Greenhouse gases in the atmosphere continue to increase and in 2008 global concentrations of such gases reached the highest levels recorded since pre-industrial times, the World Meteorological Organization (WMO) said on Monday.

Greenhouse gases trap radiation within the Earth's atmosphere causing it to warm. Human activities, such as fossil fuel burning and agriculture, are major emitters of greenhouse gases which scientists widely recognize as drivers of global warming and climate change.

After water vapor, the four most prevalent long-lived greenhouse gases in the atmosphere that are directly influenced by humans are carbon dioxide (CO2), methane (CH4), nitrous oxide (N20) and halocarbons.

According to the WMO's 2008 Greenhouse Gas Bulletin, the globally averaged mixing ratio of carbon dioxide (CO2) in 2008 was 385.2 ppm (parts per million), with an increase of 2.0 ppm from the previous year, continuing the tendency of exponential increase.

The globally averaged mixing ratio of CHR in 2008 was 1,797 ppb (parts per billion), which means an increase of 7 ppb from the previous year. While the concentration of CH4 was stable for seven years from 1999 to 2006, both 2007 and 2008 showed a significant increase.

The globally averaged mixing ratio of N2O in 2008 was 321.8 ppb, 0.9 ppb higher than in 2007, and 19 percent above the pre- industrial level.

Some halocarbons such as chlorofluorocarbons (CFCs), previously used as refrigerants, as propellants in spray cans and as solvents, are decreasing slowly as a result of the phase-out of these compounds through the Montreal Protocol on Substances that Deplete the Ozone Layer. However, concentrations of other halocarbons such as HCFCs and HFCs, which are used to substitute chlorofluorocarbons, are increasing rapidly, according to the bulletin.

"It's not really good news: concentration of greenhouse gases continue to increase, actually even a bit faster," WMO Secretary- General Michel Jarraud told reporters in Geneva.

He called for urgent global actions to prevent "the most pessimistic scenario" of global warming from happening.