Sponge study extends timeline of human-caused climate change

Human activity, especially the burning of fossil fuels, drives a substantial amount of climate change. Yet scientists haven’t pinpointed precisely when and how this human influence began, partially because temperature data from past centuries are scarce. By a common estimate, the planet has warmed about 1.2 °C since 1850–1900, a baseline period for preindustrial temperatures.

Recently, researchers analyzed long-lived sclerosponges, or Ceratoporella nicholsoni, and concluded that human-driven global warming began in the 1860s, decades earlier than scientists had previously estimated (Nat. Clim. Change 2024, DOI: 10.1038/s41558-023-01919-7). The team members, led by marine geoscientist Malcolm McCulloch at the University of Western Australia, estimate that 1.7 °C of warming has occurred since that time, nudging past international goals to keep warming below 1.5 °C.

“This study proves that there is really no debate about climate change and that the warming may have been even more extreme than the previous records we had access to have shown,” says Anastasia Yanchilina, an isotope geochemist at the California Institute of Technology who was not involved in the research.

Like tree rings, the calcium carbonate skeletons of filter-feeding sclerosponges, can indicate environmental conditions over time. Using mass spectrometry, the scientists analyzed sclerosponge samples gathered by scuba divers off the coast of Puerto Rico. First, they used uranium-series dating, which measures how much of this element has decayed into thorium, to determine each sample’s age. In this case, samples were up to 300 years old. Next, they dissolved samples in nitric acid and used mass spectrometry to analyze their strontium-to-calcium ratios, which precisely reflect ocean temperatures.

Alan Mix, a climate scientist and professor emerit at Oregon State University, as well as a former lead author for the Intergovernmental Panel on Climate Change (IPCC), praised the labor-intensive study for its precision, rigorous methods, and dataset. But he doesn’t feel it challenges the IPCC estimates that cap current warming at 1.2 °C. And he doesn’t think it jeopardizes existing international goals to limit warming. Rather, “it’s just a change in the reference period,” he says. Scientists had already known that there was a gap in past data, he says, because industrial activity began before there were reliable temperature measurements.

Mix says that adding data to fill these gaps is important, as a myriad of variables influence climate change. “People are going to want to follow this up and see how this one spot in the Caribbean applies to the world.”

“We would love to extend the area of our sclerosponge collection,” says Amos Winter, a study coauthor and Indiana State University paleoceanographer, adding that researchers are considering the Gulf of Mexico and the tropical Atlantic coast of Brazil, which are also home to sclerosponges.

Because deep ocean currents can delay the warming effects of human activities such as burning fossil fuels, future climate change is inevitable. Yet McCulloch sees the study as a call for action. “It’s clear that every increment of temperature makes things worse,” he said in a press briefing. “We’ve got to limit, as far as we possibly can, the amount of warming that’s going to happen on our planet.”