"The long-term trend is upward and strongly upward," said Barry Huebert, a University of Hawai'i professor of oceanography who specializes in atmospheric chemistry.

The Mauna Loa Observatory data is the gold standard in climate research. Since 1958, the site has been recording atmospheric CO2 levels — longer than any research facility in the world — and it's considered one of the best sites for such measurements.

The observatory, at more than 11,000-foot elevation on Mauna Loa, comprises buildings filled with and surrounded by complex scientific gear. The measurements are prized because they are taken from a site more than 2 miles high in the middle of the Pacific, distant from most human activity and far from sources of pollution.

For most of the past few thousand years, ice cores and other sources indicate the CO2 level was generally stable at about 280 parts per million, said Pieter Tans, chief scientist of the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo.

With the start of the industrial revolution, 150 years ago or so, the level started rising. By the time Mauna Loa took its first measurements in 1958, it was up to 315.6 parts per million. The CO2 level has increased each year since then. Through the 1960s, the rate of increase was less than 1 ppm per year. During the past couple of decades, it has averaged 1.5 ppm.

But occasionally, it jumps higher than that. In 1998 there was a spike. And during another spike in 2002 and 2003, the rate was 2.6 and 2.3 ppm, respectively.

Preliminary indications are that the increase for 2004 will be about 1.5, Tans said.

Last month, the overall CO2 level surpassed 380 ppm — by far the highest it has been in thousands of years. While it's increasing regularly, the rise isn't always steady, as indicated by the recent spike.

"There are natural fluctuations in climate which influence the carbon budget," Tans said.

Some of those fluctuations are seasonal changes. Every spring, when plants start growing aggressively, they take up carbon dioxide and the atmospheric level drops for a few months. Every fall, it climbs back up; each year it has been reaching a new peak.

The world climate system is complicated. Oceans take up and release carbon dioxide, and so do soils and plants. A change in the climate — such as during an El Niño event — can change the rate of transfer. And massive events like big volcano eruptions or vast wildfires can also disrupt the system.

"There are large enough sources and sinks in things like big fires and the uptake by the ocean that it doesn't take much to cause an aberration," Huebert said.

With respect to the 1998 and 2002-2003 spikes in carbon dioxide, it's notable that both periods had El Niño climate events, said Tom Schroeder, director of the University of Hawai'i's Joint Institute for Marine and Atmospheric Research.

"We often get a spike associated with El Niño cycles," he said, although he conceded that the 2002-2003 El Niño was a moderate one.

Tans said there also were big wildfires during those years. In 1998 they were in Indonesia and Malaysia, and during the more recent spike they were in Siberia, he said. Fires dump a great deal of carbon dioxide into the atmosphere, although some of that is reabsorbed as the burned areas regrow, partly fertilized by the ash from the blazes.

Less important than the recent short-term spike in carbon dioxide is the overall continuing increase in the gas, year after year. Tans said it seems inescapable that human burning of fossil fuels has an impact. Global use of fossil fuels is equivalent to 2 cubic miles of oil each year — about 6.5 billion tons of carbon.

The best scientific evidence to date is that about a third of that is taken up by the oceans, and about a sixth by soils and plants, Tans said. The remaining half ends up in the atmosphere. That, he said, probably accounts for the regular annual increase in carbon dioxide.