GENEVA (AFP) – Concentrations of carbon dioxide and other greenhouse gases in the atmosphere hit a new high in 2014, the UN said Monday, warning the resulting climate change was moving the world into "uncharted territory".

In its annual report on Earth-warming greenhouse gases in the atmosphere, the World Meteorological Organization said concentrations of carbon dioxide (CO2), methane and nitrous oxide once again broke records last year.

"Concentrations of greenhouse gases in the atmosphere are now reaching levels not seen on Earth for more than 800,000, maybe even one million years," WMO chief Michel Jarraud told reporters.

"This means we are now really in uncharted territory for the human race," he warned.

The report came as country envoys gathered Monday in Paris to iron out tough political questions ahead of a key summit tasked with sealing a climate rescue pact.

"Every year we report a new record in greenhouse gas concentrations," Jarraud said.

"Every year we say that the time is running out. We have to act now to slash greenhouse gas emissions if we are to have a chance to keep the increase in temperatures to manageable levels," he warned.

His appeal comes just weeks before a Paris summit aimed at ensuring global warming is limited to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.

Physics laws 'non-negotiable'

Jarraud urged the Paris negotiators to take a hard look at the ballooning amount of data on our changing climate and act urgently.

But since greenhouse gases can remain in the atmosphere "for centuries", there is only so much policymakers have the power to change, he said.

"It's frightening... This is a cumulative process," Jarraud said, stressing that "the laws of physics are non-negotiable."

WMO's report, which does not measure emissions of greenhouse gases but rather their concentrations in the atmosphere, showed that CO2 had risen to 397.7 parts per million (ppm) in the atmosphere last year.

That was 143 percent of levels prior to the year 1750, WMO said, adding that CO2 concentrations would likely pass the ominous 400-ppm threshold in 2016.

"We will soon be living with globally averaged CO2 levels above 400 parts per million as a permanent reality," Jarraud said.

"We can't see CO2. It is an invisible threat, but a very real one," he said.

"It means hotter global temperatures, more extreme weather events like heatwaves and floods, melting ice, rising sea levels and increased acidity of the oceans," he said.

Oceans swallow about a quarter of greenhouse gas emissions, while the biosphere sucks up another quarter.

Water vapor threat

The rising CO2 concentrations are especially worrying, WMO said, since the gas in turn hikes levels of water vapor -- itself a powerful, albeit short-lived, greenhouse gas.

This is because warmer air holds more moisture, the UN agency said, cautioning that "further increases in CO2 concentrations will lead to disproportionately high increases in thermal energy and warming from water vapor."

If CO2 concentrations were to double from their pre-industrial level of 280 ppm to 560 ppm, water vapour and clouds globally would hike global warming at three times the rate of so-called long-lived greenhouse gases, WMO said.