France - Some 150 heads of government and state on Monday kicked off a high-stakes negotiation to curb global warming and help poor countries cope with its impacts.
The Paris conference runs until December 11.
The objective: the first-ever universal climate deal to include all of the 195 nations under the umbrella of the UN Framework Convention on Climate Change (UNFCCC).
Dozens of issues have stymied the negotiations, now into their third decade. Here are the key points of contention: All countries agree that greenhouse-gas emissions which drive warming must be curbed. But by how much?
The United Nations has endorsed a global warming ceiling of two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.
But more than 100 poor countries and low-lying, small-island states - many of them in a bloc called the Climate Vulnerable Forum - are calling for a tougher goal of 1.5 C.
Timing is important too: To have a 50-50 shot at 2 C, global CO2 emissions must peak by 2025 and drop dramatically thereafter, scientists say.
The 1992 UNFCCC charter enshrines the principle that rich countries - historically responsible for warming - should do more to fix it.
That divide was codified in the 1997 Kyoto Protocol, which went into force in 2005 and placed emissions-curbing targets on industrialised nations. The United States did not ratify the plan.
Much has changed since the principle of "differentiated responsibility" was set in stone nearly 25 years ago.
Fast-growing China and India have become the world's number one and four carbon emitters, and other erstwhile "developing" countries have scaled the economic ladder.
At the same time, the European Union and US have reduced their per capita carbon emissions.
Developing nations insist that industrialised countries should do more to cut emissions, having polluted for much longer. But rich countries insist that the burden must be shared more evenly to reach the 2 C target.
The hugely contentious issue touches on all aspects of the deal - from setting emissions goals and agreeing on a review system, to the flow of climate aid.
One of the few concrete decisions to come out of the 2009 UN climate conference in Copenhagen was a pledge from rich economies to muster US$100 billion (93 billion euros) a year in financial support for poor countries from 2020.
The money will help develop technology and build infrastructure to cut emissions. It will also be used to adapt to climate impacts like rising seas and spreading deserts.
Exactly where that money will come from and how it will be distributed have yet to be worked out. Developing nations want at least half of it to come from government sources, and oppose including loans in the mix.
More recently, least-developed countries, small-island and developing states have sought additional payment for climate "loss and damage." Rich nations balk the concept of "compensation".
The proposed Paris accord will have as its backbone a roster of voluntary national pledges for reducing carbon emissions.
China, the United States and the EU - which together account for more than half of global CO2 pollution - have submitted theirs, along with more than 170 other nations.
But scientists say the sum of global commitments will not meet the 2 C objective.
As a result, some countries have proposed putting in place a process to "ratchet up" national emissions efforts and financial commitments.
There are disagreements about when the first review of national actions must take place, how often it should be repeated, and whether there should be a duty on countries to automatically ramp up action.
Some countries say the Paris agreement must articulate a longterm goal of decarbonising the world economy to encourage a shift in policy and investment away from fossil fuels. The two-degree target, they say, will otherwise remain out of reach.
But poor nations and some emerging economies counter that they should not be expected to abandon cheap energy needed to fuel growing economies and populations.