Ocean sponges suggest Earth’s been warming for longer

Some scientists are dubious of study’s assertion

Centuries-old sponges from deep in the Caribbean are prompting some scientists to think human-caused climate change began sooner and has heated the world more than they thought.

They calculate that the world has already gone past the internationally approved target of limiting global warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) since pre-industrial times, hitting 1.7 degrees (3.1 degrees Fahrenheit) as of 2020. They analyzed six of the longlived sponges — simple animals that filter water — for growth records that document changes in water temperature, acidity, and carbon dioxide levels in the air, according to a study in Monday’s journal Nature Climate Change.

Other scientists were skeptical of the study's claim that the world has warmed that much more than thought. But if the sponge calculations are right, there are big repercussions, the study authors said.

“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,” said study lead author Malcolm McCulloch, a marine geochemist at the University of Western Australia. “Basically, time’s running out.”

“It’s really a diary of — what’s the word? — impending disaster.”

In the past several years, scientists have noted more extreme and harmful weather — floods, storms, droughts, and heat waves — than they had expected for the current level of warming. One explanation for that would be if there was more warming than scientists had initially calculated, said study co-author Amos Winter, a paleo oceanographer at Indiana State University. He said this study also supports the theory that climate change is accelerating, proposed last year by former NASA top scientist James Hansen.

“This is not good news for global climate change as it implies more warming,” said Cornell University climate scientist Natalie Mahowald, who was not part of the study.

Many sponge species live long, and as they grow, they record the conditions of the environment around them in their skeletons. Scientists have long used sponges along with other proxies — tree rings, ice cores, and coral — that naturally show the record of changes in the environment over centuries. Doing so helps fill in data from before the 20th century.

Sponges — unlike coral, tree rings, and ice cores — get water flowing from all over through them so they can record a larger area of ecological change, Winter and McCulloch said.

They used measurements from a rare species of small and hard-shelled sponges to create a temperature record for the 1800s that differs greatly from the scientifically accepted versions used by the United Nations’ Intergovernmental Panel on Climate Change. The study finds that the mid-1800s were about half a degree Celsius cooler than previously thought, with warming from heat-trapping gases kicking in about 80 years earlier than the measurements the IPCC uses. IPCC figures show warming kicking in just after 1900.

It makes sense that the warming started earlier than the IPCC says because by the mid1800s the Industrial Revolution had begun, and carbon dioxide was being spewed into the air, said McCulloch and Winter. Carbon dioxide and other gases from the burning of fossil fuels are what causes climate change, scientists have established.

Winter and McCulloch said these rusty orange long-lived sponges — one of them was more than 320 years old when it was collected — are special in a way that makes them an ideal measuring tool, better than what scientists used in the midto late 1800s.

“They are cathedrals of history, of human history, recording carbon dioxide in the atmosphere, temperature of the water, and pH of the water,” Winter said.