A treaty on emissions would change some things quickly, like the retirement of fossil fuel-burning power stations

What happens this year?
In what is widely regarded as the last chance to save the world from the worst ravages of climate change, 192 nations have pledged to agree a global treaty at a two-week negotiating marathon in Paris at the end of 2015 - the most ambitious political task in history.
Each country will commit to huge cuts in carbon emissions to give the world some chance of limiting global warming to 2C, beyond which the consequences become increasingly devastating.

 A treaty on emissions would change some things quickly, like the retirement of fossil fuel-burning power stations. Photo / AP

What was achieved in 2014?
Even cynical campaigners conceded November may have seen a new era of climate-change action when America and China, the two biggest polluters, made an unprecedented pledge on emissions. Their decision was regarded as game-changing as other nations could no longer hide behind their inaction.
It came after EU-wide targets to cut emissions, and a one-off summit in New York on halting deforestation by 2030.

India and the US may announce more initiatives when President Barack Obama visits this month.
What are the obstacles?
The annual UN climate change summit in Lima in December reminded negotiators what an enormous task they face. The biggest sticking point is still the argument between developing nations and richer countries about sharing the pain.
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.