300 years of paleotemperature records preserved in sclerosponges, limestone skeletons of sea sponges. This is the material from which researchers have attempted to calculate global warming.
A unique method that produced surprising results, published recently in Nature Climate Change: “Higher terrestrial temperatures, as well as the earlier onset of industrial-era warming, would indicate that global warming would already be 1.7 °C above pre-industrial levels in 2020”, a result 0.5°C higher than the IPCC (intergovernmental panel on climate change) estimates.
Estimates of global warming are generally based on temperature data collected at the sea surface by ships.
Surveys which only began in the middle of the 19th century.
Thus, the first instrumental recordings available from 1850 to 1900 were used to define the “pre-industrial” period of the IPCC.
However, it has been established that humans have been releasing carbon dioxide into the atmosphere since “at least the early 1800s,” according to the authors, even though several large volcanic eruptions contributed to cooling the climate at that time.
Century-old thermometers
How to overcome this difficulty?
The team of researchers from the University of Puerto Rico-Mayagüez, for its part, undertook an exhaustive study of the long-lived “sclerosponges” of the Caribbean between 2007 and 2017. These organisms which live for centuries, taken at depths ranging from 33 to 91 meters, were subjected to in-depth laboratory analysis to understand their age, chemical composition and reaction to temperature variations.
The Caribbean area where the sponges live is remarkably protected from strong ocean currents and, in turn, from major climatic fluctuations, such as the Atlantic Meridional Overturning Circulation and the Southern Oscillation that is El Niño.
The data collected therefore made it possible to reconstruct temperature fluctuations over more than 300 years based on anomalies in the ratios of strontium and calcium for the period from 1961 to 1990. This information was then rigorously analyzed and adjusted to inform on temperature changes over time.
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And according to their results, therefore, the acceleration of warming triggered by human activities started, “more than 80 years before the sea temperature records”, which started in the mid-1860s. Result which considers that the overall increase was already, in 2020, +1.7°C compared to the pre-industrial period – or 0.5°C more than estimated by the IPCC.
However, many experts call for results to be interpreted with caution, given the novelty of the method.