When it comes to efforts to combat climate change, “it’s the best of times, it’s the worst of times,” said David Sandalow, the inaugural fellow at Columbia University’s Center on Global Energy Policy.

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Leaders have moved with unprecedented speed recently to combat global warming: In addition to the Paris accord, there has been progress this year on limiting international airline emissions and powerful greenhouse gases known as hydrofluorocarbons, or HFCs.

But most scientists agree that the world continues to move too slowly.

The current pledges countries have made under the Paris agreement would still allow the world to warm by 3 degrees Celsius or more above pre-industrial levels, an amount well beyond the 2-degree threshold that many experts agree is likely to trigger severe changes to the environment. In the meantime, 2016 has offered a litany of grim reminders of an already changing climate, from extensive coral death at the Great Barrier Reef to particularly sharp Arctic temperatures in a year virtually certain to become the warmest in recorded history.

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“The signals the climate system is sending are profoundly disturbing,” Sandalow said.

The consequence is that as the much-anticipated Paris agreement goes into effect Friday, the world faces a sort of race between politics and physics. The central question that will hover over the international meeting in Morocco next week is whether humans can wean themselves from fossil fuels and halt carbon emissions in time to stave off the worst effects of a changing climate. Can nations move quickly enough to combat a problem that for so long has seemed distant and intangible to many citizens?

“There’s so much that needs to be done,” said Heidi Cullen, chief scientist for the nonprofit environmental organization Climate Central. “The easy part is over. Friday, in many ways, marks the beginning of the rubber hitting the road. All of us see how much will be resting on every country’s commitment. We’re all going to be in this together, and we all have to hold each other accountable.”

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William K. Reilly, who led the Environmental Protection Agency under President George H.W. Bush and has served as president of the World Wildlife Fund, said one key challenge for countries will be to identify precisely how they plan to meet — and eventually exceed — their commitments in coming years. Another is figuring out how to fund the kind of energy transformations that will be necessary to stay “well below” the 2-degree Celsius temperature red line laid out under the Paris agreement.

“One has to hope that the engines are gearing up to ensure that those financings, which are absolutely necessary to the success of the Paris agreement, are forthcoming,” he said.

Cullen agreed that finding funding for the projects that countries must undertake to slash their emissions and transform their energy sectors will be key. “We’re going to really need to develop these green financing mechanisms,” she said, adding that it will require a mixture of public and private investment.

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Even then, the reality of effectively ending fossil fuel emissions in coming decades will be a difficult proposition. “It’s going to take actors from every level, from every sector,” she said. “It’s far from where we are now. But we’ve managed to make tremendous progress.”

The Paris climate agreement, in effect, pools together individual commitments by countries to lower their emissions, channeling that collective energy in an effort to bend the planet’s current climate trajectory. The trouble, though, is that while the current commitments help avoid a worst-case scenario of very extreme warming by 2100, they fail to put the planet on a “safe” path that would prevent serious climate impacts, such as rising seas, melting polar ice caps and more intense floods and droughts.

Meanwhile, in the scientific world, the evidence has grown ever clearer that each incremental increase in temperature, if sustained over a considerable period, corresponds to a notably different planet. In particular, researchers today are drawing more and more on analogies with warm periods in the Earth’s past, which allow them to make connections between key parameters such as planetary temperatures, carbon dioxide levels and sea levels. And the news isn’t good: For instance, there were past eras not much warmer than our own, or with comparable levels of atmospheric carbon dioxide, that had far higher seas than we have now.

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Compared with those daunting possibilities, the to-do list for Marrakesh next week appears relatively mundane. “The parties are going to roll up their sleeves and lay out the key issues in the fine print of the agreement that need to be ironed out so the architecture has meaning,” said Mariana Panuncio-Feldman, who is co-leading the World Wildlife Fund’s delegation to Marrakesh. Only in 2018, at the earliest, will a future meeting actually focus on ramping up pressure on countries to put forward more ambitious commitments that alter the trajectory further, Panuncio-Feldman said.

That’s cutting it uncomfortably close, in light of a United Nations Environment Programme report released Thursday suggesting that if the planet’s overall emissions don’t peak by the year 2020, it may no longer be possible to limit warming to 1.5 degrees Celsius, the more ambitious and aspirational target contained in the Paris agreement and a goal particularly desired by developing countries and vulnerable small island nations.

None of this factors in the potential fallout from the upcoming U.S. election. While Hillary Clinton has vowed to honor the Paris agreement and make sure the United States continues to play a leading role in combating climate change, Donald Trump has pledged to “cancel” the accord. That might technically be difficult to do, given that the agreement will already be in force in January, but a Trump administration undoubtedly could slam the brakes on the political momentum for cutting carbon emissions, at least in the United States. And such a move would reverberate in the global community that has rallied, as never before, behind the Paris process.

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“This is a binary choice,” said Reilly, the former EPA administrator, noting that Clinton has vowed to live up to the country’s commitments under the Paris agreement, while Trump has promised to “disavow” them.

“We have a choice between a strong leader on climate action and a climate denier,” Sandalow added. “The choice between Secretary Clinton and Donald Trump will make an enormous difference for the future of the planet.”

That might be true. But if Paris has revealed anything, it’s that on a global scale, Trump is an outlier. Far from questioning the reality of climate change, other world leaders have shown remarkable swiftness in the past year in trying to take the steps necessary to slow global warming.

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And that action isn’t limited to governments.

“This is the first year in history where investments in renewable energy have outpaced those in fossil fuels. So the market is moving ahead much, much faster than most people understand,” John Morton, the White House’s senior director for energy and climate change, said Thursday in a call with reporters. He added that “what we have seen in recent months, and in fact in recent years, is [the] inevitability of the transition to a low-carbon economy. And so the international community — the international business community, the international policy community — is moving forward and will continue to move forward, and there’s no questioning anymore about the commitment at both the government and policy levels.”

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