Carbon dioxide, the gas most often implicated in the threat of global warming, seems to be increasing faster than in past years, according to a report issued by the National Oceanic and Atmospheric Administration.

The concentration in the air increased at an average rate of 1.71 parts per million (ppm) per year over the last four years, NOAA's Pieter Tans said.

That is somewhat higher than the annual 1.5 ppm rise reported in much of the 1980s. And in the 1960s the increase was reported at only 0.7 ppm annually.

"The increase varies a lot from year to year. One year it can change very little, and the next year it makes up for the slow increase," Tans said in a telephone interview. "At the moment it seems to be increasing fairly fast."

Concentrations of carbon dioxide and other "greenhouse" gases are watched by NOAA's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo.

The increasing amounts of these gases in the air has caused concern in recent years because they act like the glass in a greenhouse, letting sunlight in but keeping heat from radiating into space. The gases are produced by virtually all forms of burning, but especially by automobiles, power generation, and the burning of wood such as happens in clearing of the Amazon rain forest.

Global concentrations of carbon dioxide, the most common of the greenhouse gases, have increased by about 25 percent since the mid-1800s, to just over 350 ppm.

However, scientists disagree over whether any global heating has occurred.

Climate experts say there is evidence that if carbon dioxide levels double, which could happen by the middle of the next century, global average temperatures could rise by 2 to 6 degrees Fahrenheit. The warming would not be uniform over the Earth. It would be least in the tropics and greatest at the poles, where it would melt the ice caps.

The melt water and the thermal expansion of the ocean would cause global sea levels to rise, perhaps by a foot or more.

It has been calculated that increasing the atmospheric level of carbon dioxide by 1 ppm would require the addition of 2.13 billion tons of carbon to the air.

Other greenhouse gases are much less common, though some are more powerful, molecule for molecule, in their effects:

Methane concentrations have been rising by 12 parts per billion per year, although officials said this rate may have declined slightly.

Nitrous oxide increased at a rate of 0.7 parts per billion per year.

CFC-11, one of the chlorofluorocarbon coolant compounds, was rising at 10 parts per trillion, while the rate for CFC-12 was 16 parts per trillion. These chemicals also can damage the Earth's protective ozone layer. An international agreement has been signed to reduce their use.