Ocean Sponge Skeletons Suggest a More Significant History of Global Warming Than Originally Thought

Analysis of the sea creatures’ skeletal chemistry suggests the world’s temperatures have increased by 1.7 degrees Celsius since pre-industrial times

A close-up of orange elephant ear sponge, resembling orange cauliflower attached to a rock, taken underwater.
Orange elephant ear sponge, in the Gulf of Mexico. G. P. Schmahl, NOAA

A controversial new study from the depths of the Caribbean Sea suggests that human activity has caused the world to warm more than originally thought.

An analysis of six sea sponges—centuries-old creatures with an internal chemistry that holds secrets about climate history—points to global temperatures already having increased by 1.7 degrees Celsius due to human activity, as compared to the 1.07 degree Celsius increase scientists currently agree upon.

If the new findings are corroborated, then humanity has already failed to limit global temperature rise since pre-industrial times to 1.5 degrees Celsius, an internationally accepted threshold that, if surpassed, would have particularly detrimental and largely irreversible effects on ecosystems, humans and wildlife worldwide.

The updated margin, the authors say in their study published Monday in Nature Climate Change, means humans have even less time to curb carbon emissions and warming.

“The big picture is that the global warming clock for emissions reductions to minimize the risk of dangerous climate changes is being brought forward by at least a decade,” Malcolm McCulloch, a marine geochemist at the University of Western Australia and lead author of the study, tells Seth Borenstein of the Associated Press (AP). “Basically, time’s running out… It’s really a diary of—what’s the word?—impending disaster.”

An underwater assortment of tubular sea sponges -- some yellow, purple, orange, and white.
Various species of sea sponges, approximately 60 feet underwater in the Caribbean Sea. NOAA via Wikimedia Commons

Temperature readings from the 19th century, due to imprecise equipment, have never been wholly reliable, leaving uncertainties in the complete story of global temperature rise. To fill in the gaps, trees, corals and ice cores have previously been analyzed to assess historic weather conditions, though the information they offered was also limited in scope.

Oceanic sponges, on the other hand, might offer some of the best data yet. With lifespans of up to 200 years or more, the creatures’ skeleton chemistry corresponds to the condition of their environment over time. Strontium builds up on their bones in warmer waters, and calcium builds up in cooler waters. The elemental ratio, the scientists say, is a peek into the past. With water continuously in motion through them, the sponges record a larger area of change.

“They are cathedrals of history, of human history, recording carbon dioxide in the atmosphere, temperature of the water and pH of the water,” Amos Winter, a paleo oceanographer at Indiana State University and a co-author of the study, tells the AP. “They’re beautiful. They’re not easy to find. You need a special team of divers to find them.”

McCulloch, Amos and other scientists worked with a team of deep-sea divers to closely examine six hard-shell sea sponges living between 100 and 300 feet below the Caribbean Sea, near the coasts of Puerto Rico and the U.S. Virgin Islands—areas where climate changes mimic global trends, the team says. Analysis of the specimens suggested that after a cooling period spurred by volcanic eruptions, ocean temperatures began to warm more than originally thought—about 0.5 degrees Celsius greater—in the mid-1800s.

A yellow glass sea sponge stands on a rock 8,133 feet underwater on the Sibelius Seamount in the Pacific Ocean
A yellow glass sea sponge, 8,133 feet underwater in the Pacific Ocean. NOAA Office of Ocean Exploration and Research

But not everyone is convinced by the team’s research. Several scientists have expressed skepticism over the study’s sample size, location and the framing of its results.

“I’m extremely skeptical of the idea we can overrule the instrumental global surface temperature record based on paleo-sponges from one region,” Michael Mann, a climate scientist from the University of Pennsylvania who was not involved in the study, tells the Guardian’s Graham Readfearn. “For me, it doesn’t even pass the smell test.”

“I would want to include more records before claiming a global temperature reconstruction,” Hali Kilbourne, a geological oceanographer at the University of Maryland, tells the New York Times’ Raymond Zhong.

The most widespread criticism of the study is that it incorrectly suggests the 1.5 degree Celsius threshold, specifically established in the Paris Climate Agreement, has already been surpassed. That the study proposes revised world temperatures from the mid-1800s, some outside scientists say, has no bearing on the limit set in 2015, which describes global temperature rise since the late 1800s.  

Still, the study’s warning—that global temperature rise is worsening—is corroborated by those who disagree with the framing of its results.

“Climate change is killing people now,” Friederike Otto, a climate scientist at Imperial College London’s Grantham Institute, tells the Guardian. “The slower emissions are cut, the worse the consequences will be. The world will indeed warm by 1.7 Celsius in the coming years, the level identified by the paper, if fossil fuel use is not rapidly halted.”

Get the latest stories in your inbox every weekday.