The goal has long been clear: limit global warming to 1.5C or risk climate catastrophe. But an Australian-led team of scientists say that
Key to their findings is a type of sea sponge found in the pristine waters of the Caribbean Sea. The species, known as sclerosponges, records chemical changes in its calcium carbonate skeleton, serving as a natural archive of ocean temperatures.
"It's a perfect thermometer," lead researcher Professor Malcolm McCulloch, from the University of Western Australia, told SBS News.
Previous estimates of warming are based on sea-surface temperature records that date back to the mid-1800s. But the sea sponges, which have a lifespan of roughly 300-400 years, can be used to estimate much earlier temperatures.
"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," McCulloch explained.
"They’re like a thermometer sitting in place, visibly growing the skeleton and recording the temperatures."
McCulloch says the research has been almost a decade in the making.
He and his colleagues used samples from sponges, collected off the coast of Puerto Rico, to explore changes in ocean temperatures over the past 300 years.
Sclerosponges record chemical changes in its calcium carbonate skeleton, serving as a natural archive of ocean temperatures. Source: Supplied / Clark Sherman and Amos Winter
Their estimates, based on the average surface temperature of both the ocean and land, indicate the earth may be 1.7C, possibly even 1.8C warmer since pre-industrial times.
This is about half a degree higher than the warming , which suggests global temperatures have risen by a 10-year average of 1.2C since pre-industrial times.
"The [1.5C] temperature, we think, was passed in 2010 to around 2012 … and right now, we think it's 1.7C to 1.8C," McCulloch says.
"So we're at least 0.2 degrees above that 1.5C limit — 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. The landmark international treaty pledged to keep temperatures "well below 2 degrees Celsius above pre-industrial levels," and pursue efforts to "limit the temperature increase to 1.5 degrees Celsius above pre-industrial levels".
Professor McCulloch (right) said the research has been almost a decade in the making. Source: Supplied / Clark Sherman and Amos Winter
But the data taken during that period is limited, McCulloch says.
"How do we measure temperatures back in 1850 on a ship? Well, you had to have ships going across all the oceans that try to get the average — the sailor throwing a bucket over the side trying to pull up some water and they put a thermometer in. And of course, they're very sparse records and the coverage is very limited."
Dr Georgy Falster, a postdoctoral fellow with the ARC Centre of Excellence for Climate Science at ANU, says that 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.
"Temperature 'proxy' records like this, from natural archives, [the sponges] are extremely valuable because direct temperature measurements didn’t become widespread until well into the 1900s, when global warming had already started."
She told SBS the findings suggest that human-induced global warming started as early as the 1860s.
"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."
Professor Malte Meinshausen, a climate scientist at the University of Melbourne, said while the research is "valuable", the IPCC's findings "still stand strong."
"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.
"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."
Professor Mark Howden, the director of the ANU Institute for Climate, Energy & Disaster Solutions, says the findings are similar to research carried out by his colleague, Professor Nerilie Abram.
"It's not absolutely new in that sense. But it does provide a new source of information, which confirms that very early impact," he said.
"It suggests that the total magnitude of warming since pre-industrial times — about 1.7C — is much greater than the estimates built into the UNFCCC Paris Agreement, which indicate about 1.25C warming."
Professor Howden noted that "the implied different baseline temperature does not mean that we have to recast the 1.5C and 2C temperature goals. However, it does emphasise the duration and magnitude of human impact on global systems."
"It shows that human influence is actually quite pervasive. It enhances our understanding of how much we're impacting on the globe, and how much we're impacting on global systems, such as marine ecosystems."
Professor McCulloch says his research stresses the need to start "substantially reducing emissions".
"Because we've got less and less time to do it. Every decade we slow down, then we're getting higher temperatures and higher absolute amounts of CO2 in the atmosphere. And once you have CO2 in the atmosphere, it's almost impossible to remove it."
