This is how the ocean temperature has fluctuated for 700,000 years

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Researchers from the University of Bern (Switzerland) were able to determine the average temperature of the world ocean at the time of the glacial and interglacial extremes of the last 700,000 years. The results were published in the scientific journal Climate of the past this April 14.

Volcanism, fluctuations in sunlight, greenhouse gases… Regardless of its origin, any climate change results from a disturbance in the planet’s radiative balance. In other words, an imbalance between the energy which enters the considered system and that which escapes from it. However, more than 90% of the resulting variation in heat takes place in the ocean. Its particularly high mass and heat capacity allowing it to play the role of thermostat and, thus, to temper the amplitude of the evolutions.

For these reasons, fluctuations in the temperature of the global ocean constitute a privileged parameter for the study and monitoring of climate change. However, if we have had precise measurements for about fifty years, the situation is different with the climatic periods of the past. For example, very little is known about the details of these temperatures at the scale of glacial-interglacial cycles where little direct information is available.

Reconstructing the temperature of the global ocean

Nevertheless, thanks to the work of a team of researchers from theoccript, it has been possible to reconstruct the variations in mean ocean temperature over the past 700,000 years. How did the scientists do it? The answer holds noble gases contained in air bubbles trapped in Antarctic ice, the same ones that make it possible to restore the level of CO2 atmospheric for 800,000 years. One major difference, however: the present analysis, limited to glacial and interglacial extremes, required a much larger technical base.

The prerequisites for this method are high precision measurements using a dynamic mass spectrometer which have been made possible through the considerable efforts of several doctoral students and postdoctoral researchers involved in the publication. »Reports Hubertus Fischer, one of the co-authors of the paper.

High: mean ocean temperature for 700,000 years (in anomalies compared to the Holocene, the current interglacial). Medium: atmospheric CO2 concentration. Bottom: local temperature in Antarctica. Finally, the pink bars indicate the interglacial periods. Different datasets appear on this diagram. Credits: Marcel Haeberli et al. 2021.

As the fraction of noble gases – argon, krypton, xenon – dissolved in seawater depends on its temperature, their concentration in the atmosphere will also depend on it. Therefore, it is possible to use them as indicators of thermal fluctuations of the past (especially Going through the study of their relation to molecular nitrogen). In addition, these gases have the property of being extremely well mixed on a global scale. ” We only need a single sample of polar ice to measure the average temperature of the oceans »Thus relates Hubertus Fischer. If applicable, it is the ice core taken from dome C during the project EPIC that was used.

An increasingly unclear climate past

The data obtained by the researchers signify an ever finer understanding of past climate change. Accurate to a few tenths of a degree, they show that ice ages were some 3.3 ° C below the pre-industrial level in terms of average ocean surface temperature. A value that has finally changed little over the seven oscillations that the study interval returns. On the contrary, the interglacials appear more differentiated. Those occurring before 450,000 years are for example colder by about 1 ° C than the interglacial in which we have been evolving for 10,000 years.

We can also note the variability that punctuates the alternation between the main peaks and troughs. Much of it is a mark of ocean circulation, which speeds up or slows down with changes – often abrupt – in the ice caps. The heat storage, therefore the fraction of noble gases dissolved in water, is in effect modified. ” To understand how the heat balance of the climate system is changing, we first need to understand the ocean. »Emphasizes the co-author.

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