Ancient sea sponges suggest the planet had already warmed by 1.7°C by 2020, meaning humanity may have crossed the 1.5°C threshold years earlier than conventional temperature records show.
Scientists studying centuries-old sclerosponges from the Caribbean also say industrial warming began in the mid-1860s, more than 80 years before its clear emergence in instrumental sea-surface temperature records.
The sea creatures have effectively been keeping a ‘climate diary’ beneath the ocean, preserving changes in seawater temperature within their slowly growing skeletons.
Researchers analysed six specimens of Ceratoporella nicholsoni collected between 33 and 91 metres below the surface near Puerto Rico and St Croix. These sponges build hard calcium carbonate skeletons which lock in chemical signals from the surrounding water as they grow.
By measuring the ratio of strontium to calcium through successive layers, the scientists reconstructed ocean temperatures stretching back around 300 years to 1700.
The record showed largely stable temperatures before the 1860s, interrupted by periods of volcanic cooling including the aftermath of the massive Mount Tambora eruption in 1815.
From the mid-1860s, however, temperatures began a sustained rise which the researchers link to industrial greenhouse gas emissions.
That matters because the Intergovernmental Panel on Climate Change uses 1850 to 1900 as its practical pre-industrial reference period, largely because reliable global measurements before then are limited.
The sponge study argues that this period was already warmer than the true pre-industrial climate, meaning the scale of modern warming may have been underestimated by around 0.5°C.
On that calculation, global temperatures had risen by 1.7°C above pre-industrial levels by 2020 rather than the roughly 1.2°C estimated using the conventional baseline.
The researchers also projected that 2°C could be reached by the late 2020s if emissions continued at the same rate.
Lead author Professor Malcolm McCulloch of the University of Western Australia said: “The rate of change is much faster than we thought. We’re heading into very dangerous high-risk scenarios for the future. Basically, time’s running out.”
The scientists say the Caribbean sponges are particularly useful because they live deep enough to avoid much of the short-term noise caused by seasonal changes and weather.
Temperatures in that layer of the Caribbean also closely track wider global sea-surface trends, while the sponge records matched known volcanic events and modern measurements after 1963.
The findings have, however, triggered a fierce scientific debate.
Several climate researchers have warned against using six sponges from one part of the Atlantic to redraw the entire global temperature record.
Others argue the Paris Agreement’s 1.5°C limit is tied to the established 1850 to 1900 baseline, meaning the study does not formally prove the international target has already been breached.
But the research raises a profound possibility: industrial activity may have been heating the planet for far longer and pushed global temperatures further than we thought.
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