A tiny sea sponge that lives for hundreds of years could prove global warming has raised temperatures for longer than previously projected.
Scientists have found that sclerosponges found in Caribbean waters grow for up to 300 years and possess an internal geochemical thermometer.
They store different chemicals that are dependent on water temperature, and by measuring the ratio of each chemical, scientists can gauge a temperature change.
"(The sclerosponge) serves as a natural archive of ocean temperatures," said study author Malcolm McCulloch from the University of Western Australia.
Scientists found the sponge records suggest global warming, thanks to human activity, began in the mid-1860s - several decades earlier than previously thought.
Before then in the pre-industrial period the research suggests there were stable temperatures between 1700 to 1790 and 1840 to 1860, with the gap representing cooling from volcanic activity.
"The clock of climate change has been brought forward by about a decade by our findings," Professor McCulloch said.
The Intergovernmental Panel on Climate Change estimates land and sea temperatures sat at 0.4C of warming between 1850 and 1900.
But the sea sponge research indicates it was 0.5C in the same period.
And earth may be soon be heading for 2C of warming, according to the study, exceeding the 1.5C "guardrail" set by the 2015 Paris Agreement.
Under the agreement, countries need to reduce emissions to prevent the global temperature from rising above 1.5C to avoid severe climate change ramifications.
If the research is correct, the world is already above the 1.5C safety margin.
Prof McCulloch's estimates suggest global temperatures warmed 1.7C between 2018 and 2022.
If it continues on this trajectory, the world will warm by 2.2C by the end of the decade.
"Things we were thinking would happen 10 years hence are going to be happening now," he said.
Prof McCulloch said every additional increment of warming would make climate change impacts worse.
Other climate scientists said the research reaffirmed the need for climate action and reducing carbon emissions globally.
"This implied different baseline temperature does not mean that we have to recast the 1.5C and 2C temperature goals," said Professor Mark Howden of the ANU Institute for Climate, Energy and Disaster Solutions.
"But it does emphasise the duration and magnitude of human impact on global systems."
However, other scientists argued more data was needed to support the sea sponge estimates.
"The claim that we might have overshot 1.5C is being made rather confidently, when in fact there are several uncertainties and limitations in this study which must be acknowledged," said Daniela Schmidt, a professor of earth sciences at the University of Bristol in the UK.
"There is a real uncertainty in what the mid-19th century temperatures were compared to the modern period, around 0.2C even in the instrumental record, and so that complicates our ability to make definitive statements about the crossing of the 1.5C level."
