New Sponge Study Suggests 1.5°C Climate Threshold May Already Be Past — But Experts Urge Caution

New Sponge-Based Reconstruction Extends Ocean Temperatures Back To 1700

A recent paper from researchers at the University of Western Australia Oceans Institute reconstructs Caribbean sea temperatures from about 1700 by analyzing long-lived sclerosponges. Published in Nature Climate Change, the study uses chemical ratios preserved in slow-growing sponge skeletons to estimate past sea temperatures and concludes that global warming may have reached about 1.7°C above pre-industrial levels by 2020.

How The Record Was Built

Sclerosponges grow only fractions of a millimetre per year and accumulate chemical signatures in their limestone skeletons, making them useful paleoclimate archives similar to tree rings or ice cores. The research team measured strontium-to-calcium (Sr/Ca) ratios in six Caribbean sponges to infer sea-surface temperature changes over three centuries. The authors say the Caribbean cave habitats of these sponges reduce distortion from major currents and provide a long, continuous record that predates instrumental sea-temperature observations (which began around 1850).

Findings And Claims

The paper reports that warming began earlier than indicated by the instrumental record — roughly 80 years before the IPCC's baseline timeline — and that the reconstructed record exceeded about 1.7°C by 2020. Lead author Malcolm McCulloch told the Associated Press that this shifts the clock for necessary emissions reductions, warning that time for avoiding dangerous climate impacts is now shorter.

Malcolm McCulloch: The global warming clock for emissions reductions to minimize the risk of dangerous climate change has been brought forward by at least a decade. Basically, time's running out.

Why Some Scientists Are Skeptical

Several climate experts cautioned against treating this single-region proxy as definitive proof that the global instrumental record is wrong. Critics, quoted via LiveScience and other outlets, say it is problematic to extrapolate a global temperature timeline from paleoproxies sampled in one ocean basin and from a small number of specimens. The Intergovernmental Panel on Climate Change currently estimates a long-term global warming of about 1.2°C above pre-industrial levels, based on broad instrumental and multi-proxy datasets.

Where Independent Records Stand

Independent observational datasets and recent monthly anomalies indicate parts of the climate system have already experienced 1.5°C or higher departures from pre-industrial baselines: for example, recent monthly averages (reported by New Scientist and others) have reached roughly 1.7°C above pre-industrial levels. However, a single hot month or year does not necessarily shift the long-term global average above the 1.5°C threshold.

Implications

Whether or not the sponge-based reconstruction becomes widely accepted, the study adds to evidence that the planet is warming rapidly and that rapid emissions reductions are essential to limit long-term and potentially irreversible harms to ecosystems and societies. Many scientists call for additional regional and global proxy records to confirm or refine the timeline suggested by these Caribbean sponges.

Bottom line: The sponge analysis raises important questions and underscores urgency, but policymakers and scientists will want corroborating evidence from multiple regions before rewriting global warming timelines and formal benchmarks.