A new spin on global warming.

Although the makers of quartz watches would have you believe otherwise, even the most accurate timepiece won't precisely track the passing days. Because of tiny changes in Earth's rotational rate, day length can vary as much as one-thousandth of a second over a span of a few months. A group of researchers now reports calculations suggesting that such fluctuations may provide a means of monitoring long-term climate change, such as global greenhouse warming.

Richard S. Gross and his colleagues at the Jet Propulsion Laboratory in Pasadena, Calif., analyzed the seasonal changes in day length over the last 30 years. Whereas scientists think that rearrangements of matter inside the Earth cause longer-term shifts in rotational speed, the seasonal changes are thought to occur because of variations in the strength of winds.

Gross and his colleagues found that the seasonal fluctuations in day length over the last three decades correlate with the Southern Oscillation Index- an atmospheric pressure pattern in the Pacific associated with El Nino warming. This suggests that scientists can use the alterations in day length to monitor long-term changes in the atmosphere, particularly the strength of winds that blow around the globe, Gross says.