A time machine enclosed in a giant shell, capable of reconstructing the climate from over two hundred years ago, before the rise in average global temperature caused by human activities. A study published in the scientific journal Global and Planetary Change, carried out by a group of Australian researchers, has analysed a specimen of giant tridacna. This is the largest bivalve mollusc, found exclusively in the waters of the western Pacific (in the areas of Malaysia, Indonesia and Australia) and capable of reaching 1.2 metres in length and a weight of 300 kilograms.

A diary that allows us to travel back to before climate change

Like an ancient diary, the marine animal recorded its life day by day. The alternation of day and night leaves microscopic growth lines on the shell, while the passing of the seasons produces wider bands, in a way similar to what happens in tree trunks. At the same time, seawater influences these traces through its chemical composition. The combination of these signals was analysed and deciphered by the researchers, who were thus able to reconstruct the climatic conditions of the era in the Lizard Island archipelago, located off the Australian coast of Queensland.

The first confirmation, as indicated by the study's authors in an article published in the French newspaper Sud Ouest, is linked to the fact that before the onset of the era of climate change, local conditions were cooler than they are today (with the exception of winters, which "surprisingly showed data similar to those of today").

"Today," the researchers continue, "instruments installed on land, in the oceans and in space measure the evolution of the climate. But these direct observations cover only a microscopic portion of the Planet's history. To go further back in time, we need to look in other archives." Such as those of shells with such a long lifespan.

Average summer sea temperature 0.9–1.2 degrees lower than today

The specimen examined measures 58 centimetres and was found in the Jiigurru lagoon. Dating using the carbon-14 dating method established that the mollusc lived approximately between 1785 and 1827, that is, at the end of a pre-industrial cooling period known as the "Little Ice Age".

"Under the microscope," the study's authors further explain, "we counted approximately 10,900 daily growth lines, indicating that it lived for about thirty years. We then drilled 784 tiny holes to extract powder samples from the different layers of the shell. In each of these we measured different forms of oxygen and carbon, known as isotopes. The oxygen isotopes allowed us to estimate the sea temperatures of the era, while the carbon isotopes provided information on precipitation. By cross-referencing this chemical information with the growth lines, we were able to reconstruct, month by month, the environment of the coral reef before the warming associated with industrialisation."

Information on El Niño and La Niña also reconstructed

The results indicate that 225 years ago the sea surface temperature was on average 0.6–0.9 degrees lower than it is today, on an annual basis. In particular, summers were 0.9–1.2 degrees cooler. Furthermore, the analyses indicate that at the time there was a clear alternation between wet and dry seasons, and that temperatures fluctuated following regular cycles of two or three years, consistent with local variations linked to El Niño and La Niña. These phenomena, which consist of changes in equatorial Pacific temperatures, were therefore more frequent than they are today.

According to the researchers, the Jiigurru specimen is not an isolated case, as other analyses carried out on corals or Antarctic ice have provided compatible indications. Further studies on similar shells from other locations in the Indo-Pacific region are needed, however, to understand how climatic conditions have changed from one area to another.