Measurements made based on the structure of centuries-old marine sponges suggest that the Earth may have already warmed by 1.7°C compared to the average temperature before the era of fossil fuels.
If the observations are confirmed, this means that the warming of the globe is already close to leaving behind one of the objectives of the Paris Agreement, today the main international treaty to face the climate emergency, indicating that the problem is even more urgent than before. imagined.
The data has just been published in an article in the specialized journal Nature Climate Change. “It’s as if we’ve advanced the clock on climate change by about a decade,” summarized one of the study’s coordinators, Malcolm McCulloch, from the Ocean Institute at the University of Western Australia, in an online press conference.
“This is a unique set of data, difficult to obtain, that we were able to analyze”, added the other research leader, Amos Winter, from Indiana State University (USA).
The key to the duo’s conclusions is the physical structure of the species’ sponges. Ceratoporella Nicholsoni, present in the Caribbean, near the islands of Puerto Rico and Saint Croix, and with close relatives in Brazil, according to Winter. These invertebrates, with a very primitive body structure, similar to that of corals, grow very slowly and can live for centuries, being found tens of meters deep.
Therefore, the calcareous structure of the animals — called “sclerosponges” — can be used to investigate what the environmental conditions of the ocean were like throughout their lifetime.
In fact, the variation in the proportion of the chemical elements strontium and limestone (the so-called Sr/Ca ratio) incorporated into the skeletal layers is what researchers call a paleothermometer, recording the comings and goings of water temperature with an estimated resolution of 2 years.
There is yet another advantage to using Ceratoporella Nicholsoni, say the researchers. Because it grows at depths ranging from 30 m to almost 100 m, the sponge is not subject to the more rapid and transient fluctuations that affect the water temperature near the surface. Therefore, they argue, the Sr/Ca ratio of invertebrates reflects well the general trends of warming or cooling of ocean water.
McCulloch, Winter and their colleagues combined raw data on the proportional variation of chemical elements with historical records of climate and temperature data obtained directly from the sea and on land. These last data, obtained with relatively reliable thermometers, are more recent, dating from the second half of the 19th century onwards.
But some of the sponges in the analysis are much older, dating back to the beginning of the 18th century (for comparison purposes, they began to form when gold had just been discovered in Minas Gerais and Tiradentes had not yet been born). Historical and thermometer information served to “calibrate” the origins of sponges, assigning them more specific dates.
The result, according to the scientists, follows more conventional temperature measurements quite accurately and reaches periods when there were no reliable thermometers. The limestone structure of sponges records well, for example, the periods of cooling that occurred at the beginning of the 19th century due to volcanic eruptions such as Tambora, responsible for generating the so-called “Year without Summer” of 1816.
At the same time, however, the sponges appear to capture an initial phase of warming caused by human action, with the first intensification of the burning of fossil fuels after the beginning of the Industrial Revolution, starting in the 1860s. It turns out that direct measurements of the temperature of oceans only show this process 80 years later.
Adding this data to the acceleration of the increase in temperatures on dry land (today, they calculate, twice as fast outside the oceans as inside them), the total average increase in temperature today would be around 0.5°C higher than that calculated. by the IPCC, the United Nations climate panel.
Experts consulted by Nature Climate Change, however, highlighted that it is necessary to interpret the results with caution, especially due to their implications for climate crisis diplomacy. After all, the Paris Agreement talks about keeping the planet’s warming “well below” 2°C in relation to the pre-industrial average and, if possible, around just 1.5°C. Has this limit been crossed?
“The way these findings have been communicated is flawed, with the potential to add unnecessary confusion to the public debate on climate change,” said Yadvinder Malhi of the Environmental Change Institute at the University of Oxford.
According to Malhi, data on carbon emissions produced by humans up to 1900 suggest that they would be insufficient to produce 0.5°C of warming during the 19th century. What is most likely, according to him, if the data is correct, is that good Part of this warming has a natural origin.
Either way, the Paris Agreement’s 1.5°C target is valid for the end of this century, notes Shaun Fitzgerald of the University of Cambridge’s Climate Repair Centre.
“We would only have surpassed this target if reducing carbon emissions was the only possible measure, but there are several approaches being developed to remove greenhouse gases from the atmosphere, and this could keep the target alive.”