The amount of greenhouse gases in Earth's atmosphere hit a new record high in 2012, continuing an ever-faster rise that is driving climate change, the UN weather agency said Wednesday.

"The concentrations are reaching once again record levels," Michel Jarraud, who heads the World Meteorological Organization (WMO), told reporters in Geneva.

His organisation released its annual report on greenhouse gases Wednesday, showing that concentrations of carbon dioxide (CO2), methane and nitrous oxide all broke fresh records in 2012.

Global concentrations of CO2, the main culprit in global warming, for instance reached 393.1 parts per million last year, or 141 percent of pre-industrial levels -- defined as before 1750.

The report was released a day after the UN Environment Programme warned the chances of limiting the global temperature rise to two degrees Celsius (3.6 Fahrenheit) over pre-industrial levels were swiftly diminishing, and ahead of UN climate talks that open in Warsaw next week.

The UN's two-degree target is being chased through efforts to curb Earth-warming greenhouse gas emissions, mainly caused by fossil-fuel burning to power industry, transport and farming.

"The observations from WMO's extensive Global Atmosphere Watch network highlight yet again how heat-trapping gases from human activities have upset the natural balance of our atmosphere and are a major contribution to climate change," Jarraud said.

Dave Reay, a carbon management expert at the University of Edinburgh, said that stabilising greenhouse gas concentrations was the key to successful climate negotiations, emissions regulations, and carbon markets rests.

"Despite the financial crash, and reduced emissions from some nations, the global picture is one of carbon dioxide concentrations in our atmosphere reaching a record-breaking high year after year," Reay added.

'C02 has a ratchet effect'

Experts warn that unless more is done to rein in emissions, the world faces potentially devastating effects such as more frequent megastorms, species extinctions, water shortages, crop die-offs, loss of land to the rising seas as glaciers and polar ice melt, and spreading disease.

"CO2 has a ratchet effect," said Peter Wadhams, professor of ocean physics at the University of Cambridge.

"Its influence on the climate system lasts for about 100 years, so we will be paying for our profligate use of fossil fuels for a long time to come -- so long, in fact, that we may well have now made it impossible for the planet to avoid catastrophic global warming effects, even if we make a start now on reducing CO2 emissions."

The atmospheric increase of CO2 from 2011 to 2012 was higher than the average growth rate over the past 10 years, WMO said, stressing that the global concentrations of CO2 last year were dangerously close to the symbolic 400 parts per million threshold.

That threshold was actually exceeded at several Arctic stations during the year, and the global annual average CO2 concentration looks set to cross it in 2015 or 2016, the UN agency said.

This level has not existed on Earth in three to five million years, experts say.

Concentrations of methane, meanwhile, were 260 percent of the pre-industrial level, while nitrous oxide reached 120 percent.

The WMO report said that between 1990 and 2012 there was a 32 percent increase in so-called "radiative forcing" -- the warming effect on our climate -- because of heat-trapping gases.

CO2 accounted for 80 percent of this increase.

What is happening in the atmosphere is just part of the picture.

Only about half of the CO2 emitted by human activities remains in the atmosphere, with the rest absorbed in the biosphere and in the oceans, the WMO underlined.

Jarraud noted that the UN's Intergovernmental Panel on Climate Change recently sounded the alarm over gas concentrations.

"According to the IPCC, if we continue with 'business as usual,' global average temperatures may be 4.6 degrees higher by the end of the century than pre-industrial levels -- and even higher in some parts of the world. This would have devastating consequences," he said.