate accord. It is expected to include commitments from China and India, heavy polluters spurred to rein-in their emissions and invest in clean energy by America's own commitment to slash carbon emissions from its power sector.

"The rest of the world is going to expect the U.S. to live up to its commitment [made at the Paris meeting], no matter who is in the White House," says Henrik Selin, professor of international relations at the Pardee School of Global Studies at Boston University. "If you have a president who comes in and starts rolling back the Obama initiatives, you're going to have international leaders being very unhappy about this – and they are not just countries, they are trading partners. This is not just a domestic issue, it's also very much a foreign policy issue."

And so far, he and others argue, none of the Republican candidates have offered a clear vision on climate, let alone any plan to slow warming.

"If we want to get to that low-carbon future, we have to agree that's where we're going to go, and then we can fight over the speed at which we're going to get there," Kortenhorst, of the Rocky Mountain Institute, says.