Six spongy sea creatures suggest warming could be worse than thought
Since the dawn of the industrial age, our species has warmed the planet significantly 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: ancient sponge life 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 inaccurate.
This is where the sponges come into the picture. By examining the chemical composition of their skeletons, which the creatures have steadily built up over centuries, the researchers have pieced together a new history of those first decades of warming. And it points to a surprising conclusion: Humans have raised Earth's temperature 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,” says 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 can expect the effects of a hotter Earth — deadlier heat waves, stronger storms, more destructive wildfires — to reach certain levels. If our ancestors warmed the Earth more than previously thought, the clock of dangerous climate change could actually have started sooner than we think.
With the new findings, “we may have moved things forward by a decade,” said Dr. McCulloch.
He and the research of his colleagues, published on Monday in the journal Nature Climate Change, adds other evidence This suggests that societies began warming the planet earlier than temperature data from the 19th century indicates.
Scientists and governments still use these older data as a benchmark for measuring overall warming, largely for practical reasons: they're not perfect, but they're a benchmark that everyone can more or less agree on.
That's why several researchers not involved in the new study expressed hesitation about using the Caribbean sponge data to conclude that prevailing estimates of planetary warming should be thrown out.
Measurements from a single location can only tell you so much about the global climate, says Hali Kilbourne, a geological oceanographer at the University of Maryland's Center for Environmental Science. “I would like to add more data before I can claim a global temperature reconstruction,” said Dr. Kilbourne.
The heroes of the new study are a long-lived species of sponges called sclerosponges. They are small and round, about the size of a grapefruit. They live in deep, dimly lit undersea nooks and alcoves. And they grow extremely slowly in a process that leaves chemical fingerprints of the temperature of the water that washes around them over the centuries.
The researchers examined samples of six living sclerosponges that a diving team from the University of Puerto Rico at Mayagüez collected off the coast of Puerto Rico and St. Croix, in the U.S. Virgin Islands, from a depth of up to 300 feet.
Six is not a large number of copies. But these sponges lurk so far underwater that scientists need submersibles or very capable divers to find them. Neither option is cheap.
“They're just really hard to get to,” Brad E. Rosenheim, a geological oceanographer at the University of South Florida, said of sclerosponges. All in all, scientists around the world have probably only collected something on the order of 50 members of this species, said Dr. Rosenheim, who did not collaborate on the new research.
The study authors first compared the most recent chemical changes in the sponges' skeletons with measurements of global seawater temperature over the past sixty years. The numbers lined up nicely. The researchers then sifted through the rest of the sponge data to unravel a complete history of ocean warming since 1700.
Their history suggests that ocean temperatures remained largely flat until 1790. The seas then cooled slightly due to large volcanic eruptions. And then, in the mid-1860s, they started to heat up. By the mid-20th century, the amount of warming that had occurred both at sea and on land, calculated from the sponge data, was about half a degree Celsius greater than scientists currently estimate. That gap persists to this day, according to the researchers' data.
The area these particular specimens called home is uniquely situated to tell us about the temperature of the world's oceans, 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 waters of the Caribbean closely matches the average warmth of the world's oceans. 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, said Dr. Winter. “The changes in Puerto Rico are mimicking the changes in the world.”
The new findings raise new concerns about whether governments will be able to limit global warming to 2 degrees Celsius and, if possible, 1.5 degrees Celsius, as set out in the 2015 Paris Agreement.
But the research's implications for the Paris goals are not straightforward, says Joeri Rogelj, a climate scientist at Imperial College London who was not involved in the study.
The targets represent guardrails based on scientists' predictions about how much worse the effects of global warming will become compared to conditions between 1986 and 2005, and not conditions during pre-industrial times, said Dr. Rogelj. Revised temperature estimates for the 19th century would therefore not necessarily change our understanding of whether these guardrails were breached, he said.
There is still plenty of reason to worry about how quickly we are now experiencing the damaging effects of warming, says 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're seeing today are quite surprising,” said Dr. Hegerl.