Le Bourget, Paris—The nations of the world have been talking about climate change for 23 years. In all that time they have been adding inexorably to the carbon dioxide building up in the atmosphere and doing little to slow it. But here at the 21st United Nations climate conference, 184 countries representing more than 90 percent of global CO2 pollution are getting into detailed negotiations this week over the plans they had submitted to curb global warming. And that has provided hope for the first global landmark deal.

"Success is getting an ambitious agreement that is durable, that has transparency and accountability," said John Kerry, U.S. Secretary of State at an event in Paris on December 7. "I think the stage is set."

Already, major announcements have been made, both from billionaires like Bill Gates and the U.S. government, promising large sums of money for research and development into new clean energy technologies, and for making the clean energy that is already available—solar, wind, nuclear, geothermal—cheaper. And national representatives continue to refine the text of the global deal.

Key areas of disagreement remain, such as whether the submitted plans are a final blueprint for the planet, or only a first step subject to review in 2020 or 2025, when even more aggressive cuts could possibly be made. Countries continue to weigh whether and how to monitor and verify pledges, and to debate what the long term goal is—ranging from a world free of CO2 emissions by as soon as 2060 to eliminating CO2 pollution associated with the global economy by 2200.

In addition, the negotiators continue to joust over how much money developed nations should provide to help developing countries adapt to the climate change that is already underway and to cope with losses and damages from extreme weather exacerbated by climate change. "Nothing has been decided and nothing will be left behind," said Laurence Tubiana, the French ambassador as she led the Paris climate negotiations.

The greater concern, perhaps, is whether the many national pledges go far enough, fast enough given the speed of global warming and the scale of global energy infrastructure—both existing and to be built in the next few decades. The latest report from the U.N. Intergovernmental Panel on Climate Change suggests the world must emit zero CO2 by 2070, at the latest, to have a good chance to avoid more than 2 degrees Celsius of global warming, the threshold for serious fallout.

That optimistic projection relies on technologies that actually suck CO2 back out the air, such as capturing and burying the CO2 created by burning fossil fuels. That technology is not practical yet. And even the cleaner natural gas-burning power plants that are beginning to replace coal plants in the U.S. would need to be replaced or cleaned up by then. So would the pollution associated with the massive number of fossil fuel-fired power plants expected to be built in developing countries such as India.

Despite the rapid growth in wind and solar power, the world is polluting more now than at the beginning of the 21st century, thanks to the delays and failures in developing technology to capture and store CO2 as well as the rapid growth in pollution from China. What cutting CO2 to zero in a world that continues to enjoy economic growth really means remains undiscussed, even here at the COP21. "Nobody really knows how to do this," said Guido Schmidt-Traub, executive director of the Sustainable Development Solutions Network, which works on decarbonization pathways for the world's 16 most polluting nations, including the U.S. and China.

Perhaps by 2020 or so, which is when the U.S. would like to revisit its commitment and re-pledge for 2030, according to the State Department, those conversations will have taken place in earnest. But by then it may be too late to prevent the world from surpassing its one-trillion-metric-ton carbon budget, crossing the 2 degrees C line, with more emissions to come if coal, gas and oil are not well on the way out of the global fuel supply or on their way to zero CO2 emissions.

"We're not going to do it all in one fell swoop here in Paris," admitted Kerry, who has been attending global climate talks almost since the beginning, including being the sole U.S. Senator to visit the 2009 Copenhagen climate conference. But great commitments must be made now or little hope will remain. "I am so hopeful that Paris will be a truly historic moment,” Kerry said.