A deep sea diver takes a copy of Ceratoporella nicholsoni – a sponge that takes hundreds of years to grow. Professor Malcolm McCulloch says ‘the findings show that global warming is more advanced than we thought’.Photo: Clark Sherman
Between 30 and 90 meters below the surface of the Caribbean Sea, an ancient species of sponge with a hard skeleton has been quietly recording changes in ocean temperatures for hundreds of years.
Now these sponges are at the center of a bold and controversial claim in a leading scientific journal that the planet may already have warmed by 1.7 degrees Celsius since the start of the Industrial Revolution – half a degree more than United Nations estimates . climate panel.
Several leading scientists urged caution, saying the research had “gone too far” and questioning whether such a bold claim could be made based on one sponge species in a single location.
But Prof Malcolm McCulloch from the University of Western Australia, who led the research published in the journal Nature Climate Change, said the results were robust.
“Based on the precautionary principle, our findings show that global warming is more advanced than we thought and therefore it is a wake-up call that we must continue to reduce CO2 emissions,” he said. “We will experience more severe consequences of global warming sooner than we expected.”
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Six copies were recovered with the help of deep-sea divers Ceratoporella nicholsoni – a sponge that can take hundreds of years to grow between 4 and 6 inches – were 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 temperatures on land to provide an estimate of global warming.
The sponges grow deep enough to be unaffected by natural temperature changes and in a part of the ocean, the authors said, where temperature changes closely match the global average.
When McCulloch and colleagues from Indiana State University and the University of Puerto Rico checked their data, they said the sponges matched the changes in global temperatures shown by more modern measurements.
The sponges had also recorded a sudden drop in temperature caused by a massive volcanic eruption in Indonesia in 1815, giving them confidence that the sponges were a good measure of temperature.
McCulloch said the sponges had helped overcome limitations posed by ship measurements of ocean temperatures from the 19th century, which were sparse and inconsistent.
But the sponges also showed that the current warming began in 1860. The UN 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.
Professor Amos Winter, co-author of the study at Indiana State University, said: “The message is that we are much warmer than we thought compared to the pre-industrial era. Hopefully it will help us change our view of what is happening to the world and push us to take action now.”
Professor Helen McGregor, an expert on using proxy records to reconstruct the climate of the recent past, said the study was only important because it had recorded warming at a different depth of the ocean than other proxy records, such as corals.
Their authors’ findings and claims that Earth had passed a major global warming milestone were disputed by several leading climate scientists.
Professor Michael Mann, a climate scientist from the University of Pennsylvania, said: “I am extremely skeptical of the idea that we can discard the instrumental record of global surface temperature based on paleosponges from one region. For me it doesn’t even pass the smell test.”
Mann said that while there was some evidence that as much as 0.2 degrees Celsius of warming occurred in the late 19th century, “we have come to understand that the adoption of a late 19th century baseline is really implicit when we talk about that have thresholds. [of 1.5C and 2C] Anyway, so it’s a bit of a moot point.”
Dr. Gavin Schmidt, director of the NASA Goddard Institute for Space Studies, said the sponges were a useful addition to so-called paleoclimate records, which could extend the record of temperatures before modern instruments.
“People should be careful in assuming that proxies from one part of the Atlantic always reflect the global average,” he said.
“Estimates of global mean temperatures before 1850 require multiple proxies from as wide a regional variation as possible, thus arguing that single-record records can confidently define global mean warming, as the pre-industrial period is likely too far-reaching.”
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Dr. Andrew King, a climate scientist at the University of Melbourne, said the IPCC’s choice of the period 1850 to 1900 as “pre-industrial” was a “pragmatic choice given the lack of instrumental data before 1850”.
King was “concerned by the claim” that the planet had already warmed by 1.7 degrees Celsius since the start of the Industrial Revolution.
He said there was little difference when computer simulations of the climate between 1850 and 1900 were compared with and without added CO2 from human activities.
“Only when atmospheric carbon dioxide concentrations begin to rise significantly in the 20th century will a separation become apparent. This could indicate that the period 1850-1900 is a reasonably good, but not perfect, benchmark for a pre-industrial climate.”
Professor Yadvinder Malhi from the University of Oxford said the way the findings had been communicated in the journal was flawed and could confuse the public about the status of efforts to limit global warming to 1.5 degrees Celsius.
“In spite of the [journal] headline: the results of this article 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 “tells us nothing about whether we have exceeded the 1.5 degrees Celsius temperature limit set in the Paris Agreement.”
“That limit was set as the threshold for unacceptably dangerous warming and describes the temperature increase compared to the end of the 19th century. So if this study has indeed identified warming before the mid-19th century, it doesn’t mean the planet is closer to breaking the 1.5 degrees Celsius limit, as is widely believed.
“Climate change is killing people now; the slower emissions are reduced, the worse the consequences will be. If the use of fossil fuels is not stopped soon, the world will indeed warm by 1.7 degrees Celsius in the coming years, the level indicated in the paper.”