Warming reaps earlier spring growth.

Forests and grasslands across the Northern Hemisphere are now waking from winter's slumber a full week earlier than they did just 20 years ago, according to long-term measurements of carbon dioxide in the atmosphere. These results provide the first hint that rising temperatures in recent decades have lengthened the growing season across at least half of the globe. This perspective on plant growth comes from carbon dioxide measurements made over the last 30 years on top of Mauna Loa, Hawaii, and at Point Barrow, Alaska. Concentrations of the gas fall and rise naturally throughout the year, reflecting carbon dioxide uptake by northern plants during spring and summer and then increased release of the gas by soil microbes during winter. While analyzing the gas records, Charles D. Keeling of the Scripps Institution of Oceanography in La Jolla, Calif., and his colleagues found that the seasonal swings in concentration have grown 20 percent larger at Mauna Loa and 40 percent larger at Point Barrow since the early 1960s. On top of that, the spring decline in carbon dioxide values has shifted. It now occurs about 7 days earlier than it did during the mid-1970s, the scientists report in the July 11 Nature.

"The drawdown in carbon dioxide is earlier than it was before, and that's probably the key to the whole picture, because it looks like the growing season has lengthened," says Keeling.

He suggests that rising temperatures, particularly in winter and springtime, have stimulated northern plants to grow earlier and longer. The more vigorous plant growth could explain the increased swing in the seasonal carbon dioxide cycle.

"Suppose you are up in the Canadian forest in April, for example, and the sun is up, but it's cold. So plants can't leaf out, or if they're conifers, they can't start photosynthesizing yet. But suppose it warms up. The plants really benefit. They have plenty of soil moisture and get going. They get bigger and grow longer before the season ends."

In support of the temperature link, Keeling and his colleagues note that the biggest jumps in carbon dioxide's seasonal swings followed the extremely high global temperatures during 1981 and 1990.

The new results close the loop in a feedback cycle long suspected but never demonstrated. According to theory, rising concentrations of carbon dioxide from fossil fuel burning should warm the climate, which influences vegetation and microbes and eventually affects carbon dioxide concentrations. The observations by Keeling's group indicate that climate change is indeed altering Earth's carbon dioxide cycle, although the exact cause of the recent temperature increase remains uncertain.

"When you look in your backyard, it's obvious that changes in climate would affect the biosphere. What [Keeling] has done is put that in a global perspective. He's confirming everything that we have wanted to see and that we have had no evidence for," says Inez Fung, a climate researcher at the University of Victoria in British Columbia.

Others caution that the subtle changes detected by Keeling's group must be verified by other studies. "It's a provocative suggestion, but it needs reexamination," says ecologist David Schimel of the National Center for Atmospheric Research in Boulder, Colo.