Springing a surprise

spring reaches the northern hemisphere a week earlier than it did twenty years ago. Charles Keeling of the Scripps Institution of Oceanography at La Jolla, California, us , has recorded a sudden shift in the seasonal cycle of the concentration of atmospheric carbon dioxide, resulting in the early arrival of the northern spring. The published report comes in the wake of a meeting of signatories to the Climate Change Convention in Geneva to agree on a timetable for limiting global emissions of carbon dioxide (New Scientist , Vol 151, No 2038).

Keeling had begun measuring amounts of the gas on a bi-weekly basis way back in 1958, from the Mauna Loa peak in Hawaii. Since then he has been recording a rise in average concentrations of carbon dioxide and an increasing amplitude in its seasonal cycle. Keeling reports that the amplitude of the seasonal cycle at Mauna Loa has increased by 20 per cent since the early '60s. The increase is still greater in the Arctics, measuring 40 per cent at Point Barrow in Alaska.

Every year, there is a rise in the atmospheric levels of carbon dioxide as vegetation in the northern hemisphere stops growing and releases carbon. And every spring, levels of the gas fall as plants and trees resume growth and absorb carbon dioxide in the process. The decline in atmospheric carbon dioxide in the northern hemisphere during spring seems to have shifted forward by seven days. Keeling is of the opinion that global warming has caused the early rejuvenation of the world's woodlands, particularly in the northern hemisphere (the forests of Scandinavia, Siberia and Canada), and this has led to an increased uptake of carbon dioxide by the trees. These changes, according to Keeling, "suggest largescale responses of the carbon dioxide cycle to climate change".

"Keeling's work confirms that terrestrial vegetation is much more important to the carbon dioxide cycle, and so to global warming, than we once thought," says John Grace of the University of Edinburgh, who reported last year that virgin Amazon rainforest was growing faster due to global warming.

Keeling's arguments have been further substantiated by his observation that increases in the strength of the seasonal carbon dioxide cycle has occurred in jumps, coming about two years after an increase in average temperature. He also reports the lengthening of the growing season.

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