Domatoceras, a precursor of the squid with a hard shell, thrived 443 million years ago during the early Silurian period. More than 100 million years later during the Carboniferous period, Pentamerus, a clamlike, two-shelled invertebrate, clustered on ocean floors. Both stored rare isotopes of carbon and oxygen in their calcium carbonate shells that then fossilized. By examining the percentage of such bonded rare isotopes, scientists have now confirmed the link between carbon dioxide levels and warmer ancient climates.

Certain isotopes of carbon and oxygen have extra neutrons, specifically 13C and 18O, which find each other more easily at cooler temperatures. Because scientists know the favorable bonding conditions, the number of such pairings in ancient shells provides a thermometer for different periods.

Geochemists Rosemarie Came and John Eiler of the California Institute of Technology and their colleagues counted these couplings in fossilized shells from the Silurian and Carboniferous eras and discovered, contrary to earlier findings, that the higher carbon dioxide concentrations of the Silurian were indeed linked to higher ocean temperatures.

"Attempts to quantitatively reconstruct surface temperatures at this time suggested that the two periods were similar to one another in mean global temperature," Eiler says. "Our results demonstrate that the Silurian tropical oceans were far warmer than the Carboniferous, consistent with common expectations regarding the importance of greenhouse gases for global climate."

Most previous reconstructions relied on Nobel prize winner Harold Urey's measurement of relative concentrations of 18O in calcium carbonate and seawater depending on temperature. But that method is based upon the original concentration of 18O in the oceans, which is extremely difficult to accurately reconstruct from the fossil record.

Eiler argues that the new measurements are more accurate because they do not rely on such seawater data. The team plans to refine the new method, which they describe in Nature, and use it to assess temperatures during other periods, such as the Paleocene-Eocene thermal maximum that occurred approximately 55 million years ago when there was a sudden shift from a relatively cool to an extremely hot climate as well as temperatures that prevailed during Earth's periodic mass extinctions.

The finding adds yet more weight to the contention that greenhouse gases drive climate change, and bode ill for the present increases in atmospheric concentrations of such gases. "It supports the notion," Eiler says, "that you can use simple radiative balance arguments—that is, the greenhouse effect to relate atmospheric chemistry to global temperature."