The volume of greenhouse gases causing global warming rose to a new high last year, the UN World Meteorological Organization said Tuesday, warning it is becoming increasingly unlikely the world can limit rising temperatures to UN-backed targets.

Levels of carbon dioxide (CO2) -- the single most important man-made contributor to climate change -- rose to 390.9 parts per million in 2011, which is 2.0 ppm higher than in 2010, the WMO said.

Pointing out that the worst warming gases -- CO2, methane and nitrous oxide -- had all reached new highs last year, the agency's Secetary-General Michel Jarraud said "it is getting increasingly unlikely" that a UN-backed pledge to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) could be achieved.

"Even if we were able to stop them tomorrow, these greenhouse gases will continue to have an effect for centuries," Jarraud said at the launch of the annual Greenhouse Gas Bulletin report in Geneva.

CO2 levels are at 140 percent of the pre-industrial level before 1750, Jarraud said. According to the WMO, about 375 billion tonnes of carbon have been released into the atmosphere as CO2 in the past 260 years.

"These billions of tonnes of additional carbon dioxide in our atmosphere will remain there for centuries, causing our planet to warm further and impacting on all aspects of life on earth," Jarraud said in a statement.

"Future emissions will only compound the situation," he said.

Taking the long view on data to smooth out year-on-year anomalies, the WMO showed that while carbon dioxide in the atmosphere increased on average 1.5 ppm during the 1990s, the average annual hike from 2000 to 2010 stood at 2.0 ppm.

"So it's not just increasing, it's increasing exponentially," WMO scientific officer Oksana Tarasova told reporters.

Jarraud, meanwhile, pointed out that so-called "carbon sinks", including oceans, have until now absorbed nearly half of the CO2 emitted by humans, but stressed that "this will not necessarily continue in the future."

Five major gases account for 96 percent of the warming of our climate, according to the WMO, which released its annual greenhouse gas report ahead of a new round of UN climate talks in Doha later this month.

"Between 1990 and 2011, there was a 30 percent increase in radiative forcing -- the warming effect on our climate -- because of carbon dioxide and other heat-trapping long-lived gases," the WMO said.

The levels of atmospheric methane, the second most important greenhouse gas after CO2, also reached new highs in 2011, at 1,813 ppb.

This is 259 percent of the pre-industrial level, WMO said, blaming mainly human activities like fossil fuel exploitation, cattle breeding, rice agriculture, landfills and biomass burning.

Also worrying was the increase in nitrous oxide levels, the WMO said, since its impact on climate is almost 300 times greater than that of carbon dioxide.

The gas, emitted into the atmosphere from natural and man-made sources, also plays an important role in the destruction of the ozone layer, which protects the Earth from harmful ultraviolet rays, the WMO said, indicating that its atmospheric concentration in 2011 was about 324.2 ppb, up 1.0 ppb from 2010 and 120 percent of pre-industrial levels.