Between 30 metres and 90 metres below the surface of the Caribbean Sea, an ancient sponge species that grows a hard skeleton has been quietly recording changes in the ocean temperature for hundreds of years.
Now those sponges are at the centre of a bold and controversial claim made in a leading scientific journal that, since the start of the Industrial Revolution, the planet may have already warmed by 1.7C – half a degree more than estimates used by the United Nation’s climate panel.
Several leading scientists urged caution, saying the research had “over-reached” 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 Change, said the results were robust.
“Taking a precautionary principle, our findings show that global warming is more advanced than we thought and therefore it’s a wake-up call that we have to get on with reducing CO2,” he said. “We will experience more serious impacts from global warming sooner than we had anticipated.”
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 – 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 that relates directly 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 give an estimate of global heating.
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 had also recorded a sudden drop in temperatures caused by a massive volcanic eruption in Indonesia in 1815, giving them confidence the sponges were a good proxy for temperature.
McCulloch said the sponges had helped overcome limitations from ship-based measurements of ocean temperatures of the 19th century that were sparse and inconsistent.
But the sponges also showed that current warming started in 1860. The UN’s Intergovernmental Panel on Climate Change considers the “pre-industrial” period to be between 1850 and 1900 and global heating rates are benchmarked against that 50-year period.
Prof Amos Winter, a co-author of the research at Indiana State University, said: “The take-away message is that we are much warmer than we thought we were compared to pre-industrial. Hopefully it will help change our viewpoint on what’s happening to the globe 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 had recorded warming at a different depth of the ocean than other proxy records, such as corals.
The findings and claims by their authors that the globe had breached a key warming milestone were challenged by several leading climate scientists.
Prof Michael Mann, a climate scientist from the University of Pennsylvania, said: “I’m extremely sceptical of the idea we can overrule the instrumental global surface temperature record based on paleo-sponges 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’ve come to understand that the adoption of a late 19th-century baseline really is implicit when we’re talking about those thresholds [of 1.5C and 2C] anyway, so it’s sort 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 paleo-climate records that could extend the record of temperatures back before modern instruments.
“People should be careful in assuming that proxies from one part of the Atlantic are always reflective of the global mean,” he said.
“Estimates of the global mean temperatures before 1850 require multiple proxies from as wide a regional variation as possible, thus claims that records from a single record can confidently define the global mean warming since the pre-industrial are probably overreaching.”
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 prior to 1850”.
King had “concerns with the claim” that the planet had already warmed by 1.7C since the industrial revolution started.
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’s only when atmospheric carbon dioxide concentrations start to significantly increase in the 20th century when a separation becomes clear. This would suggest that the 1850-1900 period is a pretty good, although not perfect, proxy for a pre-industrial climate.”
Prof Yadvinder Malhi, at 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 keep global heating to 1.5C.
“Despite the [journal] headline, the results of this paper do not show that we have already exceeded the Paris climate targets.”
Dr Friederike Otto, a climate scientist at Imperial College London’s Grantham Institute, said the study “does not tell us anything about whether we have exceeded the 1.5C temperature limit set in the Paris Agreement”.
“That limit was established as the threshold of unacceptably dangerous warming and describes temperature rise relative to the late 19th century. So if this study has indeed identified warming from before the mid-1800s, that doesn’t mean the planet is any closer to breaking the 1.5C limit as it is widely understood.
“Climate change is killing people now; the slower emissions are cut, the worse the consequences will be. The world will indeed warm by 1.7C in the coming years, the level identified by the paper, if fossil fuel use is not rapidly halted.”