Paris, France—The long-running clash between climate science and climate politics again took center stage at the Paris summit on Friday as the talks headed into overtime and activists prepared an unauthorized march near the Arc de Triomphe on Saturday “because climate justice won’t wait for governments—it is up to us to keep fossil fuels in the ground.”

Friday began with the French hosts, United Nations officials, rich nation governments, and US and UK media expressing confidence that an ambitious agreement will be reached, though not before Saturday, a day later than scheduled. “I’m optimistic,” the BBC quoted French Foreign Minister Laurent Fabius as saying, adding that Fabius had told reporters he was “sure” the 196 assembled governments would approve a new text that would be “a big step forward for humanity as a whole.” The New York Times reported that the draft text “skates on the edge of historical significance,” adding that the biggest missing piece is “clear language on monitoring and verifying whether and how countries will follow through on their promises to cut emissions.”

At noon, however, an all-star international panel of climate scientists delivered a far harsher judgment, warning that the current text needs massive improvement to deliver on its stated goal of limiting temperature rise to “below 2 degrees C or 1.5 C above pre-industrial levels.”

“The current text is weaker than the final agreement that came out of [the failed] Copenhagen [summit in 2009],” said Kevin Anderson, deputy director of the Tyndall Center for Climate Change Research in the UK, to a packed-to-bursting press conference at the Le Bourget convention center. Noting that the text does not even contain the words “fossil fuels,” Anderson added, “It is not consistent with science. It calls for peaking greenhouse gas emissions ‘as soon as possible.’ That is not consistent with a 2 degrees limit. Negotiators are praising this text as ‘practical,’ but for whom? For poor, non-white people in the southern Hemisphere, it falls somewhere between dangerous and deadly. But we still have 24 hours here to pull something more serious together.”

China and the United States, as the world’s leading climate and economic superpowers, obviously will be critical to whatever outcome emerges over the next 24 hours. Many developing nations have been unhappy with the US position, which they charge does not reduce heat-trapping emissions anywhere near fast enough to honor a 2 degrees C goal (much less a 1.5 degree C goal) and does not provide the scale of financial assistance poor and vulnerable nations need to cope with intensifying climate impacts and shift to zero-carbon economic development. The US has pledged, in what is known in Paris summit lingo as its INDC (Intended Nationally Determined Contribution) to reduce its emissions by 26 to 28 percent from 2005 levels by 2025.

The Chinese vice foreign minister, Liu Zhenmin, told a press conference late on Friday afternoon that, “The US Secretary of State said that his government would face domestic difficulties if the INDCs are included in the Paris agreement.” Liu added, “We must have the United States on board for a successful Paris agreement. We need to find a solution that is acceptable to all.”

The Nation was told separately that Kerry specifically blamed “resistance in the Congress” for the relative weakness of the US position in Paris. A request for comment from the US delegation’s press office is pending.

“Rather than blame Secretary of State Kerry and President Obama, who I think understand the climate crisis and want to do what they can to reach a just agreement in Paris, we should blame Charles and David Koch, because it is their funding of climate deniers in Congress that has made it impossible for the US to be more ambitious at this summit,” said Victor Menotti, director of the International Forum on Globalization.

“To achieve the 1.5 C limit, we’d need complete de-carbonization of the world economy by 2050,” Hans Schellnhuber, founder of the Potsdam Institute for Climate Impact Research and former chief climate-science adviser to the German government, told The Nation. “That means that, once we leave Paris, every country should set up a plan to de-carbonize its economy.”

“To limit warming below 1.5 degrees C, there is no scenario available that says that we can delay action to 2020 and beyond,” said Joeri Rogelj of the International Institute for Applied Systems Analysis in Austria, who authored one of the few scientific studies to analyze a 1.5 C scenario. “We need a global peak of emissions by 2020 to limit warming to 1.5 degrees C.”

Such a schedule is light-years ahead of what the current text calls for. Although the point has gotten little attention in media coverage, it is arguably among the most critical facing the negotiations: How soon and how vigorously must emissions reductions begin?

The current text is based on voluntary commitments submitted by 186 governments—Intended Nationally Determined Commitments, or INDCs. These pledges in most cases cover the period from now until 2030—and, as has been widely reported, would yield a temperature rise of roughly 3 degrees, a catastrophic amount. Many poor and vulnerable countries and civil-society groups have urged that the Paris accord call for much greater commitments, starting much sooner. But the current text only envisions convening a “dialogue” in 2019 that would “take stock” of the collective efforts of all nations to de-carbonize, but would not single out high emitters nor impose obligations for additional action.

Another huge concern raised by civil society: The current text has gutted or outright removed language stipulating that human rights, gender equality, indigenous people’s rights and ecosystem integrity are fundamental to making a climate accord work in the real world. “The people we’re here to represent are being left out of this document, by state parties, and we’re looking for them to rise to the occasion,” said Roberto Borrero of the Indigenous People’s Caucus, a coalition of the indigenous people’s groups attending the summit. “The United Nations estimates that there are 370 million indigenous people in the world. Our lands cover 22 percent of the earth’s surface and contain 80 percent of the earth’s biodiversity. Climate change threatens the very survival of our people and we want to see language [to that effect] restored to the legally binding portion of the agreement.”

The world—and the United States in particular—has one possible trick up its sleeve to help keep the 1.5 C target in reach, but it would mean banning shale gas, said Robert Howarth, a professor at Cornell University in New York who has done some of the most cutting-edge research on fracking. Shale gas is composed largely of methane, a greenhouse gas that has escaped notice in the official proceedings at the Paris summit, where the focus is overwhelmingly on carbon dioxide. But methane is actually a much more powerful trapper of heat than is carbon dioxide over the short term—which, paradoxically, means that reducing methane emissions offers a much quicker way to reduce the increase in the total concentration of heat-trapping gases in the atmosphere.

“If we continue methane production at current rates, the world will run up against the 1.5 degrees limit in 12 to 15 years,” Howarth told The Nation. “If we stop producing methane, which means stop doing fracking of natural gas and oil, the world wouldn’t run up against that limit for about 50 years. So we could buy ourselves 25 to 35 years of time, which is critical. That could allow us to improve our political and socioeconomic responses to climate change and de-carbonize our societies accordingly. But if we’re serious about a 1.5 degrees target, or even the 2 degrees target, we can’t keep on fracking.”