The Earth may have passed 1.5 degrees Celsius of warming and in fact could soon reach 2 degrees, say international researchers who studied 300-year-old sea sponges in the Caribbean. The study is considered controversial. 

The researchers studied the sea sponges off Puerto Rico. Samples of centuries-old sclerosponges in the eastern Caribbean showed that human-caused warming started in the mid-1860s, about eight decades before the period indicated by sea surface records.

The sea temperatures off Puerto Rico vary less than in other parts of the Caribbean.

A rise in temperature of 2 degrees is possible by 2030, said the paper, in Nature Climate Change.

Hotter land temperatures, together with the earlier onset of industrial-era warming, indicate that global warming was already around 1.7 degrees C above pre-industrial levels by 2020, they said. “Our result is 0.5 °C higher than [UN Intergovernmental Panel on Climate Change] estimates, with 2 °C global warming projected by the late 2020s, nearly two decades earlier than expected,” they added. 

Sclerosponges have calcium carbonate exo-skeletons and are long-lived. The chemical composition of their skeleton can indicate temperature changes because of minerals they absorb at varying rates from the marine environment.

"The industrial era of warming commenced earlier than we thought – in the mid 1860s," Malcolm McCulloch, a professor of isotope geo-biochemistry at the University of Western Australia and lead author of the study, told reporters.

"Since then, the increasing global mean surface temperatures, which means global warming, has been half a degree greater than the current accepted estimates," he said.

“It’s a bit of a wake-up call,” he said. With this new research, “We may have brought things forward by about a decade.”

Satellites and sensors in recent decades have typically measured temperature increases, but the global warming in the 1800s is harder to assess.

Other researchers note that the study was too small – only six sponges of the rare and hard-to-find sponges were studied, and only from one location in the world’s oceans and they said that you cannot extrapolate from one location. The sponges are only found at great ocean depths.

“I would want to include more records before claiming a global temperature reconstruction,” Hali Kilbourne, a geological oceanographer at the University of Maryland Centre for Environmental Science, told the New York Times.