TRANSCRIPT
The goal has long been clear: limit global warming to 1.5 degrees or risk climate catastrophe.
But that threshold may have already been breached, according to new research by an Australian-led team of scientists.
Key to their findings is a type of sea sponge found in the Caribbean Sea.
The species, which have a lifespan of about 300 to 400 years, changes the chemical composition of its skeleton as the temperature changes.
Lead Researcher, Professor Malcolm McCulloch says they act as a perfect ocean thermometer.
"Because they live for so long — they've grown in layers, continually through time — we actually have a continual record of changes back through time. And in this case, we've gone back to the 1700s."
Professor McCulloch, a researcher in coral reefs at the University of Western Australia, says the research has been almost a decade in the making.
He and his team used samples from the sponges, collected off the coast of Puerto Rico, to explore changes in ocean temperatures over the past 300 years.
They say their findings suggest global temperatures may have already risen by 1.5C since the pre-industrial period, and the goal to keep warming below 2C could be exceeded by the end of the decade.
"That temperature, we think, was passed in 2010...around 2010 to 2012, on our record. Right now, we think it's 1.7C - 1.8. So we're at least 0.2 degrees above the 1.5 kind of limit if you like, or the target that everyone knows about."
Aiming to limit temperature rises to 1.5C or below was a key goal of the 2015 Paris Climate Agreement.
That agreement uses the Intergovernmental Panel on Climate Change, or IPCC's pre-industrial warming baseline temperature from 1850-1900 , when global temperatures were first officially recorded.
But the researchers say the sponges provide records as far back as 1700, and using this earlier pre-industrial baseline, their estimates suggest the earth has warmed by around 1.7 degrees, about 0.5 degrees more than the IPCC figure of 1.2 degrees.
But some climate scientists - not involved in the study - say the new findings do not discredit the current Paris Agreement goals.
Professor Mark Howden is the Director of the Institute for Climate, Energy & Disaster Solutions the Australian National University.
He says the different baseline temperature does not mean that we have to recast the 1.5 and 2 degree Paris goals.
"When we're making big policy decisions, we have to have a very good evidence base. So we want to take this study, and then add to it other studies to provide a more concrete base from which we need to change."
The University of Melbourne's Malte Meinshausen a lead author of the latest IPCC report, says:
"A single new paleo record off the coast of Puerto Rico is a valuable addition to the large evidence of warming. But it is just that, one study among hundreds."
Other experts say the research gives a clearer idea of when human-induced global warming actually started
Dr Georgy Falster is Postdoctoral Fellow with the ARC Centre of Excellence for Climate Science at ANU.
"Previously, it was thought that we started seeing this global warming pattern emerge from around the early 1900s. But this suggests that it was several decades before that."
All can agree on one thing - the need to accelerate action towards net zero emissions.
