The fossilized shells of tiny plankton have filled in a missing piece in the picture of global warming.

By studying the chemical makeup of these shells, scientists have confirmed that temperatures rose sharply at the tropics during the earth’s most dramatic global heat wave — the Paleocene-Eocene Thermal Maximum, which began about fifty-five million years ago. The amount of temperature increase at the tropics was the only part of the global-warming scenario that hadn’t been confirmed until now.

A key measurement was the shells’ ratio of magnesium to calcium. The higher that ratio, the higher the temperature of the water in which the shells formed. “If there is a dramatic increase in that ratio, then you can assume that there was some sort of warming,” says Amanda Brill, a recent Carolina master’s graduate in geological sciences who worked on the study.

Brill generated fragmentation data, which helped show that the findings from the rest of the study were reliable. If the shells had been highly fragmented, that would have meant that the shells had dissolved enough over the years that the calcium/magnesium data might have been distorted.

Brill’s work was tedious. She’d divide the shells into groups of 300, then use a microscope to examine them one by one. Out of the group, she’d count how many were fragmented, then use those numbers to generate percentages — out of all the shells, what percentage had broken into pieces? James Zachos of the University of California Santa Cruz led the study. Timothy Bralower, former chair of geological sciences at Carolina and now a professor at Pennsylvania State University, was cochief scientist on the 2001 drilling expedition of the JOIDES Resolution that recovered the sediment cores containing the fossilized plankton shells from the tropical Pacific. The work confirms current models that scientists use to predict the effects of rising atmospheric carbon dioxide caused by industrial emissions and other pollutants. It means, Zachos says, that “Greenhouse climate theory is alive and well.”