It's fall 2020, and the presidential campaign in the U.S. is happening against the backdrop of extreme weather events the world over.

In the U.S., wildfires are burning — fueled in part by hotter, drier conditions out West. Hurricanes are plaguing the Caribbean. And the Arctic is seeing its second-lowest ice cover ever.

And the climate world — activists, diplomats, scientists, and business people — anxiously awaits the fate of the best diplomatic shot to limit global warming: the 2015 Paris agreement.

"This election is really going to be very, very critical for whether or not we're actually able to meet the Paris agreement goals and secure the future of the vulnerable around the world," said Rueanna Haynes, a former Trinidad and Tobago climate negotiator who's now a senior legal adviser at Climate Analytics, a policy nonprofit.

The pact reached five years ago was the first universal, binding accord to address the climate crisis. In it, almost every country agreed to keep the increase in the globe's average temperature to "well below" 2 degrees Celsius (3.6 degrees Fahrenheit).

In 2017, during a Rose Garden speech in which he called the deal "draconian," President Donald Trump announced he was pulling the U.S. out of the emissions mitigation framework. But Democratic presidential nominee Joe Biden says he'll put the U.S. on course again if he's elected.

"I'll bring us back into the Paris agreement," Biden said in a speech last week during a historic West Coast wildfire season. "I'll put us back in the business of leading the world on climate change. And I'll challenge every other country to up the ante on climate commitments."

'Quite detrimental'

U.S. involvement — and pushing others to make bolder pledges — is crucial.

In the Paris agreement, governments agreed to set targets for reducing greenhouse gas emissions. But the targets themselves aren't legally binding, and so far, they're not nearly ambitious enough.

Climate Action Tracker, a collaboration of two research organizations that tracks how much warming we're on track to see, estimates the world will warm around 2.7 C (4.9 F) by 2100 if countries hit their current pledges and carbon-cutting targets.

The accord was built on the idea that once countries sign-on, they name emissions reduction targets, report their progress and pressure each other to increase their ambition in the next round of target-setting.

"In a process like that, it's quite detrimental that the second-largest emitter pulls out," said Håkon Sælen, a senior researcher at the Center for International Climate Research, referring to the U.S. withdrawal.

Sælen and colleagues modeled what that might mean in the long term, and found that eventually, it will lower other nations' trust and willingness to reduce their own emissions.

"One thing we find is that, over time, the effect on other countries' emissions is actually even larger than the direct effect on U.S. emissions," Sælen said.

So far, the globe has warmed about 1 C (1.8 F) since the pre-industrial era, according to the Intergovernmental Panel on Climate Change.

The worsening impacts include hotter heatwaves, more intense hurricanes, and rising sea levels that make storm surges more damaging.

The changing climate poses an existential threat to some of the world's most vulnerable people, who are coming to expect more unprecedented events like Hurricane Dorian, a Category 5 hurricane that intensified rapidly last year and pounded the Bahamas for a day and a half.

'Technologically prohibitive'

The 2020 election would determine federal environmental policy for the next four years. But science tells us four years could make a major difference.

"In the next decade, we have to more than halve present emissions to be on a climate-safe pathway," said Bill Hare, a climate scientist and CEO of Climate Analytics. "If we don't achieve significant reductions in the next four or five years, it's going to make the task extremely expensive, if not technologically prohibitive."

The U.S. withdrawal from the Paris agreement doesn't officially take effect until the day after the U.S. election, due to a delay period written into the agreement.

But in an effort to roll back what he calls "job-killing" regulations, President Trump has spent the last four years reversing the domestic energy and environmental policies that would have helped the U.S. meet its Paris climate pledge to cut emissions by about a quarter by 2025.

Those rollbacks include rules that would have limited greenhouse gas emissions from power plants, increased auto emissions standards, and required oil and gas companies to limit methane leaks.

This domestic policy matters, as the U.S. is the world's second-biggest emitter of heat-trapping greenhouse gasses.

On the global stage, however, the U.S. represents more than just its carbon footprint.

"The U.S. plays a critical role in the global economy," Hare said. "It's also a key source of technological innovation. And diplomatically, if the U.S. stays out of the global efforts to ramp up action, then I think a lot of countries will hide behind the U.S."

Hare points to coal-reliant Poland, Jair Bolsonaro's pro-development Brazil, and a so-called "gas-powered" economic recovery in Australia, as examples of places where action on climate change could lag without U.S. pressure.

Yet, a global backslide is looking slightly less likely after Chinese President Xi Jinping announced more ambitious carbon-cutting targets in his video address to the United Nations General Assembly, stepping into a climate leadership vacuum the U.S. left years ago.

The announcement will send "positive shockwaves" through diplomatic circles, Helen Mountford, vice president for climate and economics at the World Resources Institute, said in a statement.

'Huge mass movement'

President Trump's list of second-term priorities includes no mention of climate change. Biden is framing a vote for him as a vote to reestablish the U.S. as a global leader on climate change.

His platform includes $2 trillion in spending on climate change and pledges to push for an end to global fossil fuel subsidies and pressure China to lower the carbon footprint of its global development initiatives.

Progressive activists want Biden to be more aggressive by, for example, banning fracking for natural gas. But they mostly see him as a moderate with whom they can work.

"I think that a Biden election, plus a huge mass movement which forces him to take action, could be significant in really starting to make faster the transition to green energy," said Eyal Weintraub, a youth climate organizer in Argentina.

Weintraub is part of a generation of Greta Thunberg-inspired activists who see themselves as part of a climate movement that crosses borders. Even though Weintraub can't vote in the U.S., he plans to watch in Buenos Aires on election day — and its aftermath.

"It's the most important election almost in the history of the climate crisis," Weintraub said. "If you're a climate activist, you need to have your eyes on the television that night."

This article originally appeared at PRI's The World.