Since the dawn of the industrial age, our species has warmed the planet much more than today’s most widely accepted estimates imply, according to a team of scientists who have compiled detailed new information about Earth’s past climate from a unusual source: centuries-old sponges. living in the Caribbean Sea.
Satellite and sensor networks have measured with great precision the increase in temperatures in recent decades. But to assess the full arc of global warming, scientists often combine this data with 19th-century thermometer readings that were often irregular and inaccurate.
This is where sponges come in. By examining the chemical composition of their skeletons, which the creatures steadily built over centuries, researchers have pieced together a new story of those first decades of warming. And it points to a surprising conclusion: Humans have raised global temperatures by a total of about 1.7 degrees Celsius, or 3.1 Fahrenheit, and not 1.2 degrees Celsius, the more commonly used value.
“It’s kind 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 ancestors warmed the planet more than previously believed, then the clock on dangerous climate change may have started earlier than we think.
With the new findings, “we may have moved things forward by about a decade,” Dr. McCulloch said.
His and his colleagues’ research, published Monday in the journal Nature Climate Change, adds to other evidence suggesting that societies began warming the planet earlier than 19th-century temperature records indicate.
Scientists and governments still use those older records as a benchmark for measuring total warming, largely for practical reasons: They’re not perfect, but they’re a yardstick that everyone can more or less agree on.
That’s why several researchers not involved in the new study expressed doubt about using Caribbean sponge data to conclude that prevailing estimates of global warming should be discarded.
Measurements from any location can only tell so much about the global climate, said Hali Kilbourne, a geological oceanographer at the University of Maryland Environmental Science Center. “I would like to include more records before attempting a global temperature reconstruction,” Dr. Kilbourne said.
The heroes of the new study are a type of long-lived sponge called sclerosponges. They are small and round, about the size of a grapefruit. They live in deep, poorly lit underwater corners and niches. And they grow extremely slowly in a process that leaves chemical signatures of the temperature of the waters around them over centuries.
The researchers examined samples of six live sclerosponges that a diving team from the University of Puerto Rico at Mayagüez collected off the coasts 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 copies. But these sponges hide so far underwater that scientists need submersibles or highly trained divers to find them. Neither option is cheap.
“They’re just very difficult to get to,” Brad E. Rosenheim, a geological oceanographer at the University of South Florida, said of sclerosponges. In total, scientists around the world have probably only collected something on the order of 50 members of this species, said Dr. Rosenheim, who did not work on the new study.
The study authors first compared the most recent chemical changes preserved in sponge skeletons with measurements of global sea surface temperatures from the past six decades. The numbers matched up very well. The researchers then worked with the rest of the sponge data to uncover a complete history of ocean warming dating back to the 1700s.
Its history suggests that ocean temperatures remained largely stable until 1790. Then the seas cooled somewhat due to major volcanic eruptions. And then in the mid-1860s, they started to warm up. By the mid-20th century, the amount of warming that had occurred in both sea and land, calculated using 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 global ocean temperatures, said Amos Winter, a professor of earth and environmental systems at Indiana State University who worked on the study.
Previous research has shown that the temperature of Caribbean waters closely tracks the average heat of oceans around the world. And, because sclerosponges live so deep beneath the waves, the waters around them don’t fluctuate in temperature as much as those on the surface.
“It’s probably one of the best areas” to study larger ocean trends, Dr. Winter said. “The changes in Puerto Rico imitate the changes in the world.”
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 in the 2015 Paris Agreement.
But the study’s implications for the Paris goals are not straightforward, said Joeri Rogelj, a climate scientist at Imperial College London who was not involved in the research.
The targets represent barriers based on scientists’ predictions about how much worse the effects of global warming will be compared to conditions between 1986 and 2005, not conditions during pre-industrial times, Dr. Rogelj said. Therefore, revised temperature estimates for the 19th century would not necessarily change our understanding of whether these safety barriers have been breached, he said.
There are still many reasons to worry about how quickly we are experiencing the harmful consequences of warming, said 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 we are seeing today are quite surprising,” Dr. Hegerl said.