Climate change: Caribbean sponges indicate earlier onset of global warming

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Ceratoporella nicholsoni: species that grows slowly and can live for several centuries

Photo: Andoni Alvarez / NMNH / Office of Education and Outreach / Smithsonian

According to a research group, global average surface temperatures could already be 1.7 degrees Celsius above the temperature level before the industrial revolution: Australian and US scientists have derived this from studying the skeletons of sponges in the Caribbean Sea.

In their study published in the journal Nature Climate Change, the group suspects that global warming began in the 1860s - earlier than the Intergovernmental Panel on Climate Change (IPCC) models. At 1.7 degrees Celsius, it would also be 0.5 degrees Celsius higher than the IPCC estimate. However, other researchers express some sharp criticism of the study.

"Human-made emissions are driving global warming, but the temperature increase compared to pre-industrial levels is uncertain," writes the research team led by Malcolm McCulloch from the University of Western Australia. Since regular temperature measurements only began in the 19th century, and in some regions only in the 20th century, climate research relies on estimates and models for earlier temperature values. McCulloch and his team now tried to reconstruct a consistent temperature curve using sponge skeletons.

Skeletons store environmental conditions

The sponges belong to the species Ceratoporella nicholsoni, which grows very slowly and can live for several centuries. The chemical composition of their calcareous skeleton can provide information about past environmental conditions.

The research group collected the sponges from locations around the Caribbean island of Puerto Rico in water depths of 33 to 91 meters and examined them for the two elements calcium (Ca) and strontium (Sr), which accumulate in their skeletons. “The Sr/Ca ratio acts like a historical thermometer with lower values ​​in warmer periods and higher values ​​in cooler periods,” explains Wenfeng Deng from the Chinese Academy of Sciences in Guangzhou in a commentary also published in “Nature Climate Change”. In this way, the sponges provided a temperature curve that dates back to the year 1700.

The team used the temperatures of the Caribbean water around the sponges as proxy data: From these values, they drew conclusions about the global temperatures of the sea surface water and, in turn, about the global average temperatures of the entire earth's surface.

In fact, such “proxy” data has already been used in other studies to reconstruct sea temperatures. However, corals would usually be analyzed for this, notes Anton Eisenhauer from the Helmholtz Center for Ocean Research Kiel (Geomar) in an independent classification. The coralline calcifying sponges used here are the better and more stable climate archive: they would grow in greater water depths and represent the mixed surface water mass of the ocean, while corals only reflected the surface temperature of the seawater in the upper water depth of around five meters. “If you want to see long-term trends, coralline sponges are much more suitable, as was the case in the study,” says Eisenhauer.

Eisenhauer considers the study's conclusions from the measured values ​​in the coralline sponges to global, historical sea temperatures to be "valid". "What is still missing is confirmation of the results by sponges in other areas and independent groups."

“No conclusive evidence”

Jochen Marotzke from the Max Planck Institute for Meteorology in Hamburg, on the other hand, is skeptical: "The work does not provide any conclusive evidence that the sponge skeletons in a single location say anything about the global mean temperature." Above all, the conclusion of the study is that The world has already warmed by 1.7 degrees Celsius, which is unsustainable.

Helge Goessling from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) in Bremerhaven describes the study's methodology as largely solid. "Nevertheless, I have methodological concerns that raise doubts about the results overall." For example, it is not sufficiently proven that the sea temperature of a single region - in this case the Caribbean - generally reflects the long-term trend of the global average temperature. For these and other reasons, he is not convinced that the climate model projections need to be significantly corrected.

Mojib Latif from Geomar also speaks of uncertainties in the methodology - but also of a very academic discussion: "In my opinion, it is already far too warm on Earth, regardless of whether we are 'officially' still below or above 1.5 degrees Celsius." The effects of global warming, which has already been realized, are catastrophic, says Latif: "In my opinion, we should not discuss tenths of a degree and distract from the urgency of trade."

ani/dpa