NEW YORK – Since the dawn of the industrial age, our species has warmed the planet by considerably more than today’s most widely accepted estimates imply, according to a team of scientists who have gleaned detailed new information about earth’s past climate from an unusual source: centuries-old sponges living in the Caribbean Sea.

Networks of satellites and sensors have measured the rising temperatures of recent decades with great precision. But to assess the full arc of global warming, scientists typically combine this data with 19th-century thermometer readings that were often spotty and inexact.

This is where the sponges come in. By examining the chemical composition of their skeletons, which the creatures built up steadily over centuries, the researchers have pieced together a new history of those earliest decades of warming. It points to a startling conclusion: Humans have raised global temperatures by a total of about 1.7 deg C, not 1.2 deg C, the most commonly used value.

“It’s a bit of a wake-up call,” said Dr Malcolm McCulloch, a geochemist at the University of Western Australia and one of the scientists who worked on the new research.

Climate researchers look at the total amount by which humanity has warmed the planet to predict when we might expect the effects of a hotter earth – deadlier heat waves, stronger storms, more destructive wildfires – to reach certain levels. If our forebears heated the globe more than previously believed, then the clock on dangerous climate change might effectively have started earlier than we think.

With the new findings, “we may have brought things forward by about a decade”, Dr McCulloch said.

He and his colleagues’ research, published on Feb 5 in the journal Nature Climate Change, adds to other evidence suggesting that societies started warming the planet earlier than 19th-century temperature records indicate.

Scientists and governments still use those older records as the benchmark for measuring total warming, largely for practical reasons: They are not perfect, but they are a yardstick that everyone can more or less agree on.

That is why several researchers who were not involved in the new study expressed hesitation about using the Caribbean sponge data to conclude that prevailing estimates of the planet’s warming should be tossed out.

Measurements from any single location can tell you only so much about the climate worldwide, said Dr Hali Kilbourne, a geological oceanographer at the University of Maryland Centre for Environmental Science.

“I would want to include more records before claiming a global temperature reconstruction,” Dr Kilbourne said.

The heroes of the new study are a long-lived type of sponge called sclerosponges. They are small and round, about the size of a grapefruit. They dwell in deep, dimly lit undersea nooks and niches, and they grow extremely slowly in a process that leaves chemical fingerprints of the temperature of the waters that wash around them through the centuries.

The researchers examined samples from six live sclerosponges that a diving team from the University of Puerto Rico at Mayaguez collected off the shores of Puerto Rico and St Croix, in the United States Virgin Islands, from depths of up to 91m.

Six is not a large number of specimens. But these sponges lurk so far underwater that scientists need submersibles or highly capable divers to find them. Neither option is cheap.

“They’re just very hard to get to,” Dr Brad Rosenheim, a geological oceanographer at the University of South Florida, said of sclerosponges. All in all, scientists worldwide have probably only ever collected about 50 members of this species, said Dr Rosenheim, who did not work on the new study.

The study’s authors first compared the most recent chemical changes preserved in the sponges’ skeletons against measurements of global sea-surface temperatures from the past six decades. The numbers lined up nicely. The researchers then worked through the rest of the sponge data to unspool a complete history of ocean warming going back to 1700.

Their history suggests that ocean temperatures stayed mostly flat till 1790. The seas then cooled somewhat because of major volcanic eruptions. In the mid-1860s, they began to warm. By the middle of the 20th century, the amount of warming that had taken place across both sea and land, when calculated using the sponge records, was about 0.5 deg C greater than scientists currently estimate. That gap has persisted to this day, the researchers’ data shows.

The area these particular specimens called home is uniquely situated to tell us about ocean temperatures globally, said Dr Amos Winter, a professor of earth and environmental systems at Indiana State University, who worked on the study.

Past research has shown that the temperature of the Caribbean’s waters closely tracks the average warmth of the oceans worldwide. And because sclerosponges live so deep beneath the waves, the waters around them do not fluctuate in temperature as much as those at the surface.

The waters there are “probably one of the best areas” to study larger ocean trends, Dr Winter said. “The changes in Puerto Rico mimic the changes in the globe.”

The new findings raise fresh concerns about whether governments will be able to limit global warming to 2 deg C and, if possible, 1.5 deg C, as stipulated under the 2015 Paris Agreement.

But the study’s implications for the Paris goals are not straightforward, said Dr Joeri Rogelj, a climate scientist at Imperial College London, who was not involved in the research.

The targets represent guard rails based on scientists’ predictions about how much worse the effects of global warming will get compared with conditions between 1986 and 2005, not conditions during pre-industrial times, Dr Rogelj said.

Revised temperature estimates for the 19th century, therefore, would not necessarily change our understanding of whether these guard rails have been breached, he said.

There is still ample reason to be concerned about how quickly we are now experiencing the harmful consequences of warming, said Dr Gabi Hegerl, a climate scientist at the University of Edinburgh, who was also not involved in the study.

“Some of the impacts of climate change that we’re seeing today are quite surprising,” Dr Hegerl said. NYTIMES