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Changes in the chemistry of Caribbean sea sponges suggest a breach of the 1.5C rise in the long-term global average temperature will occur sooner than expected, according to new research that has sparked fierce debate.
The potentially critical threshold could be reached by the late 2020s, according to the projections based on 300 years of ocean records and published in Nature Climate Change on Monday.
The Paris Agreement in 2015 set a goal of limiting global warming to 2C, and ideally 1.5C, from pre-industrial levels. This long-term measure is distinct from annual fluctuations that may be greater than 1.5C in any single year.
The paper focuses on the sclerosponge, whose long lifespan means it can be used to monitor historical sea temperatures. Temperature changes leave measurable chemical imprints in the sponge’s calcium carbonate skeleton.
Scientists not involved in the paper questioned whether world temperature trends could be extrapolated from data from a single species and region.
Malcolm McCulloch, a co-author of the paper who specialises in coral reefs at the University of Western Australia, said the projections included margins of error but nevertheless suggested that climate change was happening faster than thought.
“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 McCulloch. “So this is a major change to the thinking about global warming.”
The sponge data showed that warming of its ocean environment linked to human activity began as early as the 1860s, or about 80 years earlier than suggested by instrument measurements at the sea surface, the paper asserts.
This finding, together with rising land temperatures, suggested the global temperature rise had already reached 1.7C above pre-industrial levels by 2020, the paper said.
This would indicate that the “guardrail” of limiting global warming to 1.5C in the 2015 Paris accord had already been passed several years before, McCulloch argued. The sponge analysis indicated the 2C level of warming would be passed later this decade unless there were “major reductions in emissions”, he added.
The Intergovernmental Panel on Climate Change, the UN scientific body, has estimated that global surface temperatures were at least 1.1C higher in 2011-20 than during its reference period of 1850-1900.
It concluded in a 2021 report, signed off by scientists from almost 200 countries, that the 1.5C level could be reached by 2040, depending on emission levels.
Greenhouse gas emissions would need to fall by almost half by 2030 for the rise to be limited to 1.5C, the IPCC said. Instead they have continued to rise.
Scientists said the sponge experiment was reasonable in itself, but many thought it insufficient to justify such wide-ranging conclusions about global warming trends.
The research “illustrates how temperatures in the Caribbean started to rise over the industrial period”, said Gabi Hegerl, professor of climate system science at the University of Edinburgh. But the planet-wide interpretation “overstretches it”.
“[A] single location cannot substitute global data, as climate varies across the globe, which is why the only way to measure global temperature is to get data from across the globe,” Hegerl said.
The papers findings could “add unnecessary confusion to public debate on climate change”, said Yadvinder Malhi, professor of ecosystem science at the University of Oxford.
Joeri Rogelj, research director at the Grantham Institute, said that while the new research “allows us to peek centuries in the past and imagine how warm it was in the Caribbean when Napoleon was conquering Europe”, the suggestion that the research “directly changes how we see the goals of the UN Paris Agreement is quite a stretch”.
“Relabelling the warming that has occurred until today by using a different starting point does not change the impacts we are seeing today, or the impacts we are aiming to avoid.”
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