For much of the past year, the U.N. climate talks in Paris were viewed as something of a global party. Finally, after years of failed negotiations, world leaders had some signs of progress they could point to that showed they were finally taking the issue of climate change seriously. Thousands of environmental activists planned to travel to the French capital to remind the 190 participating countries of how much further they had to go to reach a truly historic agreement. Al Gore’s plan to throw a celebrity-studded concert gave the whole event a celebratory atmosphere. Now, of course, in the wake of the terrorist attacks in Paris, the tone of the conference has become much more subdued, and many of the ancillary events, including the massive People’s Climate March, have been cancelled due to security concerns.

Despite the attacks, however, world leaders have remained committed to the talks. President Barack Obama will attend the conference’s opening ceremony, along with a delegation that includes Secretary of State John Kerry, Energy Secretary Ernest Moniz, EPA Administrator Gina McCarthy, Interior Secretary Sally Jewell, Agriculture Secretary Tom Vilsack, and NOAA Administrator Kathryn Sullivan. “The most powerful tool we have to fight ISIL is to say that we’re not afraid,” Obama said last week. “I think it is absolutely vital for every country, every leader to send a signal that the viciousness of a handful of killers does not stop the world from doing vital business, and that Paris—one of the most beautiful, enticing cities in the world—is not going to be cowered by the violent, demented actions of a few.”

And the business of the climate talks is indeed vital. If we do not move swiftly to curb carbon emissions, global temperatures are on pace to rise more than 4 degrees Celsius by the end of the century—an outcome that would have severe environmental repercussions. In recent months, countries have put together national pledges for curbing emissions, formally called Intended Nationally Determined Contributions (INDCs), which lay out a course through 2030. The United Nations estimates that these pledges will add up to 2.7 degrees Celsius of warming by 2100—assuming live up to all of their promises over the next few decades—though other analyses have suggested that the total will be closer to 3.5 degrees. Either way, the U.N.’s Christiana Figueres, the chair of the Paris conference, has admitted that these proposals are still “by no means enough.”

Plenty of other necessary climate policies won’t make it into the final text. For example, there’s no mention of carbon pricing in the draft proposals, deflating hopes that the world would rally around a mechanism for reflecting the true cost of fossil fuels. And groups of nations are still jostling for something more ambitious than what’s been promised. A number of developing nations that stand to lose the most from a warming planet have come together in recent weeks to urge just that. They expect rich nations to pay up for causing climate change in the first place.

But here, at the start of the conference, let’s be optimistic. There’s been some progress in the past few weeks on some of the big sticking points going into the talks. One question was whether countries would be willing to agree to review their commitments every five years, to see how else they could ramp up ambitions. China, which originally was skeptical of this idea, joined the French in a bilateral agreement supporting the five-year reviews in November. India submitted its INDC in October—meaning that every major polluter in the world is officially in for  a theoretical agreement, which could cover up to 90 percent of global greenhouse gasses for the first time in history.

Outside the formal structure for the talks, the past few months have delivered a remarkable number of success to the environmental movement. In the U.S., Obama has added energy to climate activists’ campaign to keep fossil fuels in the ground, by formally rejecting the Keystone XL pipeline in November. For the movement, the pipeline represented the president’s last undecided, yet most powerful symbolic decision on climate. Divestment—another major priority—is also gaining ground. The German company Allainz announced days before the Paris talks it would divest from fossil fuels, out of $2 trillion in assets, adding pressure to banks and lenders to reconsider their ties to coal.

Many other obstacles remain and could undermine the rest of the agreement, however. One of those is the legal structure for it. Earlier this month, John Kerry told The Financial Times that any agreement in Paris will not be legally binding, which is probably correct, given that not all countries are on board, including the U.S. (as it would be impossible to get it ratified by the Senate). European nations and many of the most vulnerable nations to climate change, however, think a nonbinding agreement is tantamount to admitting failure. “If the agreement is not legally binding, there is no agreement,” French President François Holland told reporters in response to Kerry’s comments.

Congressional Republicans are also trying to show the world they are not on board. Senate Majority Leader Mitch McConnell’s staff this year reached out to foreign embassies to detail his plan to undo Obama’s efforts. “Con­sid­er­ing that two-thirds of the U.S. fed­er­al gov­ern­ment hasn’t even signed off on the Clean Power Plan and 13 states have already pledged to fight it, our in­ter­na­tion­al part­ners should pro­ceed with cau­tion be­fore en­ter­ing in­to a bind­ing, un­at­tain­able deal,” McConnell warned foreign officials.

And in the last month, Republicans tried to derail the Paris deal before we even have one. The GOP-controlled Senate voted 52-46 on two bills to overturn the EPA’s ability to rein in greenhouse gasses—the centerpiece tool used by Obama to tackle domestic emissions. And it’s still unclear just how the U.S. can pay for the $500 million the Obama administration is trying to secure for climate financing, out of a $3 billion total pledge, without House or Senate approval.

Still, Republicans can’t do much more in the short-term. Their biggest threat is the presidential election. If they gain the White House, the next president could undermine Obama’s actions quickly, which could cripple the broader agreement.

For the next twelve days, though, our focus will be on the officials in Paris. There is much for negotiators to haggle about: the divide between developed and developing nations on how to pay for climate change’s damages and adaptation; the ways to hold countries accountable so they stay on track to meet their pledges; and how nations plan to ramp up their financial commitments to reach the $100 billion a year of promised climate financing.

These are the issues that have doomed previous climate deals. Let’s hope that world leaders can work through their disagreements this time around—for all our sakes.