Next week, in the waning days of what is set to be the hottest year on record, world leaders meet on the outskirts of Paris for a summit that seeks nothing less than to steer the global economy away from its ever-growing reliance on fossil fuels.
The challenge is enormous and has proven elusive in the past. The U.N.-sponsored talks are aimed at getting 195 countries to agree on a path for cutting the greenhouse gas emissions which scientists say have raised global temperatures and begun upending the earth's climate.
Opening the summit at Le Bourget on Nov. 30, heads of government from big carbon burning countries such as U.S. President Barack Obama and China's Xi Jinping will seek common cause with leaders from the smallest emitters in Africa and island states.
When it concludes two weeks later on Dec. 11 - give or take a couple of days for last-minute wrangling - their negotiators are likely to claim success in committing both rich and developing nations to weaning the world off the coal and oil resources that gave rise to the Industrial Revolution.
"Done right, it will shape the economy of the 21st century," said Andrew Steer, head of the World Resources Institute think-tank. Done wrong, critics warn, the consequences could be catastrophic.
For climate scientists who overwhelmingly say that continuing to burn carbon even at today's pace will raise global temperatures by several degrees, a weak agreement will trigger inhospitable changes to the earth's climate systems.
A hotter planet would see dire - if hard to perfectly predict - effects: rising seas, more intense storms and droughts on land and extinction for vast numbers of life forms in warmer, more acidic oceans.
Yet an array of other voices contend that severing the global economy from its foundations on coal, oil and gas risks unleashing pain of its own: rising energy costs that would deny the world's poor affordable power essential to improving their lives, and wound entire industries in wealthy countries.
Reconciling those forces has stumbled in past UN-backed talks. The last attempt to strike a global agreement collapsed in rancor in Copenhagen in 2009, when a few developing countries balked at a deal they said did not go far enough in requiring industrialized nations to cut their emissions.
Chastened by Copenhagen and aware that another failure could dissolve any remaining appetite for collective action, expectations for Paris have been kept lower.
And the mood will be somber, amid tight security after the attacks that killed 130 people in Paris.
FROM DRAFT TO DEAL
Negotiators still have to resolve deep differences in a 51-page draft but much of the work, including new policies and regulations meant to curtail high-carbon energy use, has already been done back home. About 170 countries have submitted plans for curbing emissions beyond 2020 - including some like Sudan or Bolivia that blocked the deal in Copenhagen.
China, reluctant to submit to any outside oversight of its carbon pledges six years ago, has promised to steer its coal-powered economy onto a greener path.
And there will be no repeat of the 1997 Kyoto Protocol that mandated specific reductions for rich nations. The agreement is unlikely to carry the force of international law, something the European Union wants dearly but the United States opposes.
Instead, most nations now seem willing to commit to reviews of their policies every five years as a means of holding each other to account.
For those pushing a tough accord, the urgency has been cranked up by the latest temperature data: 2015 is on track to be the warmest since records began in the mid-19th century.
Several scientific studies project that pledges made so far will - at best - hold the world to temperature rises of anywhere from 2.7 degrees to 3 or even 3.5 above pre-industrial times by 2100. That's well above an agreed 2-degree UN limit.
But there is optimism, too. Leaders of all major emitting countries have expressed support for an accord. Businesses, city mayors and religious leaders including Pope Francis have urged greater action to protect the environment.
Even Europe's major oil companies, such as BP and Royal Dutch Shell, say they favor a price on carbon. "It's almost inconceivable that there won't be an agreement given the number of leaders who have called for it," says Alden Meyer of the Union of Concerned Scientists.
And big shifts in energy use are under way.
Campaigns calling for investors to divest from high carbon industries have added pressure on an already squeezed coal industry. U.S. coal companies including Patriot Coal Corp and Walter Energy Inc have filed for bankruptcy as tighter regulations, falling energy prices and an economic slowdown in China have taken a toll.
The UN says investment in renewable energy has grown 500 percent since 2004 to $270 billion in 2014, and prices have fallen sharply. Britain, home of the Industrial Revolution, now plans to phase out coal-fired power plants by 2025. Even some OPEC nations are feeling the benefits of falling prices of renewables such as solar photovoltaics (PV).
"Solar PV is cheaper than gas - even at Abu Dhabi prices," said Ahmad Belhoul, chief executive of the United Arab Emirates green energy firm Masdar.
Many developing nations say the biggest obstacle to a deal in Paris is to secure new financing to help curb their greenhouse gas emissions and adapt to changes in their climate, building flood defenses on rivers, for example, or shifting to drought-resistant crops.
Rich nations promised in Copenhagen to mobilize $100 billion a year in climate finance by 2020, but are resisting targets for higher amounts beyond that. By one estimate finance reached $62 billion in 2014.
Indeed success in Paris may ride on that age-old argument of who pays: most developing countries have made their own promises contingent upon financing from the wealthier states.
(Additional reporting by Sarah McFarlane in London, editing by Jonathan Leff, Bruce Wallace and David Evans)