Think global warming might change things a little? You haven't seen anything compared to 50 million years ago.

Though Antarctica is year-round one of the coldest places on Earth, and the continent's interior is the coldest place, with annual average land temperatures far below zero degrees Fahrenheit, during the Eocene epoch, 40-50 million years ago, there was a period with high concentrations of atmospheric CO2 and consequently a greenhouse climate.  

The CO2 was not the only culprit, there have been periods where CO2 was 10X what it is now without spiking the heat, but it sure didn't help things. It meant that some parts of ancient Antarctica were as warm as today's California coast, and polar regions of the southern Pacific Ocean registered 21st-century Florida heat.

Credit: Yale

Writing in the Proceedings of the National Academy of Sciences, the authors say their new measurements, using a technique  called carbonate clumped isotope thermometry, can help improve climate models used for predicting future climate.

"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,"  says co-author Hagit Affek, associate professor of geology&geophysics at Yale.

Peter M.J. Douglas,
lead author and postdoctoral scholar at the California Institute of Technology, says that by measuring concentrations of rare isotopes in ancient fossil shells, they found that temperatures in parts of Antarctica reached as high as 17 degrees Celsius (63F) during the Eocene, with an average of 14 degrees Celsius (57F) — similar to the average annual temperature off the coast of California today.

Eocene temperatures in parts of the southern Pacific Ocean measured 22 degrees Centigrade (or about 72F), researchers said — similar to seawater temperatures near Florida today.

Today the average annual South Pacific sea temperature near Antarctica is about 0 degrees Celsius.

These ancient ocean temperatures were not uniformly distributed throughout the Antarctic ocean regions — they were higher on the South Pacific side of Antarctica — and researchers say this finding suggests that ocean currents led to a temperature difference.

"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 Douglas. "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 scientists measured the abundance of two rare isotopes bound to each other in fossil bivalve shells collected by co-author Linda Ivany of Syracuse University at 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, the researchers said. They combined these results with other geo-thermometers and model simulations.

"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 said. "This combined result provides a fuller picture than either approach could on its own."