Scientists Use Sea Sponges to Study Global Warming Back to 1700

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. And it points to a startling conclusion: Humans have raised global temperatures by a total of about 1.7 degrees Celsius, or 3.1 Fahrenheit, not 1.2 degrees Celsius, the most commonly used value.

“It’s a bit of a wake-up call,” said Malcolm T. 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 Monday 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 aren’t perfect, but they’re a yardstick that everyone can more or less agree on.

That’s why several researchers who weren’t 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 only tell you so much about the climate worldwide, said Hali Kilbourne, a geological oceanographer at the University of Maryland Center 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 Mayagüez collected off the shores of Puerto Rico and St. Croix, in the U.S. Virgin Islands, from depths of up to 300 feet.

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,” Brad E. Rosenheim, a geological oceanographer at the University of South Florida, said of sclerosponges. All in all, scientists worldwide have probably only ever collected something on the order of 50 members of this species, said Dr. Rosenheim, who didn’t 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 through 1790. The seas then cooled somewhat because of major volcanic eruptions. And then, 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 half a degree Celsius 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 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 don’t fluctuate in temperature as much as those at the surface.

“It’s 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 degrees Celsius and, if possible, 1.5 Celsius, as stipulated under the 2015 Paris Agreement.

But the study’s implications for the Paris goals aren’t straightforward, said Joeri Rogelj, a climate scientist at Imperial College London who wasn’t involved in the research.

The targets represent guardrails 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 preindustrial times, Dr. Rogelj said. Revised temperature estimates for the 19th century therefore wouldn’t necessarily change our understanding of whether these guardrails 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 Gabi Hegerl, a climate scientist at the University of Edinburgh who also wasn’t involved in the study. “Some of the impacts of climate change that we’re seeing today are quite surprising,” Dr. Hegerl said.