Agreements on climate change -- to paraphrase what the 19th-century German statesman Otto von Bismarck said about law-making -- are like sausages.

It's best not to know how they are made.

On December 11 2015, 195 states are scheduled to strike a deal in Paris to curb the fossil-fuel gases imperilling Earth's climate system.

The outcome will be determined in the coming months by Bismarck-style sausage-making -- a long, slow grind, and with many questionable ingredients.

What emerges will prompt future generations to either praise us for taming the carbon monster or curse us for short-sightedness and greed.

The stakes are "nothing less than the shape of the climate regime for the next several decades," says Alden Meyer of the Union of Concerned Scientists (UCS), a veteran US climate monitor.

"2015 will set the stage for the living conditions of our grandchildren - and their grandchildren, too," believes Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research (PIK) in Germany.

Elliot Diringer of the Center for Climate and Energy Solutions (C2ES), a US think-tank, predicts "a tough year ahead" in what is already a notoriously troubled UN process.

Half a dozen negotiation phases take place before Paris, the climax of a four-year bid to seal a global deal to take effect by 2020.

The first will be in Geneva next month, when countries must slim down a sprawling blueprint for the Paris pact, the legacy of a just-finished marathon in Lima.

After that, countries have a rough deadline for the first quarter of 2015 for putting voluntary emissions-curbing pledges on the table.

That's when the haggling starts in earnest, along with the toxic risk of finger-pointing and nit-picking.

Which countries are doing enough to fight climate change, and which countries are failing to pull their weight?

Developing countries say rich economies must do most.

After all, goes this argument, they bear historic responsibility for global warming, as they gorged on cheap and plentiful coal, oil and gas to power their prosperity.

Rich countries retort that the carbon division is out of date. It was based on the realities of 1992, when the UN Framework Convention on Climate Change (UNFCCC) was born at the Rio Earth Summit.

Today, developing countries -- led by China, the world's number one carbon polluter -- account for around 60 percent of global emissions, thus making them the sources of tomorrow's warming.

The years-old "differentiation" row bedevilled Lima, but Diringer says there are hopes it may not be such a nightmare in 2015.

China has shown the way to other developing emitters by signing a bilateral deal with the United States, the number two polluter, while Europe is challenging other rich parties with its own 2030 pollution goals, he argues.

"What's most important now is for other countries to declare their contributions to the Paris agreement," says Diringer.

"As long as others follow the lead of the US, China and the European Union, we should have a decent shot at a meaningful global deal."

- Political heat -

After the summer break, the talks will be pressured by civil society and leaders such as UN Secretary General Ban Ki-moon and, reportedly, Pope Francis. Mobilisation on this scale was last seen in the run-up to the ill-fated Copenhagen Summit in 2009.

By November 1, the UNFCCC will unveil a report that tots up the pledges to see how close they get to the coveted goal of limiting global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.

If the tally falls badly short, that will set the scene for some frenzied work in Paris. What mechanisms can be added to ensure that the target is met?

Then there is money. For any kind of deal to emerge in Paris, poorer countries will demand that rich economies flesh out a vow to provide at least $100 billion annually in climate finance by 2020.

For the second time in six years, the nation-state system will be on trial to see if it can fix a global environment problem.

If it fails once more, interest will swing more and more to bilateral and regional action and carbon-cutting and adaptation measures by cities, businesses and individuals.

"You have to remember that these international negotiations do not represent everything that's happening on climate," said Pascal Canfin of the World Resources Institute (WRI).