No, ice core study does not show CO2 is 'not the cause of warming' | Fact check

The claim: Ice core data shows CO2 does not cause global temperature changes

A Dec. 20, 2023, Instagram video (direct link, archive link) shows clips from the movie "The Great Global Warming Swindle" in which Ian Clark and the late Frederick Singer − both scientists critical of human-driven climate change − talk about CO2 and ice cores. Clark references a 2003 ice core study that reported temperatures increased hundreds of years before CO2 levels in Antarctica about 240,000 years ago.

"CO2 clearly cannot be causing temperature changes," says Clark.

Text included in the post reads, "How inconvenient. Ice core surveys show carbon dioxide is not the cause of warming."

The video, which also circulated on TikTok, garnered more than 10,000 likes in two months.

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Our rating: False

The post misrepresents the findings of the 2003 ice core study, which does not preclude the possibility of CO2-driven warming during the study period. A greenhouse gas increase isn't the only thing that can initiate climate change, but greenhouse gas emissions from human activity are driving modern warming.

Ice core data consistent with CO2-driven warming

The conclusions of the 2003 study have "often been misunderstood," Nicolas Caillon, lead author on the study and researcher at the French National Centre for Scientific Research, told USA TODAY in an email.

The study used a chemical analysis of Antarctic ice core samples to investigate climate changes 240,000 years ago. The authors reported an apparent temperature increase in Antarctica roughly 800 years (plus or minus 200 years) prior to an atmospheric CO2 increase.

However, the authors were clear that this finding does not mean CO2 did not also cause warming during that timeframe. Instead, they suggested that changes in Earth's orbit initiated warming in Antarctica and caused the release of stores of CO2 (through various natural processes), which then caused more warming.

Warming events like the one 240,000 years ago "take about 5,000 years to be complete," Caillon said. "The lag is only 800 years. All that the lag shows is that CO2 did not cause the first 800 years of warming, out of the 5,000-year trend. The other 4,200 years of warming could, in fact, have been caused by CO2 as far as we can tell from this ice core data."

While an increase in atmospheric greenhouse gases initiated modern global warming, it is well known that other things can initiate climate change.

For instance, "changes in the amount of summer sunshine, due to changes in the Earth's orbit around the sun ... have long been known to affect the comings and goings of ice ages," Caillon said.

Fact check: No, Antarctic sea ice data doesn't show 'global warming narrative is falling apart'

The social media post also ignores other relevant studies. For instance, while the 2003 study only reports temperature change in Antarctica, a 2012 study found that global temperature warmed after CO2 levels rose at the end of the last ice age, according to Edward Brook, an Antarctic ice core researcher at Oregon State University.

CO2 causes warming regardless of how it gets into the atmosphere, Richard Alley, a climate scientist at Penn State, told USA TODAY.

"CO2 can be released to the atmosphere by some processes that are triggered by warming," he said in an email. "Or CO2 can be released by processes not related to warming. Whatever process adds CO2 to the atmosphere, the physics and the models and the recent data and the history show that the CO2 warms the climate, which does not care how the CO2 got into the air."

'Overwhelming' evidence CO2 release by humans drives modern global warming

While changes in Earth's orbit around the sun can initiate climate change, scientists can tell this is not what is causing modern warming.

Orbital cycles "operate on long time scales, ranging from tens of thousands to hundreds of thousands of years," NASA reports. "In contrast, Earth’s current warming has taken place over time scales of decades to centuries."

Additionally, "Earth is currently in an interglacial period − a period of milder climate between Ice Ages. If there were no human influences on climate, scientists say Earth’s current orbital positions within the Milankovitch cycles  predict our planet should be cooling, not warming," NASA noted.

However, there is compelling evidence that modern warming is driven by an increase in the concentration of atmospheric greenhouse gases due to human activity – specifically industrial pollution. Researchers have long understood that CO2 (and other greenhouse gases) warm the Earth by slowing the escape of heat into space, and the amount of warming matches what scientists would expect based on the increased CO2 humans have added.

Researchers have also been able to demonstrate that their understanding of greenhouse gas and planetary physics is sound by accurately predicting greenhouse gas-driven warming decades in advance.

"Climate models published since 1973 have generally been quite skillful in projecting future warming," Carbon Brief reports. "While some were too low and some too high, they all show outcomes reasonably close to what has actually occurred, especially when discrepancies between predicted and actual CO2 concentrations and other climate forcings are taken into account."

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USA TODAY reached out to the Instagram user who shared the post for comment but did not receive a response.

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This article originally appeared on USA TODAY: Ice core study misused in flawed climate change assertion | Fact check