Carbon dioxide marches to an uneven beat.

As the global population of people and cars climbs ever higher, so do emissions of carbon dioxide, the notorious greenhouse gas. Since 1958, when scientists first started monitoring its concentration, carbon dioxide has accumulated in the atmosphere almost in lockstep with the growing use of gasoline and other fossil fuels.

But in the last 15 years, the carbon dioxide buildup has surged ahead of fossil fuel emissions and then lagged behind--changes that have puzzled atmospheric scientists and hampered efforts to predict climate.

A team of researchers now reports measurements that may explain why concentrations of this gas have followed such an uneven upward path. Charles D. Keeling of the Scripps Institution of Oceanography in La Jolla, Calif., and his colleagues blame the carbon dioxide antics on Earth itself. Minor climate fluctuations significantly alter the ability of ocean water and land surface to absorb and emit carbon dioxide, they report in the June 22 Nature.

The large swings exhibited by the ocean and continents caught scientists off guard, says Keeling: "So far, everything that's happened has been a surprise."

Keeling and his cohorts measure carbon dioxide concentrations at Mauna Loa, Hawaii, and at the South Pole. In the 1980s, they noticed that the accumulation of carbon dioxide accelerated even as high petroleum prices slowed the rise in fossil fuel use. After 1990, the pattern reversed. Carbon dioxide continued to accumulate in the atmosphere, but at a rate slower than that predicted from fossil fuel emissions.

To trace the cause, the researchers turned to measurements of carbon isotopes in air. Carbon dioxide produced from fossil fuel combustion and from land vegetation is isotopically light--it has a low ratio of carbon-13 to carbon-12, compared to the carbon dioxide in air. Carbon dioxide released from the oceans, however, has an isotopic ratio close to that of air.

From the isotopic measurements, Keeling and his colleagues conclude that the oceans and land surface swing between accelerating and slowing the carbon dioxide buildup in air. The two tend to balance each other: During El Nino events, for instance, the oceans absorb more carbon dioxide than usual, but the land surface emits more.

According to the researchers, the carbon dioxide surge seen in the 1980s resulted from an oceanic warming, which depressed the amount of gas absorbed by the upper layer of water. When global temperatures reached record highs in 1990 and 1991, the warming spurred vegetation on the continents to absorb additional carbon dioxide, causing the slowdown in accumulation witnessed in 1992 and 1993. The eruption of Mt. Pinatubo in mid-1991 could also have put a temporary brake on the carbon dioxide rise by cooling off the globe. The findings have important implications, says Keeling, because researchers must understand how carbon dioxide accumulates in the atmosphere before they can forecast how much climate will warm in response to the buildup of greenhouse gases.

Other researchers question the new study, however. Pieter P. Tans of the National Oceanic and Atmospheric Administration in Boulder, Colo., says the isotopic data do not provide a clear picture because the observed changes are small. He remains unconvinced that the oceans and land surface vary so widely in the amount of carbon dioxide they absorb and emit. "I'm not sure such anomalies can be real," says Tans.

Whatever the cause, carbon dioxide has continued to surprise scientists. Last year, the rate of accumulation of fossil fuel emissions surged ahead once again, ending the 2-year slowdown.