Poor countries argue rich nations put most of the CO2 into the atmosphere so they should cut the most and give the developing world a chance to catch up. Richer nations accept the point but argue the developing world now produces a huge and growing portion of CO2, which we just can't put into the atmosphere.
Other problems are whether the treaty will be legally binding and emissions internationally monitored.
What are the obstacles beyond the negotiations?
Communication is a big one. Despite the overwhelming scientific consensus that the climate is warming and humans are largely responsible, the sceptics are effective at sowing seeds of doubt, especially as reining in emissions is inconvenient.
The collapse in oil and gas prices also threatens the green agenda, making renewable energy sources such as wind and solar even more expensive by comparison to fossil fuel power, meaning that even bigger subsidies are likely to be needed to attract investment. It's a tough sell for politicians with short-term goals.
What will help the cause?
The weather has played a role in softening the public's appetite for the big lifestyle sacrifices after drastic emissions cuts. November saw thousands of rallies and marches on climate-change in more than 150 countries.
In the UK, the worst floods since 1953 last year were a wake-up call, while 2014 is looking to have been the warmest on record.
Climate scientists are also upping their communications game, engaging with the public through the arts instead of with graphs and numbers. They are also challenging climate sceptics more robustly.
Business is playing a part, too. The movement among pension funds to divest of investments in fossil fuel companies has mushroomed to hundreds of institutions. Renewable technology such as solar and onshore wind are reducing their costs, making them more competitive.
What would happen if a treaty was agreed to limit global warming to 2C?
With the world on course to record a temperature increase of about 5C by the end of this century compared to pre-industrial levels, a successful treaty would change things overnight, noticeable more as a rapid change in the collective mindset rather than on the street. But share prices for fossil fuel companies could begin crashing, as their reserves would need to stay in the ground and become virtually worthless. Entrepreneurial activity would soar, involving the kind of research intensity usually reserved for world wars, leading to a complete overhaul of the transport and energy network.
By 2050, electric vehicles would be the norm and trucks would switch to gas. Coal and gas would be quickly phased out, with wind, solar and nuclear taking up most of the slack, and remaining fossil fuel stations would need technology that sucked emissions out of the sky.
Virtually every building would adopt low-power LEDs for lighting. Flying would be extraordinarily expensive, the cost of meat would soar and virtually everything would be made from recycled material.
There would certainly be all sorts of shifts in behaviour that we can't even imagine at the moment, some of which would fail dismally while others will prove revolutionary.
-Independent
