The world's nations must urgently ramp up commitments to cut planet-warming carbon emissions to avoid "human tragedy", the United Nations warned on Thursday.

As they stand, these commitments -- which kick in from 2020 -- would still allow average global temperatures to climb as high as 3.4 degrees Celsius (6.1 degrees Fahrenheit) by 2100, a recipe for massive climate damage, the UN Environment Programme (UNEP) said in its annual "Emissions Gap" report.

"If we don't start taking additional action now, beginning with the upcoming climate meeting in Marrakesh, we will grieve over the avoidable human tragedy," said UNEP head Erik Solheim.

UN climate negotiators from 196 countries -- tasked with implementing the landmark Paris climate pact entering into force on Friday -- meet in Marrakesh, Morocco, from November 7-18.

"The growing number of climate refugees hit by hunger, poverty, illness and conflict will be a constant reminder of our failure to deliver," Solheim said in a statement.

The UNEP report tracks the so-called global carbon budget -- the total amount of greenhouse gases humanity can add to the atmosphere without pushing temperatures above the threshold of destructive warming.

Going into the Paris climate summit last December, countries had agreed to a maximum of 2C (3.6F) above pre-industrial era levels.

But a maelstrom of climate-enhanced natural disasters -- including deadly storm surges, droughts, floods and wildfires -- prompted nations to lower the danger threshold to "well below" 2C, and even 1.5C (2.7F) if possible.

These more ambitious targets mean that the "remaining carbon dioxide budget is now considerably lower," the report said. 

As a result, CO2 emissions projected for 2030 -- including existing carbon-cutting pledges -- must be slashed an additional 25 percent to have a reasonable chance of avoiding crippling climate impacts, it concluded.

Even if all national plans for curbing greenhouse gases are fully implemented, including those conditional on financial aid, the carbon budget that would give humanity a two-thirds chance of staying under the 2.0 C ceiling would be all used up within 15 years.

And under a scenario that would yield 50/50 odds of capping global warming at 1.5C, the budget would be depleted well before 2030. 

CO2 emissions from the energy sector were flat in 2014 and 2015 despite a strong uptick in global GDP, an encouraging sign that economic growth can be decoupled from an increase in carbon pollution.

But total greenhouse gases from all sources continued to show a steady increase in 2014, according to the UNEP report.

2015 was the first year in which average surface temperatures were a full degree Celsius above the pre-industrial benchmark.