World leaders gathered in Paris on Monday for the latest chapter in what is now a decades-long series of meetings, negotiations and proposals on how to collectively stave off catastrophic climate change.

Hopes are high that this 21st Conference of the Parties, or COP21 , will represent a turning point in the evolution of the international response. The global effort has been fraught with disappointment since its formal launch in 1992 at the Earth Summit in Rio de Janeiro, where the Rio Convention included the adoption of the international treaty known as the United Nations Framework Convention on Climate Change .

Despite small successes and even major milestones over the subsequent 20 COPs, or conferences of the parties -- including approval in 1997 of the Kyoto Protocol , the first global agreement for mandatory reductions in greenhouse gas emissions -- the annual gatherings have generally failed in their chief objective to stabilize the warming of the atmosphere.

In fact, the 2015 summit in Paris will come at the end of a year that's on track to be the hottest ever recorded -- not to mention one filled with severe droughts, floods and storms, the kind of extreme weather that scientists say will only become more commonplace as man-made emissions continue to push the world's climate along its dangerous path.

But there is cause for optimism as well: The 2015 talks also coincide with a heightened public awareness of climate change, and a momentum for action that surpasses what we've seen previously. This past year brought the largest climate demonstration in history, a landmark U.S.-China climate deal and pledges from a dozen major corporations to invest over $140 billion in efforts to curtail carbon dioxide emissions.

"People around the world have a much keener sense of both the risks and the opportunities presented by climate change, and that's translated into stronger political will," said Elliot Diringer, executive vice president of the nonprofit Center for Climate and Energy Solutions .

That kind of willpower was arguably absent when delegates met in 2009 for COP15, the 15th Conference of the Parties in Copenhagen, Denmark. While many people were optimistic ahead of that meeting, it was ultimately plagued by the same failed attempts and artificial promises as previous summits.

A central challenge over the years has been the lack of agreement on who bears responsibility for climate change and who should shoulder the cost of addressing it. Developing countries have tended to argue that they need not curb their emissions if the heaviest historical polluters -- such as the U.S. and Europe -- refuse to do so themselves.

Fortunately, the framework set up in 1992 allows the parties to continually assess and revise the global plan. "It forces countries to go back to the table every year to evaluate where we have or have not gone," said Diringer. "We've effectively had a couple decades of experimentation, and the international effort has evolved -- reaping 20 years of lessons."

Jon Hovi, a climate expert at the University of Oslo, has been following the negotiations since COP7, the 2001 climate meeting in Marrakesh, Morocco. He noted the current move away from Kyoto's legally binding approach to a looser plan that should allow countries the flexibility to decide how they will contribute.

"The new approach entails that targets are not enforceable. Hence, countries are now free to commit to deeper emissions reductions without having to fear punitive consequences if they should fail to reach their targets," Hovi said. "At the same time, lack of enforcement also means that countries are less likely to take their targets seriously -- particularly if they see that other countries also shirk."

Katharine Hayhoe, director of the Climate Science Center at Texas Tech University, highlighted what she sees as another key aspect of the momentum shift leading up to Paris. "For decades now, climate negotiations have been primarily based on science and policy," she told The Huffington Post in an email. "As a scientist, I'd be the first to say that facts matter. But in doing so, we've left out one of the most important things we use to make our decisions -- our hearts."

"Every major world religion or faith has issued statements and calls to action on climate change. And given that the vast majority of people in the world (and going to COP) belong to one of those religions, I am hopeful that we can see change," added Hayhoe, who noted that she will be attending the Paris meeting with the Union of Concerned Scientists to "represent scientists who speak to both the moral and the scientific urgency of acting on climate."

Diringer, a former journalist, said he'd recently looked back at the stories he filed from that first Rio Earth Summit in 1992. "They exuded a certain hopeful glow," he said. "It felt like an important moment when the whole world recognized these urgent global challenges and then set out to address them."

Some of that initial optimism may have faded over the years. But Diringer is one of a number of experts who see emerging glimmers of hope. "Countries are bringing greater political will than ever to Paris," he said. "Now, we'll see if the process can deliver."

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