NOAA: Greenhouse Gases Still Building Up

The greenhouse gases widely blamed for raising the planet's temperature are still building up in the atmosphere.

The National Oceanic and Atmospheric Administration said Monday there was a continuing increase in carbon dioxide and nitrous oxide in the air last year, though methane leveled off.

The agency said and there was a decline in two chlorofluorocarbons, gases blamed for the ozone hole over the Antarctic.

Overall, NOAA said, its annual greenhouse gas index "shows a continuing, steady rise in the amount of heat-trapping gases in the atmosphere."

The index was set at a base value of 1.0 in 1990. For 2005 it was 1.215, the agency said.

Greenhouse gases, produced by industry, automobiles and other engines, tend to trap heat from the sun at the planet's surface, reducing the amount of heat radiated back out into space.

The result has been a gradual increase of temperatures over recent decades, raising concerns that human activities will result in a damaging climate change.

Most of the increase is attributed to carbon dioxide, which the agency said accounts for 62 percent of radiative warming by greenhouse gases.

Carbon dioxide increased from an average of 376.8 parts per million in 2004 to 378.9 ppm in 2005. The preindustrial CO2 level is estimated to have been about 278 ppm.