Scientists analysed 300 years of ocean temperature records from marine sponges and discovered global warming has increased by 0.5 degrees more than was previously estimated.

In the study, published in Nature Climate Change, scientists extracted ocean temperature records preserved in the calcium carbonate skeletons of long-lived sclerosponges.

Researchers from The University of Western Australia, Indiana State University and University of Puerto Rico discovered industrial-era warming began in the mid-1860s, consistent with that expected from historical records but more than 70 years earlier than suggested by records from ship-based measurements of sea surface temperatures.

Lead author Emeritus Professor Malcolm McCulloch, from UWA’s Oceans Graduate School and Oceans Institute, said the finding showed that global warming, the combined average of surface ocean and land warming, had been underestimated by about 0.5 degrees, mostly during the initial 19th century stage of the industrial era when coverage by shipping was still limited.

“So rather than the Intergovernmental Panel on Climate Change estimate of average global temperatures having increased by 1.2 degrees by 2020, temperatures were in fact already 1.7 degrees above pre-industrial levels,” Professor McCulloch said.

Last year was an extreme year in terms of weather events with record-breaking global warming almost reaching the 2 degrees limit – or, before these new findings, what was previously the 1.5 degrees limit.

The findings raise the question of whether the global mean surface temperature has or will soon exceed the Paris Agreement, aimed at keeping warming to below 2 degrees.

“If current rates of emissions continue, average global temperature will certainly pass 2 degrees by the late 2020s and be more than 2.5 degrees above pre-industrial levels by 2050,” Professor McCulloch said.

“The now much faster rates of land-based warming also identified in the study are of additional concern, with average land temperatures expected to be about 4 degrees above pre-industrial levels by 2050.

“Keeping global warming to no more than 2 degrees is now the major challenge, making it even more urgent to halve emissions by early 2030, and certainly no later than 2040.”