It's now crunch time in Paris - will the world reach an unprecedented agreement to combat future climate change? Herald science reporter Jamie Morton explains the ABCs of the COP21 conference.

A is for "Ambition"

As the world edges closer to a historic climate accord, New Zealand is preparing to step back while the big players attempt to thrash a final deal out.

Overnight in Paris, where close to 200 nations have gathered for the UN COP21 conference, momentum built further with the release of a new draft agreement that was scrubbed of three quarters of the big sticking points holding up negotiations.

By the end of this weekend, it's widely expected the world will have an unprecedented agreement to slash greenhouse gas emissions and support the poor, low-lying countries most threatened by climate change.

The Ministry of Foreign Affairs and Trade's climate change ambassador in Paris, Jo Tyndall, pointed out there were still plenty of hurdles to jump over the next few days.

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One of them was the simple matter of what level of temperature rise the world was willing to accept.

In the lead-up to the summit, experts argued that keeping this century's global warming below the critical 2C "point of no return" would be a diplomatic struggle.

Indeed, the latest analysis from the credible Climate Action Tracker added up all of the emissions-cutting 160 pledges laid on the table and worryingly found they still amounted to a 2.7C limit, rather than the officially agreed 2C cap.

However, its authors argued, that figure could be slashed if all governments decided to follow through on their conditional targets and scrapped any plans for coal plants.

And if that sounds like a tall order, the narrative at COP21 surrounding temperature-focused goals has been overtaken by a contagious call - sparked by understandably worried vulnerable Pacific countries - to aim even higher toward a 1.5C limit.

More than 100 nations have backed the idea, among them Canada, whose surprise endorsement sparked a surge of excited press releases from green groups.

Some observers are however concerned the 1.5C target was an unhelpful red herring that threatened to undermine the immensely complex global effort that managed to bring together 195 countries in the first place.

Victoria University's Professor Dave Frame was worried people were now talking more seriously about the 1.5C figure without first committing to a cumulative limit consistent with 2C.

"I think that's very unwise."

Similarly, Dr Matthew Watson, of the University of Bristol, said an agreement geared toward a 1.5C target would have "significant and, worryingly, unstated consequences".

"Current scientific thinking suggests we cannot limit warming to less than 1.5C without large scale intervention in the climate system - implicit in this agreement, then, is acceptance of some form of climate engineering."

Australia has gone as far as supporting a UN-led review of the idea, while New Zealand has drawn fire from environmental quarters for similarly holding back.

Ms Tyndall believed the 1.5C issue could be tackled through what she called a "language solution" wired into the final agreement, which is due for adoption at 6pm Friday but more likely to be signed off on Saturday or Sunday.

The latest draft of the text, released overnight New Zealand time, carried one cleverly-worded option to stick to the officially-agreed 2C but "scale up" global efforts to keep warming below 1.5C.

It's expected that such subtle phrasing will help negotiators navigate their way through long-term goals, which include potential five-yearly reviews and the bigger political challenge of getting action even before the Paris Agreement's post-2020 commitments kick in.

Interestingly, the working draft even mentions a goal of achieving zero emissions by the end of the century - but many experts say that still lies well beyond the horizon of today's political and technological landscape.

Still, there are strong and near universal expectations that the final agreement should set out clear and defined ambitions for the future - and not let the world down with another watery document like that which sunk the anti-climactic 2009 Copenhagen summit.

As one headline put it: "Words matter. Meaning matters most."

With COP21 on the home stretch, the French presidency has taken charge of the final process, and there's widespread faith that the host country's famed knack for diplomacy will get the deal over the line.

The hope isn't misplaced: in the space of just a few days, France proved itself able to whittle down over 1,600 flagged points of disagreement in the draft text to 228 - a 79 per cent reduction.

B is for "Binding"

It's a strange question that only something as messy as COP21 could offer: how, exactly, can non-binding targets still be binding?

Despite top diplomats telling media the final agreement will contain plenty of legally binding measures, it's likely the final agreement won't actually force countries to achieve what they pledged.

For the US particularly, it's become increasingly improbable that the White House could push such a hard-core measure through a Republican-dominated Congress.

Instead, the agreement should feature some sort of non-binding target in which countries are still assessed against the pledges they've made.

