University of California researchers have solved a longstanding mystery for scientists trying to understand how Earth's climate can quickly shift between cold and warm modes.

The mystery revolves around the source of a rapid change in the geochemistry of oceanic carbon that occurred as the last ice age ended, between 16,000 and 20,000 years ago.

Based on analyses of carbon stored in tiny fossil seashells, geologists Howard Spero of UC Davis and David Lea of UC Santa Barbara suggest that the chemical change occurred because of dramatic shifts in ocean circulation. They have linked the timing of this event to changes recorded in the ocean, in Antarctic ice cores and on the continents.

Climate-change experts say these changes reflect the types of events that could occur because of global warming related to human activities.

"An understanding of the relative timing of this event is critical because the greenhouse gases that humans are producing are likely to affect not only the warming of the atmosphere but also the circulation of the oceans," Spero said. "Understanding the order of events that occurred when Earth warmed quickly in the past can help us model what might happen if the Earth continues to warm into the future."

"This link will enable us to better establish the relationship between changes in atmospheric carbon dioxide and global climate change," said Lea.

The paper is published in the April 19 issue of the journal Science. The study was funded by the U.S. National Science Foundation and the Hanse Institute for Advanced Study, a non-profit private foundation based in Germany.