Carbon-cutting pledges from 146 nations for a universal climate rescue pact leave the "door open" to capping global warming below the danger threshold, the United Nations said Friday, a month ahead of crunch talks in Paris.

But even if these 10-to-15 year plans are fulfilled, humanity will have used up three-quarters of its carbon "budget" by 2030 and must slash greenhouse gas output even more to avoid devastating climate impacts, the UN's Climate Change Secretariat warned.

"An unprecedented world-wide effort is underway to combat climate change, building confidence that nations can cost-effectively meet their stated objective of keeping a global temperature rise to under 2 C," it said in an assessment of the country pledges.

At the same time, "much greater emissions reductions efforts... will be required" to meet the two degrees Celsius (3.6 degrees Fahrenheit) target endorsed by the UN 195-nation climate body, it said.

The longer we wait, the harder and more expensive it will become to cut back the fossil fuel emissions that drive climate change.

The Secretariat's 66-page review comes exactly one month before the November 30 to December 11 Paris summit tasked with finalising a historic global pact.

As they stand, the pledges place the world on track for warming of some 2.7 C by 2100 -- "by no means enough, but a lot lower than the estimated four, five or more degrees of warming" that would have otherwise take place, said UN climate chief Christiana Figueres.

If countries commit in Paris to periodically revising ambition upward, the goal stays within reach, she added.

The so-called Intended Nationally Determined Contributions, or INDCs, will be a pillar of the Paris pact, which would be the first to bind all the world's nations in a single action plan.

- 'From catastrophe to disaster' -

The UN reviewed 146 INDCs submitted by October 1, including all developed nations and three-quarters of developing ones. Collectively, they cover 86 percent of the world's greenhouse gas emissions.

Another eight pledges have been submitted since then.

Taken together, the carbon reduction schemes would cause per average capita emissions to decline by up to nine percent over the next 15 years.

If commitments are met, combined annual emissions in 2025 will be about 55.2 billion tonnes of carbon dioxide equivalent (GtCO2e) -- a measure used to group different greenhouse gases such as carbon dioxide, methane and nitrous oxide -- compared to some 50 GtCO2e today.

By 2030, the figure will be 56.7 GtCO2e, showing that global emissions -- while slowing -- would still be on an upward trajectory.

The UN Environment Programme has previously estimated that emissions must fall to about 32-44 GtCO2e by 2030 if we are to have a better-than-even chance at hitting the 2 C goal.

"As the report makes clear, to stay below 2 degrees -- much less the 1.5 degrees that many countries are calling for -- the Paris agreement must have meaningful provisions designed to quickly ramp up the level of ambition," Alden Meyer, a climate analyst at the Union of Concerned Scientists, told AFP.

To stay under the 2 C threshold scientists estimate that humanity's has a total CO2 budget of about 1,000 gigatonnes.

Taking the INDCs into account, that allowance would be 54 percent spent by 2025, and 75 percent by 2030, the report said.

Even if parties do not ramp up their pledges until as late as 2030, the possibility of a 2 C limit "still remains," said the report.

However, "this could be achieved only at substantially higher annual emission reduction rates and cost," compared to action now.

Reacting to the report, anti-poverty NGO Oxfam said the pledges were a step in the right direction, but "they only take us from a 4 C catastrophe to a 3 C disaster."