Climate skeptic's claim about CO2 levels, ice ages and animals misleads | Fact check

Kate S. Petersen, USA TODAY
·8 min read
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Smoke billows from a large steel plant on Nov., 4, 2016, in Inner Mongolia, China. Over the industrial era, the amount of carbon dioxide in the atmosphere has increased by about 40%, according to the U.S. Global Change Research Program.

The claim: The 'six great ice ages' began when CO2 was higher than now; all plants and animals die when CO2 levels are cut in half

A July 4 post (direct link, archive link) on X, formerly Twitter, shows a video in which Ian Plimer, a skeptic of human-driven climate change with ties to the mining industry, minimizes the significance of modern atmospheric CO2 concentrations.

The video's caption, which quotes Plimer, reads: "Six of the six great ice ages started when we had more carbon dioxide in the atmosphere than now. We have 0.04% of that gas in the atmosphere ... Well that means nothing to me because the atmosphere has changed in its carbon dioxide content from over 20% to now, which is really low in geological time. If we halved it, all plant life would die, and animals would die."

The post was shared more than 300 times in seven weeks on Facebook, according to Crowdtangle, a social media metrics tool. It was reposted more than 9,000 times on X.

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Our rating: Partly false

Plants and animals have existed when CO2 levels were half of modern levels. The ice age element of the claim is trickier to analyze since the post is unclear what specific time periods the six "great ice ages" are meant to reference, but there have likely been times when Earth's climate got colder even though CO2 levels were higher. However, the Quaternary glaciation, which some scientists consider a "major ice age," is thought to have begun when the concentration of CO2 in the atmosphere was lower than it is today.

'Ice age' can refer to different kinds of events

Though the post presents the concept of "six great ice ages" and their CO2 levels as settled science, experts say they are still learning about CO2 levels on ancient Earth and that the phrase "ice age" can refer to different types of events.

Neither Plimer nor the social media user responded when USA TODAY asked which six "great ice ages" they were referencing.

"It becomes very difficult to reconstruct climate in the deep past," Ashleigh Hood, a sedimentologist and paleo-environmental scientist at the University of Melbourne, told USA TODAY in an email. "Plimer makes it out to be concrete evidence, but it’s still very much a work in progress to reconstruct past atmospheric conditions and climates."

While the Earth is billions of years old, the best climate records are for the last hundreds of thousands to millions of years, according to Hood.

"Before this ... evidence is harder to find and more altered and more controversial and complicated," she said.

Richard Alley, a geosciences professor at Pennsylvania State University, told USA TODAY that he was unsure which six time periods the post was referencing and that the phrase "ice age" can be used to describe different kinds of events. These can include very long periods when the Earth sustains significant ice cover, but it can also reference times within broader cold periods when the ice grows larger.

Hood said the way scientists use the phrase "ice age" often depends on the scale and focus of their research. She said there have been four broad "intervals" of ice over Earth's history and that these likely began when CO2 concentrations were higher than now (420 ppm as of July 2023).

Fact check: Human greenhouse gas emissions, not El Niño, drive climate change

Yige Zhang, a paleoclimatologist at Texas A&M University, told USA TODAY that atmospheric CO2 concentrations were lower than present day at the beginning of the Quaternary glaciation, which began about 2.7 to 2.6 million years ago. The Quaternary glaciation is one of five time periods that Zhang says are commonly understood to be "major ice ages."

"I'm not sure what 'six ice ages' he was referring to," Zhang told USA TODAY in an email.

Over the last roughly 2.6 to 2.7 million years there have also been around 40 "glacial cycles" (which can also be called "ice ages"), according to Marcus Lofverstrom, a hydrology and atmospheric science professor at the University of Arizona. He said all of these cycles are thought to have begun when CO2 concentrations were below 420 ppm.

Climate dynamics on Earth changed as sun grew brighter

While CO2 is a greenhouse gas that slows the escape of heat into space, an older Earth with more CO2 could still be colder than the present because the sun was dimmer millions and billions of years ago, according to Alley.

"The CO2 level at which the Earth is cold enough for ice has been dropping as the sun brightened," he said.

Other processes, including geological carbon cycling and changes in Earth's orientation to the sun, can also cause Earth's climate to warm or cool over tens of thousands to hundreds of thousands of years.

Fact check: Greenhouse gases, not Milankovitch cycles, drive modern global warming

The post claims that past CO2 concentrations have been as high as 20%. However, it is not certain that CO2 levels were ever that high, and if they were, it would have been billions of years ago, according to Hood.

The "early Earth had an atmosphere very different from today, and we think it was made up of a large part CO2 and methane, but no oxygen," she said.

If the present-day atmospheric CO2 concentration rose to 20%, "the oceans would likely boil," Lofverstrom said.

History shows plants and animals have survived in lower CO2 atmosphere

The post also claims that all plants and animals die when atmospheric CO2 levels are half of current levels. Since contemporary levels are about 420 ppm, that would mean plants and animals would die off at 210 ppm.

However, there have been many times plants and animals have lived in atmospheric CO2 levels that low or lower, Lofverstrom said in an email.

"The CO2 concentration at the Last Glacial Maximum, about 21,000 years ago, was around 185 ppm," he said. "Broadly similar CO2 concentrations, 180 to 210 ppm, occurred during the ... last several million years − in the Pleistocene. There were likely time periods with even lower CO2 concentrations further back in time."

NASA also reports many times over the last 800,000 years that CO2 levels dipped below 210 ppm, with plants and animals continuing to survive.

Fact check: Humans are responsible for a significant amount of CO2 in the atmosphere

The contemporary rise of CO2 in Earth's atmosphere, which is due to human activity, is causing significant and rapid global warming. Some of the consequences of this warming − increased heat wave frequency, sea level rise and polar and glacial ice melt − have already been observed.

The rate at which humans are increasing CO2 in the atmosphere does not give ecosystems time to adapt, according to Hood.

"Even though CO2 has varied substantially through Earth’s history, it has generally changed on relatively long timescales (millions or billions of years) compared to the rate of our current warming," she said. "If you look at events in Earth’s history which have environmental and climate change at the same rapid rate of today’s change, these are basically all mass extinction events."

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This article originally appeared on USA TODAY: Past Earth could be cold with high CO2 because of dim sun | Fact check