Can sponges tell whether the planet has warmed more than we thought?

A handful of centuries-old sponges from deep in the Caribbean are causing 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 since pre-industrial times, hitting 1.7 degrees as of 2020. They analyzed six of the long-lived 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.

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.

What did the sponge reveal?

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 the study lead author Malcolm McCulloch said. McCulloch is a marine geochemist at the University of Western Australia.

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 (IPCC). 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.

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 mid- to late 1800s.

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

Other skeptical voices

University of Pennsylvania climate scientist Michael Mann, who wasn't part of the study, has long disagreed with the IPCC's baseline and thinks warming started earlier. But he was still skeptical of the study's findings.

"In my view, it begs credulity to claim that the instrumental record is wrong based on paleo-sponges from one region of the world. It honestly doesn't make any sense to me," Mann said.

In a news briefing, Winter and McCulloch repeatedly defended the use of sponges as an accurate proxy for world temperature changes. They said that except for the 1800s, their temperature reconstruction based on sponges matches global records from instruments and other proxies like coral, ice cores and tree rings.

And even though these sponges are only in the Caribbean, McCulloch and Winter said they are a good representation for the rest of the world because they're at a depth that doesn't get too affected by the warm and cold cycles of El Nino and La Nina, and the water matches well with global ocean temperatures.

Princeton University climate scientist Michael Oppenheimer, who also wasn't part of the sponge study, said even if the McCulloch team is right about a cooler baseline in the 1800s, that shouldn't really change the danger levels that scientists set in their reports. That's because the danger levels "were not tied to the absolute value of preindustrial temperatures" but more about how much temperatures changed from that time, he said.

Although the study stopped at 2020 with 1.7 degrees Celsius in warming since pre-industrial times, a record hot 2023 pushes that up to 1.8 degrees, McCulloch said.

"The rate of change is much faster than we thought," McCulloch said. "We're heading into very dangerous, high-risk scenarios for the future. And the only way to stop this is to reduce emissions. Urgently. Most urgently."

(With input from AP)