Between 30 meters and 90 meters below the surface of the Caribbean Islands Sea, an ancient sponge species that grows a hard skeleton, has been silently recording changes in ocean temperature for hundreds of years.
Now those sponges are at the center of a bold and controversial claim made in a leading scientific journal that the planet may have already warmed by 1.7C since the start of the Industrial Revolution – half a degree more than the United Nations’ estimates were used. climate panel.
Several leading scientists urged caution, saying the research had “overreached” and questioned whether such a bold claim could be made based on one sponge species from a single location.
But Prof Malcolm McCulloch of the University of Western Australia, who led the research published in the journal Nature Climate Changesaid the results were robust.
“On a precautionary basis, our findings show that global warming is more advanced than we thought and therefore it is a wake-up call that we need to continue reducing CO2,” he said. “We will experience more severe impacts of global warming sooner than we expected.”
With the help of deep sea divers six specimens of Ceratoporella nicholsoni – a sponge that can take hundreds of years to grow between 10cm and 15cm – has been removed from areas off the coast of Puerto Rico and the US Virgin Islands.
As the sponges grow, they store strontium and calcium in a ratio directly related to the temperature of the water around them.
McCulloch and colleagues reconstructed global ocean temperatures over the past 300 years from signals found in the sponges and then combined them with land-based temperatures to provide an estimate of global warming.
The sponges grow deep enough to be unaffected by natural fluctuations in temperature and in an area of the ocean, the authors said, where temperature changes closely match the global average.
When McCulloch and colleagues at Indiana State University and the University of Puerto Rico checked their data, they said the sponges matched the changes in global temperature seen from more modern measurements.
The sponges also recorded a sudden drop in temperatures caused by a massive volcanic eruption in Indonesia in 1815giving them confidence that the sponges were a good proxy for temperature.
McCulloch said the sponges helped overcome limitations of ship-based measurements of ocean temperatures from the 19th century that were sparse and inconsistent.
But the sponges also showed that current warming began in 1860. The UN’s Intergovernmental Panel on Climate Change considers the “pre-industrial” period to be between 1850 and 1900 and global warming rates are compared to that 50-year period.
Prof Amos Winter, a co-author of the research at Indiana State University, said: “The takeaway message is that we are much warmer than we thought we were compared to pre-industrial times. Hopefully it will help change our view of what is happening to the world and make us act now.”
Prof Helen McGregor, an expert on using proxy records to reconstruct the climate of the recent past, said the study was important if only because it recorded warming at a different depth of the ocean than other proxy records , such as corals.
‘Extremely skeptical’
The findings and claims by their authors that the world has breached an important warming milestone have been challenged by several leading climate scientists.
Prof Michael Mann, a climate scientist from the University of Pennsylvania, said: “I am extremely skeptical of the idea that we can dominate the instrumental global surface temperature record based on paleospongs from one region. For me it doesn’t even pass the smell test.”
Mann said while there was some evidence that as much as 0.2C of warming occurred in the late 19th century“we found that the acceptance of a late 19th century baseline is really implicit when we talk about those thresholds [of 1.5C and 2C] anyway, so it’s kind of a moot point”.
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Dr Gavin Schmidt, director of the NASA Goddard Institute for Space Studies, said the sponges were a useful addition to so-called paleoclimate records that could extend the record of temperatures back before modern instruments.
“People should be careful to assume that proxies from one part of the Atlantic always reflect the global average,” he said.
“Estimates of global average temperatures prior to 1850 require multiple proxies of as wide a regional variation as possible, thus claiming that records from a single record can confidently define global average warming as the pre-industrial probably overreaches.”
Dr Andrew King, a climate scientist at the University of Melbourne, said the IPCC’s choice of 1850 to 1900 as a “pre-industrial” was a “pragmatic choice given the lack of instrumental data before 1850”.
King had “concerns about the claim” that the planet has already warmed by 1.7C since the industrial revolution began.
He said when computer simulations of the climate between 1850 and 1900 were compared with and without added CO2 from human activities, there was little difference.
“It is only when atmospheric carbon dioxide concentrations begin to increase significantly in the 20th century that a divide becomes apparent. This suggests that the period 1850-1900 is a fairly good, though not perfect, proxy for a pre-industrial climate.
Prof Yadvinder Malhi, at the University of Oxford, said the way the findings were communicated in the journal was flawed and could confuse the public about the status of efforts to keep global warming to 1.5C.
“Despite the [journal] head, the results of this paper do not show that we have already exceeded the Paris climate goals.”
Dr Friederike Otto, a climate scientist at Imperial College London’s Grantham Institute, said the study “doesn’t tell us anything about whether we have exceeded the 1.5C temperature limit set in the Paris Agreement”.
“That limit is set as the threshold of unacceptable dangerous warming and describes temperature rise relative to the late 19th century. So if this study indeed identified warming from before the mid-1800s, it does not mean that the planet is any closer to breaking the 1.5C limit as it is commonly understood.
“Climate change is killing people now; the slower emissions are reduced, the worse the consequences will be. Indeed, the world will warm by 1.7C in the coming years, the level identified by the paper, if the use of fossil fuels is not stopped quickly.”
