Concentrations of the main greenhouse gases in the atmosphere have reached their highest level since pre-industrial times, the World Meteorological Organization (WMO) said on Wednesday.

Concentrations of the gases continued to build up in 2009 -- the latest year of observations -- despite the economic slowdown, the UN weather agency said in its latest Greenhouse Gas Bulletin.

Rises in the amount of greenhouse gases increase radiation in the atmosphere, warming the surface of the Earth and causing climate change.

'The main long-lived greenhouse gases including carbon dioxide, methane and nitrous oxide have reached their highest recorded levels since the beginning of the industrial age, and this despite the recent economic slowdown,' WMO Deputy Secretary-General Jeremiah Lengoasa told a briefing.

The findings will be studied at a UN meeting in Cancun, Mexico, from November 29 to December 10 to discuss climate change.   

Total radiative forcing of all long-lived greenhouse gases -- the balance between radiation coming into the atmosphere and radiation going out -- increased by 1.0 percent in 2009 and rose by 27.5 percent from 1990 to 2009, the WMO said.   

The growth rates for carbon dioxide and nitrous oxide were smaller than in 2008, but this had only a marginal impact on the long-lasting concentrations.   

It would take about 100 years for carbon dioxide to disappear from the atmosphere if emissions stopped completely.

Carbon dioxide is the single most important greenhouse gas caused by human activity, contributing 63.5 percent of total radiative forcing. Its concentration has increased by 38 percent since 1750, mainly because of emissions from burning fossil fuels, deforestation and changes in land use, the WMO said.

Natural emissions of methane due for example to the melting of the Arctic icecap or increased rainfall on wetlands -- themselves caused by global warming -- are becoming more significant, it said.

This could create a 'feedback loop' in which global warming releases large quantities of methane into the atmosphere which then contribute to further global warming.   

These natural emissions could be the reason why methane has increased in the atmosphere over the past three years after nearly a decade of no growth, the WMO said.   

Human activities such as cattle-rearing, rice planting, fossil-fuel exploitation and landfills account for 60 percent of methane emissions, with natural sources accounting for the rest. -Reuters