A study of ancient fossil shells has revealed that the Antarctic was once as warm as the California coast.

Yale University scientists came to their conclusion after trying a different model to gauge historic temperatures.

Their findings underscore the potential for seesawing temperatures at the Earth's poles and the associated risk of melting polar ice and rising sea levels.

According to Hagit Affek, associate professor of geology and geophysics and a co-author of the study, the research could help improve climate models used for predicting future climate changes.

"Quantifying past temperatures helps us understand the sensitivity of the climate system to greenhouse gases and especially the amplification of global warming in polar regions," he said.

The study focused on Antarctica during the Eocene epoch, around 40 to 50 million years ago, when Earth had a greenhouse climate with high concentrations of atmospheric CO2.

By measuring concentrations of rare isotopes in ancient fossil shells, the team found that temperatures in parts of Antarctica reached as high as 17C (around 63F) during the Eocene. The new measurement technique is called carbonate clumped isotope thermometry.

The average temperature was estimated to be 14C (57F), similar to the average annual temperature off the coast of California today. Eocene temperatures in parts of the southern Pacific Ocean measured 22C - similar to modern-day seawater temperatures off Florida.

These ocean temperatures were not uniformly distributed throughout the Antarctic ocean regions. Researchers attributed the higher temperatures on the South Pacific side of Antarctica to ocean currents.

"By measuring past temperatures in different parts of Antarctica, this study gives us a clearer perspective of just how warm Antarctica was when the Earth's atmosphere contained much more CO2 than it does today," said Peter Douglas, the lead author of the study.

"We now know that it was warm across the continent, but also that some parts were considerably warmer than others. This provides strong evidence that global warming is especially pronounced close to the Earth's poles. Warming in these regions has significant consequences for climate well beyond the high latitudes due to ocean circulation and melting of polar ice that leads to sea level rise."

To determine the ancient temperatures, the team measured the abundance of two rare isotopes bound to each other in fossil bivalve shells collected on Seymour Island, a small island off the northeast side of the Antarctic Peninsula. The concentration of bonds between carbon-13 and oxygen-18 reflect the temperature in which the shells grew.

"We managed to combine data from a variety of geochemical techniques on past environmental conditions with climate model simulations to learn something new about how the Earth's climate system works under conditions different from its current state," Affek added.