Data from sponges suggest the world has passed its climate limit, a new study says

CNN
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Using sponges collected off the coast of Puerto Rico in the eastern Caribbean Sea, scientists calculated ocean temperatures over a period of 300 years and concluded that the world has already passed one critical limit to global warming and is rapidly heading toward another.

These results, published Monday in Nature Climate Change Journal, troubling but also controversial. Other scientists say the study contains too many uncertainties and limitations to draw such firm conclusions, and could ultimately confound public understanding of climate change.

The sponges — which grow slowly, layer by layer — can act like time capsules of data, allowing glimpses into what the oceans were like hundreds of years ago, long before modern data existed.

Using samples of hard sponges, which live for centuries, a team of international scientists was able to calculate ocean surface temperatures 300 years ago.

They found that human-caused warming may have started earlier than currently assumed, and as a result, the global average temperature may have already risen by more than 1.5 degrees Celsius above pre-industrial levels. The findings also suggest that global temperature could exceed two degrees of warming by the end of the decade, researchers say.

Under the 2015 Paris Agreement, countries pledged to limit global warming to below two degrees above pre-industrial levels, with an ambition to limit that to 1.5 degrees. The pre-industrial era – or the state of the climate before humans started burning large amounts of fossil fuels and warming the planet – is usually defined as the period from 1850 to 1900.

The study's authors believe that their findings indicate that the pre-industrial era must date back to between the 18th century and the 1760s. Changing that baseline means the world has actually warmed by at least 1.7 degrees (scientists say long-term global warming currently lies between 1.2 and 1.3 degrees).

“The big picture is that global warming, and the urgent need to cut emissions to reduce the risks of dangerous climate change, has been advanced for at least a decade,” said Malcolm McCulloch, lead author of the study and a marine geochemist at the university. Western Australia said in a press conference. “So, this is a big change in thinking about global warming.”

However, many climate scientists have questioned the study's findings, especially the use of a single species of sponge from a single location in the Caribbean to represent global temperatures. Gavin Schmidt, a climate scientist at NASA, said estimating the average global temperature requires data from as many locations as possible, as climate varies across the planet.

“Claims that records from a single record can confidently determine average global warming since the pre-industrial era are likely an exaggeration,” he said in a statement.

Gaby Hegerle, professor of climate system science at the University of Edinburgh, said the study was “a nice new record of how temperatures in the Caribbean started to rise during the industrial period.” But she added in a statement that “the interpretation regarding global warming targets is overstated.”

Some went further. Yadvinder Malhi, professor of ecosystem science at the University of Oxford's Environmental Change Institute, said the way the findings were communicated was “flawed” and had “the potential to add unnecessary confusion to the public debate on climate change”.

One of the study's co-authors defended its power and said that temperature changes in the part of the Caribbean where the sponge came from always mimicked changes around the world.

“This is probably one of the best fields if you're trying to figure out the global average on Earth,” said Amos Winter, a professor of geology at Indiana State University. He said that ocean temperatures in the region are mostly affected by pollution resulting from the heating of the planet, and not by natural climate fluctuations such as the El Niño phenomenon.

Whatever the baseline for measuring global warming, what remains clear, experts say, is that the effects will worsen with every fraction of a degree of warming.

“It is exciting to see new research that allows us to glimpse centuries into the past,” Guiri Rogelj, director of research at the Grantham Institute at Imperial College London, said in a statement. But he added, “Renaming the warming that has occurred to date using a different starting point does not change the impacts we are seeing today, or the impacts we aim to avoid.”

Winter hopes the study will serve as a call to action. “We hope this will help change our perspectives on what is happening in the world, and make us act now, not wait for some disaster to happen to change our habits.”