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Ceratoporella sea sponge. (Photo: National Museum of Natural History/Smithsonian Institution)
Ceratoporella sea sponge. (Photo: National Museum of Natural History/Smithsonian Institution)

ScienceFebruary 11, 2024

Has the world already blown past 1.5°C?

Ceratoporella sea sponge. (Photo: National Museum of Natural History/Smithsonian Institution)
Ceratoporella sea sponge. (Photo: National Museum of Natural History/Smithsonian Institution)

A new study using sea sponges as ‘natural thermometers’ suggests yes – but other scientists challenge the claim.

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Ancient sea sponges from the Caribbean Sea have sparked a climate science controversy this week, centred around the question: have we already passed 1.5°C of warming?

Here’s what the research entailed: six specimens of slow-growing sponges were collected from the ocean floor near Puerto Rico, between 30 and 90 metres deep. The sponges’ hard skeletons are a kind of “natural thermometer”, inscribed with a record of changing ocean temperatures dating back 300 years. These sponge thermometers showed that warming began around 1860, during the Industrial Revolution, and that, to date, we’ve accrued 1.7°C of warming.

That’s 0.5°C more than the IPCC’s estimate of 1.2°C, and exceeds the 1.5°C target enshrined in the Paris Agreement. “The big picture is that the global warming clock for emission reductions to minimise the risk of dangerous climate change has been brought forward by at least a decade,” said Malcolm McCulloch, lead author of the study. So, does this mean we’re cooked? Not so fast.

Scientists have criticised the framing of the study, pointing out that data from a single location shouldn’t be generalised to the whole planet. “A single new paleo record off the coast of Puerto Rico is a valuable addition to the large evidence of warming,” said Malte Meinshausen, professor in climate science at the University of Melbourne. “But it is just that, one study among hundreds. The IPCC’s findings still stand strong.”

Then there’s the problem of shifting baselines. Usually, global heating is measured relative to the average temperature between 1850 and 1900, yielding the IPCC’s estimate of 1.2°C. This study hones in on a different historical baseline: the year 1860, when the temperatures began to tick upward according to the sponges. “The absolute degree of warming will always depend on the baseline and different groups have different definitions,” said Daniela Schmidt, Professor of Earth Sciences from the University of Bristol. “The absolute number should not be the focus of the discussion, though.” Carbon Brief has a thorough analysis of criticisms and limitations of the study.

Ultimately, whether our speedometer says 1.2 or 1.5 or 1.7, we’re still racing towards dangerous levels of heating. The overarching message is the same: slam the brake on emissions, because every fraction of a degree avoided makes a difference.

Keep going!