Extract from ABC News
Stationary sea sponges are sparking debate in the climate science world with new research into the underwater creatures suggesting the planet could be a lot hotter than we thought.
The world's nations have been committed for the past decade to keeping global warming well under an average 2 degrees Celsius increase, compared with temperatures before the Industrial Revolution.
But establishing exactly what temperatures were in the pre-industrial 1700s has been difficult.
Now scientists behind a new paper in Nature Climate Change say they have an answer.
They found that Caribbean sea sponge skeletons have effectively recorded temperatures for about 300 years, giving us an idea of both past and future temperatures.
The findings, based off six samples of the sclerosponge species Ceratoporella nicholsoni from Puerto Rico, suggest the planet may already be 1.7C warmer, based on the average surface temperature of both the ocean and land.
This is half a degree higher than the recent 10-year average of 1.2 C of warming reported by the World Meteorological Organisation since pre-industrial times.
The 1.7C of warming reported in the new research would put Earth beyond the preferred international policy of limiting the temperature increase to 1.5C.
And the researchers behind the sponge work warn the world could be on track to 2C of warming by the end of this decade.
What makes a sponge a thermometer?
About 10 years ago one of the study's authors Professor Malcolm McCulloch, a coral reef geochemistry expert from the University of Western Australia, was looking into ocean acidification.
Thinking sclerosponges from Puerto Rico could be a good collection to investigate, he got in contact with Indiana State University oceanography and palaeoclimatology scientist Professor Amos Winter.
Acidification took a back seat as they noticed something else might be going on with the sponges collected from depths between 33 and 91 metres just off the coast of Puerto Rico.
Every year the sclerosponges lay down about 0.25 millimetres of calcified skeleton.
"They calcify differently from corals in that these sponges tend to pump a lot of seawater through their tissue and extract particulates to feed on," Professor McCulloch said.
"The sclerosponges are unique ... and don't seem to manipulate the calcifying environment too much. So they're pretty much a direct record of the ambient seawater."
Looking into the chemistry of the sponges, the scientists measured the ratio of the element strontium to calcium in each calcified layer.
Using the United Kingdom's Meteorological Office's sea surface temperature database, known as HadSST4, the researchers found a correlation between the strontium-calcium ratio and temperatures from 1961 to the present — the ratio corresponded inversely with temperature records.
By calibrating the sponge data with the sea surface temperature records they could then use older calcified layers on the sponges to establish temperatures back to 1700.
They found pre-industrial temperatures were lower than the figures used by the Intergovernmental Panel on Climate Change, the official global authority on climate change.
The sponge data seems to track with major global climate modifiers like the massive eruption of Indonesia's Tambora volcano in 1815, which led to a volcanic winter as temperatures dropped around the planet because of ash in the atmosphere
The researchers suggested there were stable periods of pre-industrial temperatures from 1700 to 1790, and 1840 to 1860, before the onset of human-induced warming in the 1860s.
Georgina Falster, a palaeoclimatologist at the Australian National University who was not involved in the study, said we should have high confidence in the sponge-based global temperature record.
"Ocean temperature records like this, from natural archives, are extremely valuable because ocean surface temperature measurements from ships didn't become widespread until well into the 1900s – when global warming had already started," Dr Falster said.
"There is a long overlap between the sponge record and the period of high-quality sea surface temperature observations [1964-2022] and the two records match extremely well during this time."
Josephine Brown, a climate scientist at the University of Melbourne, said the study provided a valuable new source of information about long-term temperature trends.
"Whether or not we have passed 1.5C warming relative to a particular baseline, there is an urgent need for drastic reductions in greenhouse gas emissions," Dr Brown said.
"Every additional fraction of a degree of warming will have larger impacts, so we need to aim to get to net zero emissions as rapidly as possible."
Making global assumptions from a single location
Professor McCulloch said the Caribbean could be used to calculate a global average of temperature warming because there were less variables than other parts of the ocean with no big upwellings or major phenomenon like the El Niño Southern Oscillation.
"You wouldn't want to do it [the research] in the Arctic ocean or Antarctic because we know those waters are changing rapidly," he said.
"If you came on a spaceship and said 'Where am I going to test the temperature [of the planet]?' this is where I'd do it."
But not all scientists are convinced you can take the findings from one location and make a global assumption.
Malte Meinshausen, a climate scientist at the University of Melbourne and a lead author of the last IPCC report, said while a new palaeoclimate record from Puerto Rico was valuable it was just one among hundreds of climate studies.
"It's studies exactly like this that highlight the merit of the IPCC, in which hundreds of scientists comb through thousands of scientific studies to distil robust findings," Professor Meinshausen said.
"The IPCC's findings still stand strong.
"We are close to 1.5C warming and there is no reason for complacency on the path towards net zero emissions."
Gavin Smidt, NASA's Goddard Institute for Space Studies director, said people should be careful in assuming a proxy from one part of the Atlantic reflected a global average.
"Estimates of the global mean temperatures before 1850 require multiple proxies from as wide a regional variation as possible," he commented.
"Claims that records from a single record can confidently define the global mean warming since the pre-industrial are probably overreaching.
"There is a real uncertainty in what the mid-19th Century temperatures were compared to the modern period ... so that complicates our ability to make definitive statements about the crossing of the 1.5C level."
Professor McCulloch and Professor Winter hope climate modellers and the IPCC take a look at their findings.
But ultimately they hope greater strides in decarbonisation will be taken.
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