A newly published study conducted by a collaborative team of French and Swiss researchers is suggesting a flaw in the way past ocean temperatures have been estimated. This discovery claims that oceans may have been much cooler over the past 100 million years than records suggest, meaning that the recent spike in ocean temperatures might be more significant and alarming than we thought.

"Oceans cover 70 percent of our planet," explains Anders Meibom, one of the authors of the study from the University of Lausanne. "They play a key role in the earth's climate. Knowing the extent to which their temperatures have varied over geological time is crucial if we are to gain a fuller understanding of how they behave and to predict the consequences of current climate change more accurately."

Over the last few decades, scientists have developed increasingly sophisticated methods to track our ocean's temperature. From a global network of monitoring stations called Argo floats to newer satellite based technology, every day offers a clearer picture of how climate change is warming our oceans.

In order to estimate how ocean temperatures have varied over millions of years, scientists traditionally measure the presence of an oxygen isotope (oxygen-18) found in tiny marine fossils called foraminifera. It was generally thought that the oxygen-18 content found in the calcareous shells was fixed and unchanging over time. These measurements suggested that our global ocean temperature was around 15° Celsius (27° F) warmer 100 million years ago than today.

The new study claims this measurement technique is flawed and shows that the level of oxygen-18 present in foraminifera can in fact change over time. The scientists demonstrated this by exposing the fossils to high temperatures in artificial seawater containing oxygen-18, and through chemical analysis found that it was in fact able to absorb the isotope.

"What appeared to be perfectly preserved fossils are in fact not," says Sylvain Bernard, the study's lead author. "This means that the paleotemperature estimates made up to now are incorrect."

The scientists claim the previous 15-degree temperature shift identified in the fossil record can be explained by a process called re-equilibration. This refers to rises in temperature of up to 30° Celsius (54° F) that occur during sedimentation, causing the foraminifera re-equilibrate with the surrounding water.

"To revisit the ocean's paleotemperatures now, we need to carefully quantify this re-equilibration, which has been overlooked for too long," says Meibom. "For that, we have to work on other types of marine organisms so that we clearly understand what took place in the sediment over geological time."

After running new computer simulations, the team suggest that current paleotemperature estimations have been overstated and ocean temperatures over the past 100 million years may have actually been much more stable. The striking implication of the study is our currently rising ocean temperatures are much more anomalous that previously thought. If global ocean temperatures were in fact relatively stable over the past 100 million years then our current, more accurate measurements are exponentially more disturbing, suggesting a nearly 1 degree rise over the past century.

The new research was published in the journal Nature Communications.