What would a strong climate pact look like?

Ministers from 195 nations entered the home stretch Wednesday of decades-long UN climate talks tasked with delivering a pact that can protect humanity from the ravages of runaway global warming.

But how will the world know if the agreement, due Friday, is up to the task?

Here are a few benchmarks for a deal that could make a real difference, according to scientists, policy analysts and advocates:

- Ramping up pledges -

Voluntary pledges to slash greenhouse gas emissions submitted by all but a handful of nations are not enough to meet the UN target of holding global warming to two degrees Celsius (3.6 degrees Fahrenheit) above pre-Industrial Revolution levels, the threshold for climate catastrophe, according to scientists.

This is why putting in place a mechanism for ramping up the promises is critical. But sharp disagreement persists on when that process should start and what nations will be required to do.

A strong outcome would start the "ratcheting" process even before the five- and 10-year pledges enter into force in 2020, and be accompanied by an open-book system to verify actions taken, climate analysts say. It should apply to all countries, not just rich ones, though giving developing nations some leeway to build up their technical knowhow.

- A new global economy -

To back up the 2C target, the Paris agreement must articulate a long-term goal for transforming the global economy. Scientists have set some markers: no additional CO2 in the atmosphere by about 2060, and no greenhouse gases at all by 2080.

Shooting for an even lower temperature limit of 1.5C -- favoured by nations vulnerable to rising seas and extreme drought -- would require phasing out fossil fuels even sooner, or overshooting the mark and rowing back.

But negotiators have struggled mightily to find a magic formula that is clear enough to signal the need to purge fossil fuels from the world economy but also broad enough to bridge the national interests of oil exporters, emerging giants, rich nations and the world's most climate-vulnerable countries.

Calling for greenhouse gas emissions to peak "as soon as possible" followed by a "rapid reduction" -- terms under consideration for the pact -- are too vague, say scientists. A more ambitious wording, they say, might call for "decarbonisation" and for achieving net zero emissions shortly after the middle of the century.

- Putting money on the table -

Coming into the Paris talks, developing countries were already assured of receiving $100 billion (91 billion euros) in climate finance per year from 2020, a pledge dating back to the fraught 2009 climate summit of Copenhagen.

But all the details behind that headline figure were left to be worked out.

From the point of view of the recipients -- especially the poorest and most climate-vulnerable -- a strong Paris deal would make it clear that the $100 billion pledge was a floor not a ceiling, and would grow over time.

If rapidly emerging economies that have moved closer to full industrialisation -- such as China and Mexico -- were among the donors, even on a purely voluntary basis, it would also strengthen the deal.