Extract from ABC News
Ice core data from Antarctica has been used to calculate a new pre-industrial baseline for human-induced warming. (Supplied: Dr John Cherry)
In short:
Scientists say data from Antarctica ice core shows world has already warmed 1.49 degrees Celsius due to human activity.
The study calculates a pre-1700 baseline based off carbon content in the ice.
What's next?
Researchers behind the study want politicians and scientists to use their modelling as a baseline rather than a commonly used 1850-1900 time period when the Industrial Revolution had already started.
The world has already warmed 1.49 degrees Celsius because of humans compared to pre-industrial levels, according to a new study of carbon dioxide content in Antarctic ice cores.
Research published today in Nature Geoscience said when compared to a pre-1700 baseline, the planet reached the temperature increase at the end of 2023.
The figure is higher than globally accepted estimates of about 1.3C of warming, as highlighted in the United Nations's latest emissions gap report, which is based on historical temperature observations from 1850 to 1900.
The latter methodology is used by the world authority on global warming, the International Panel on Climate Change (IPCC), for its reports which inform international climate policy.
The IPCC's approach has guided policy to meet the 2015 Paris Agreement targets after nations around the globe agreed to try to keep warming below 2C and to aim for 1.5C.
Ice cores from places such as Antarctica and the Arctic Circle can help provide insights into historical global temperatures. (ABC News: Jane Norman)
But Andrew Jarvis, a climate scientist at Lancaster University and co-author of the new study, said the climate policy and scientific communities should rethink what we consider the pre-industrial baseline.
"We know that there's warming baked into the 1850 to 1900 estimate, simply because that is not the beginning of the Industrial Revolution," he said
"Significant things had already happened by that point and we're offering a way out there to a more scientifically secure baseline to operate from."
Calculating a new baseline
Scientists adopted the 1850 to 1900 time period for the simple reason it had the earliest reliable temperature data from land and sea observations around the world.
Reliable and consistently recorded temperatures pre-1850 that cover the Industrial Revolution, which occurred from about 1760 to 1840, are sparse.
But Dr Jarvis thought he could reach a pre-1700 baseline before the Industrial Revolution began by using Antarctic ice cores.
Dr Jarvis said other scientists had been able to measure the concentration of CO2 in bubbles in layers of ice that corresponded with different time periods.
"They can ... then tie the [CO2] concentration of the bubbles to the air concentration at that point in time," he said.
By using the CO2 ice records from Law Dome in East Antarctica, Dr Jarvis and his co-author, climate physicist Piers Forster from the University of Leeds, could then estimate historical temperatures.
The records from Law Dome, which is about 120 kilometres inland from the Australian research base Casey Station, span centuries — from 1300 to 1700.
"You then knit that data together with the actual concentration measurements that are made since 1959 … they form a near-perfect continuous line between the ice core and the direct CO2 observations," Dr Jarvis said.
"We have provided a new, scientifically defensible way of coming up with the pre-industrial baseline against which you're measuring the warming."
An example of the kinds of bubbles in a slice of ice core which are analysed for CO2 content. (Supplied: Australian Antarctic Division)
Dr Jarvis said the method could still be run with the existing baseline period of 1850 to 1900.
By using the study's methodology for this time period it also showed, like studies using sea and land temperature, there had been 1.3C of warming but with less statistical uncertainty.
"I think that first and foremost, a method that we can all buy into that provides the most robust estimates of the human-induced warming is critical for the process," Dr Jarvis said.
"You're coming into things like COP29 and you really need to be armed with a contemporary estimate that has the lowest uncertainty possible so that you know where you are in relation to your objectives."
Dr Jarvis said the method could also provide timely estimates.
He said using CO2 data from October, their method suggested human-induced warming had hit 1.53C.
Drill implements are used to bring ice cores to the surface in places like Antarctica. (Supplied: Australian Antarctic Division)
Andrew King, a University of Melbourne climate scientist who was not part of the study, said the research was interesting and useful, but the Paris Agreement had not necessarily been breached yet.
"This new study doesn't mean we have failed to keep to the Paris Agreement but rather that the baseline that has been used doesn't account for some pre-1850 warming," he said.
"This was already known from other studies but this study provides another line of evidence.
"Work to understand the implications of 1.5C global warming is typically based on the 1850-1900 baseline or something similar and we're still not at that level yet."
Dr King said regardless of what baseline was used, there was a desperate need to reduce greenhouse gas emissions and decarbonise more quickly.
"This would help us to avoid the most dangerous climate change impacts," he said.
Professor Forster said climate change should not be pushed down the international political agenda, but put on the top of the priority list instead.
"It (1.5C warming) should be a wake-up call to countries to really get on with the business of decarbonisation and also supporting other countries as well," he said.
"The tipping points are hard to predict and exactly when they will be.
"We have to try to prevent every increment of warming."
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