The temperature records of deep-water sponges may indicate that climate change is accelerating faster than we thought.

A group of century-old sponges from the depths of the Caribbean Sea could contain clues about the impact of man-made climate change. Some scientists believe that warming started earlier and has progressed more rapidly than previously thought.

Researchers estimate that the world has already exceeded the internationally approved target of limiting global warming to 1.5°C since pre-industrial times, reaching 1.7°C as of 2020. They analyzed six long-lived sponges – simple animals that filter water – for growth records documenting changes in water temperature, acidity, and carbon dioxide levels in the air, according to a study published in the journal Nature Climate Change on Monday.

Other scientists were skeptical of the study’s claim that the world has warmed much more than previously thought, but if the sponge’s calculations are correct, there are significant implications, the study’s authors said.

«It may be ‘running out of time’ to reduce emissions,» said the study’s lead author, Malcolm McCulloch, a marine geochemist at the University of Western Australia. «The bigger picture is that the time clock for global warming emissions reductions to minimize the risk of dangerous climate change is running at least a decade ahead.»

«We have a decade less than we thought,» he continued. «It’s really a diary of… impending disaster.»

In recent years, scientists have observed more extreme and damaging weather conditions – floods, storms, droughts, and heatwaves – than expected for the current level of warming. One explanation for this would be that there has been more warming than scientists had initially calculated, according to study co-author Amos Winter, a paleo-oceanographer at Indiana State University. He says this study also supports the theory that climate change is accelerating, as proposed last year by former NASA scientist James Hansen.

«This is not good news for global climate change, as it implies more warming,» said Natalie Mahowald, a climate scientist at Cornell University who was not involved in the study.

How do sponges provide clues about global warming?

Many sponge species live a long time and, as they grow, they record environmental conditions in their skeletons. Scientists have long used sponges, along with other proxies (tree rings, ice cores, and corals), to naturally record environmental changes over the centuries. Doing so helps to fill in data before the 20th century.

Sponges – unlike corals, tree rings, and ice cores – allow water to flow from all around them, so they can record a larger area of ecological change, explain Winter and McCulloch.

They used measurements of a rare species of small, hard-shelled sponges to create a temperature record for the 19th century that differs greatly from the scientifically accepted versions used by the United Nations Intergovernmental Panel on Climate Change (IPCC). The study reveals that in the mid-19th century, the temperature was approximately half a degree Celsius cooler than previously thought, and that warming caused by heat-trapping gases occurred about 80 years earlier than the IPCC measurements used. IPCC figures show that warming began shortly after 1900.

It makes sense that warming began earlier than the IPCC says because the Industrial Revolution had started by the mid-19th century, and carbon dioxide was being released into the air, say McCulloch and Winter. Carbon dioxide and other gases from burning fossil fuels are what scientists have determined to cause climate change.

Winter and McCulloch say that these long-lived, rust-colored sponges (one of which was over 320 years old when collected) are special in a way that makes them an ideal measurement tool, better than what scientists used in the mid-to-late 19th century.

«They’re cathedrals of history, human history, recording carbon dioxide in the atmosphere, water temperature, and water pH,» says Winter. «They’re beautiful,» he adds. «They’re not easy to find. You need a special team of divers to find them.»

This is because they live between 33 and 98 meters deep in the dark, explains Winter.

Do sponges provide accurate records of climate change?

The IPCC and most scientists use 19th-century temperature data that came from ships whose crews took temperature readings by lowering wooden buckets to collect water. Some of those measurements could be biased depending on how the collection was done; for example, if the water was collected near a hot steam locomotive. But sponges are more accurate because scientists can trace small regular deposits of calcium and strontium in the critters’ skeletons. Warmer water would produce more strontium compared to calcium, and cooler water would produce higher proportions of calcium compared to strontium, explains Winter.

University of Pennsylvania climate scientist Michael Mann, who was not part of the study, has long disagreed with the IPCC’s baseline and believes that warming began earlier. But he is still skeptical of the study’s findings.

«In my view, it would be credulity to affirm that the instrumental record based on paleo-sponges from one region of the world is incorrect. Honestly, it makes no sense to me,» says Mann.

In a press conference, Winter and McCulloch repeatedly defended the use of sponges as an accurate indicator of global temperature changes. They said that, except in the 19th century, their sponge-based temperature reconstruction matches global instrument records and other proxies such as coral, ice cores, and tree rings.

Although these sponges are only found in the Caribbean, McCulloch and Winter said they are a good representation for the rest of the world because they are at a depth that is not too affected by cold and warm cycles. El Nino and La Nina and the water line up well with global ocean temperatures.

How do sponges change the perspective on global warming?

Princeton University climate scientist Michael Oppenheimer, who also did not participate in the sponge study, says that even if McCulloch’s team is right about a cooler baseline in the 19th century, that really shouldn’t change the danger levels that scientists set in their reports. This is because danger levels «were not tied to the absolute value of preindustrial energy.» temperatures «But more about how much the temperatures changed since then.

Although the study stopped in 2020 with a warming of 1.7°C since preindustrial times, according to McCulloch, by 2023 the temperature will increase to 1.8°C.

«The pace of change is much faster than we thought,» says McCulloch. «We are heading towards very dangerous high-risk scenarios for the future. And the only way to stop this is to reduce emissions. Urgently. Most urgent.»