What happens when you bring together close to 200 nations for one of the most pivotal summits in history?

Organised chaos.

And COP21 should be.

An attempt to unite the planet on an issue so nuanced and geo-politically charged as climate change is going to require more than a quick sit-down over coffee.

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Hence the miniature city that has emerged in an outlying suburb of Paris called Le Bourget, about 45 minutes on the RER train line from the Eiffel Tower.

A day at COP21 begins with a rail trip to Le Bourget station and then another crowded coach-ride to the Parcs Des Expositions.

As the kebab shops and brasseries roll past the window, you might find yourself awkwardly squashed between a Polish negotiator, a Canadian professor or a Dutch protester.

The venue's total area is slightly smaller than Mt Smart stadium, but the sheer scale of the conference is bewildering.

The first thing you see is a series of massive buildings, visible from the highways that flow into Le Bourget.

Just as imposing is the security: the police and gendarmerie who guard the entrances sport berets, boots and assault rifles.

A group of people visit the American stand displayed at the COP 21, the United Nations conference on climate change at Le Bourget. Photo / Getty Images
A group of people visit the American stand displayed at the COP 21, the United Nations conference on climate change at Le Bourget. Photo / Getty Images

Admission to the conference comes only after an airport-style X-ray scan and multiple credential checks.

Outside the main entrance, activists hand out flyers carrying bold font messages that bark at you: 650 billion a year to finance climate chaos!

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Along the main thoroughfare between the main buildings, other demonstrators chant amid a constant flow of delegates in black suits and thick winter coats.

At any one time, there are 50 or so places you could choose to be: high-powered panels on climate finance, film screenings, mini expos and Ted-X-like plenary talks in cavernous theatres.

Outside one, a familiar voice booms into the foyer area.

It turns out to be that of America's best-known climate campaigner, silhouetted against a large stage screen.

In his signature brand of oration, Al Gore is comparing the world's atmosphere to an "open sewer" being pumped with more than 100 million tonnes of global-warming pollution each day.

Former US Vice President and environmental activist Al Gore delivers a speech during the opening of 'Action Day' at the COP21 United Nations conference. Photo / Getty Images
Former US Vice President and environmental activist Al Gore delivers a speech during the opening of 'Action Day' at the COP21 United Nations conference. Photo / Getty Images

He highlights the peril of climate change with footage of atomic bomb blasts -- the amount of pollution being trapped in the atmosphere is apparently equal to the energy release of 400,000 Hiroshima blasts every day -- and images of people wading through floodwater.

Gore is far from the only big name in town: billionaires Bill Gates, Sir Richard Branson and Mark Zuckerberg have turned up to launch climate campaigns. Delegates also saw an unlikely presentation from Arnold Schwarzenegger and US State Secretary John Kerry.

US actor and former California governor Arnold Schwarzenegger gives a press conference on carbon price as part of the COP 21 UN conference on climate change. Photo / Getty Images
US actor and former California governor Arnold Schwarzenegger gives a press conference on carbon price as part of the COP 21 UN conference on climate change. Photo / Getty Images

And the traffic in Le Bourget -- by usual standards a typically French mess of honking trucks and cars trying to turn in every direction at once -- has been made yet more of a nightmare by police having to escort constant motorcades through.

In the media centre, thousands of computers have been set up for the mass of international journalists, racing to filing updates on the negotiations.

When one American journalist casually asks about dinner plans, his stressed colleague angrily snaps at him.

Other reporters are just as unhappy to be interrupted by an activist walking between them and handing out bracelets symbolising the loss of rainforests.

Most rooms at the conference are accessible, but those that remain shut off are where the real action is happening.

Each country gets their own slice of the venue. China and the US each have numerous rooms, plus stage areas for regular press conferences.

Perhaps a reflection of its tiny place in the scheme of things, New Zealand's small delegation room is hidden well away in a far corner of the warehouse-like Hall 3.

Its door opens only for daily briefings to the scattering of Kiwi observers who have come to the other side of the world to track our negotiations.

Activists, academics and environmentalists at home have roasted New Zealand's climate pledge but the country's role in the COP21 blockbuster is barely noticeable.

Cast as the bad guys are so-called "obstructionists" like Saudi Arabia -- official delegation members discussing hold-out attempts by some nations never name them -- and Canada has become the protagonist through its surprise move of backing a new global warming limit of 1.5C.

As for the central plot, Labour's climate spokesperson Megan Woods, attending as part of the official New Zealand delegation, jokes the script could have been written a long time ago.

In Paris time on Wednesday, the French presidency is expected to formally receive a revised text of a global agreement to slash emissions beyond 2020 and to support developing countries' mitigation efforts.

People visit the American stand displayed at the COP 21, the United Nations conference on climate change at Le Bourget. Photo / Getty Images
People visit the American stand displayed at the COP 21, the United Nations conference on climate change at Le Bourget. Photo / Getty Images

A handful of big stumbling blocks stand in the way of sign-off; and Ms Woods expected these would be familiar issues such as what emissions reductions big developed countries would commit to, or what cash would be available to the most vulnerable nations and who would pay what.

Another knotty problem was how reductions targets that weren't binding could still be policed to ensure action.

Despite positive buzz about getting the deal done, it's unclear when or if the talks will hit a crisis point.

There's chatter this might happen on Thursday night, allowing enough time for the bomb to be defused in time for the 6pm, Friday, scheduled end of COP21.

But veterans of these conferences, none of which have ever ended on time, know UN climate negotiations almost always carry on well into over-time, and sometimes into the next day.

They only need to point to the conference bus timetable -- the last COP21 coach doesn't roll out of Le Bourget until Monday.

• Jamie Morton travelled to Paris with the support of the NZ Science Media Centre and the Morgan Foundation.