The following are the main obstacles in the UN climate talks, according to a background paper issued for a 45-nation meeting that began in Paris on Monday.
The five-page document was prepared by France, which will host the year-end conference tasked with forging a global agreement on climate change.
This is the term for agreeing the scale of curbs in greenhouse-gas emissions that drive warming.
The UN Framework Convention on Climate Change (UNFCCC) has embraced a goal of capping the rise in Earth's mean temperatures at a maximum of 2.0 Celsius (3.6 Fahrenheit) over pre-industrial levels.
Poor countries and low-lying small island states, which will be hit first and hardest by climate change, say 2 C is not good enough, and favour a tougher UNFCCC goal of 1.5 C.
Exactly how to reach a particular outcome –- whether 2 C or 1.5 C –- has also yet to be determined.
The proposed Paris accord will depend on voluntary national pledges for reducing carbon emissions.
China, the United States and the European Union (EU), which together account for more than half of global carbon dioxide (CO2) pollution, have already submitted theirs.
But the sum of global commitments is unlikely to immediately meet the 2 C objective.
As a result, some countries are insisting on a stringent review process to progressively ramp up ambition and monitor progress. Others object.
Another sticking point is apportioning responsibility—"differentiation" in climate jargon.
The 1992 UNFCCC charter enshrines a principle that rich industrialised countries historically caused the problem, and should thus do more to fix it.
Much has changed since then: China and India have become the world's No. 1 and No. 4 carbon emitters respectively, and some countries labelled "developing" 25 years ago have moved swiftly up the economic ladder.
One of the few concrete decisions to come out of the Copenhagen conference in 2009 was a pledge from rich economies to muster $100 billion (92 billion euros) per year in financial support for poorer countries from 2020.
But where that money will come from and how it will be distributed still has to be worked out.
More recently, the world's poorest nations have presented an additional demand for "loss and damage" compensation for climate-driven impacts that can no longer be avoided. Rich nations have objected.
Also undetermined is the legal status of the accord to be finalised in in December.
For the moment, the UN is leaving most options open, saying it could be a "protocol, another legal instrument, or an agreed outcome with legal force."
© 2015 AFP