Evidence suggests that global temperatures have already risen by 1.5°C since the pre-industrial era.

A groundbreaking study led by an Australian research team challenges previous estimations and underscores the urgent need for climate action. 

Unlike traditional methods that rely on sea-surface temperature records dating back to the mid-1800s, this study employs a novel approach by analysing chemical changes in the skeletons of long-lived sea sponges.

This particular type of sea sponge grows a hard carbonate skeleton. The ratio in which the sea sponges store strontium versus calcium in their skeletons is directly related to the temperature of the water they grow in. 

These marine organisms, found in the Eastern Caribbean, have recorded centuries of temperature data, offering a new lens through which to view historical climate change. 

According to the researchers, the analysis of sponge samples from the past 300 years indicates that the Paris Agreement 1.5°C threshold, relative to pre-industrial levels, has already been met.

The paper - accessible here - has sparked a variety of expert opinions.

Professor Malte Meinshausen - a Professor in Climate Science at The University of Melbourne - says “at first look, this single new study seems to say that the IPCC radically underestimated warming”. 

“However, it is studies exactly like this that highlight the merit of the IPCC, in which hundreds of scientists comb through thousands of scientific studies to distil robust findings,” he said.  

“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. The IPCC's findings still stand strong.

 “We are close to 1.5°C warming and there is no reason for complacency on the path towards net zero emissions.”

Dr. Georgy Falster, a postdoctoral fellow at the Australian National University, says the study is useful in identifying the early start of human-caused global warming.

“For scientists to be able to calculate how much global temperature has risen above pre-industrial levels, we have to know exactly what the pre-industrial global temperature was,” he says. 

“Temperature 'proxy' records like this, from natural archives, are extremely valuable because direct temperature measurements didn’t become widespread until well into the 1900s, when global warming had already started. 

“This new record of global temperature from sea sponges starts in 1700 - long before the industrial era when humans started releasing large amounts of CO2 into the atmosphere.

“This means that the sea sponges are a 'natural thermometer' that provide an accurate pre-industrial temperature baseline.”

Professor Mark Howden - Director of the ANU Institute for Climate, Energy & Disaster Solutions (ICEDS) - says the paper suggests “that the total magnitude of warming since pre-industrial times (about 1.7°C) is much greater than the estimates built into the UNFCCC Paris Agreement (which indicate about 1.25°C warming)”. 

“This implied different baseline temperature does not mean that we have to recast the 1.5°C and 2°C temperature goals but it does emphasise the duration and magnitude of human impact on global systems. 

“It will be important to draw from other, similar, data sources beyond this one region to establish the global nature of these relationships."