Many have pointed out that previous attempts at this have often failed - some countries have fallen well short of what they previously offered to do before 2020 - so the crunch will come down to some type of peer-review mechanism.

That has shifted the focus to transparency - and even the possibility of naming and shaming slacker countries.

This hasn't washed well with some big polluting countries, which are framing the move as an intrusion of sovereignty.

Some have suggested the agreement could create a new global climate watchdog or tribunal, similar to the UN nuclear disarmament office, but the matter is more likely to be addressed with a function built into the accord itself.

So what would motivate nations to stick to this honesty box approach?

COP21 has already signaled that international reputations will be a strong motivator, and there's real optimism that the gathering of 150 world leaders at the summit's opening was more than just an impressive photo opportunity.

C is for "Cash"

Of all the sticking points, financing was always going to be the most predictable roadblock.

A much-lauded centerpiece of COP21 is the drive to create a climate fund that would ultimately generate an annual $100 billion for developing countries to tap into.

Yet surrounding it has been incessant wrangling over what proportion should be contributed by rich countries, which are already being scolded for their underwhelming support for those nations now facing the wrath of climate-driven disasters.

Earlier at the conference, the four most powerful developing nations - China, Brazil, India and South Africa - teamed up to reject the economic methodology behind a key OECD report that found two-thirds of the $100 billion pot had already been mobilised by rich countries.

The diverse G77 bloc, led by South Africa, have also been vocal in demanding that $100 billion be a starting point, and that cash from developed nations for adaptation not be voluntary aid but a strict obligation.

But the final agreement may yet let developed nations off the hook with a weak clause enabling rich countries to commit funds voluntarily and, as the wording states, "when in a position to do so".

Another trigger point has been whether rich nations will accept liability for the so-called "loss and damage" sustained by poor, low-lying countries.

For nations that have experienced more than their share of climate change, such as the Marshall Islands and Bangladesh, the topic has been emotionally-charged and ripe for heated headlines.

For developed nations, the notion of having to accept liability - and forking out billions of dollars in compensation - has been sore point, if not a terrifying prospect.

A business-focused publication being circulated around COP21, The Bottom Line, warned developing countries not to over-play their hands.

"Developing countries must understand that the Paris Agreement is itself the financial package they need to drive down emissions, build resilience and open a path to equitable access to sustainable development," it stated.

"They ought to not lose sight of this as they bargain over sources of finance."

Added to the mess is the conundrum of "differentiation" that frames the intricate divisions, dynamics and responsibilities between developed and developing countries.

Quandaries raised so far have ranged from wealth sharing to a re-balancing of emissions production - but all come back to the central clash of who pays for what.

And even once a climate funding system is established, there's the more practical problem of whether small countries even have the structures in place to receive and make use of international funds for mitigation and adaptation.

In our part of the world, New Zealand has a great chance to guide its tiny Pacific nations through the process and help them secure the cash they now desperately need.

Will New Zealand get what it wants?

New Zealand's two big bargaining points at COP21 have been around land use and access to carbon markets - both crucial to meeting its post-2020 pledge of lowering emissions by 30 per cent from 2005 levels and 11 per cent from 1990 levels before 2030.

Particularly, the Government sought clarity and certainty in the agreement that would give it unrestricted access to global carbon markets and better guidance around its current land and activity-based approach to emissions accounting.

However, Ms Tyndall said, it was unlikely the Paris agreement would give New Zealand what it sought.

"We won't get absolutely everything we are after. Ideally, we would have liked a well-formed and detailed set of principles and guidelines included in the agreement for both carbon markets and the land sector," she said.

"The negotiation isn't mature enough to have that level of detail, but we are still reasonably confident that there will be the basis there to develop the kinds of principles and guidelines we have in mind.

"I think if we can get a provision in the agreement that contains those essentials, and an accompanying decision that sets up a work programme to develop principles further, we will be in a really good space."

Meanwhile, she expected New Zealand's role at the conference to be minimal from here on in, as the French presidency turned to the big nations to steer the conference toward agreement.

"A lot of this process to date has been every country having a seat at the table... but at the end, a small number of countries will need to be brought on board and by and large, those are the big economies that don't include New Zealand.

"So we have no expectation of being in the small rooms at the very end of this process, where the final deal is cut."

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