CO2 is small proportion of atmospheric gases, but still drives global warming | Fact check

Kate S. Petersen, USA TODAY
·6 min read
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The claim: Human-contributed CO2 is only .0016% of the atmosphere, so it can't cause climate change

A Feb. 6 Facebook post (direct link, archive link) lists the purported percentages of various gases in the atmosphere:

  • Nitrogen: 78.08%

  • Oxygen: 20.94%

  • Argon: 0.93%

  • CO2 Total: 0.04%

  • CO2 Natural: 0.0384%

  • CO2 Human: 0.0016%

"CO2 is a trace gas in our atmosphere," the post is captioned. "Numbers say that all the nonsense we're subjected too in reference to it's effect and our contribution to the amount in the atmosphere is a sham (sic). Check them out for yourself."

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Our rating: False

While it makes up a small percentage of total gases in the atmosphere, extensive evidence shows that CO2 and other greenhouse gases emitted by human behavior are driving global warming. The post also understates the amount of CO2 humans have contributed to the atmosphere.

Total amount of CO2 in atmosphere more important than percentage

Claiming that a proportionally small amount of CO2 can't meaningfully impact climate is a logical fallacy and a "commonly used technique to confuse people," Josh Willis, a NASA climate scientist, previously told USA TODAY.

Small amounts of a substance can have a big impact. For example, the percentage of CO2 in the atmosphere is much higher than the percentage of arsenic per body weight necessary to kill a person.

Further, the proportion of CO2 in the atmosphere isn't as important as the absolute amount of CO2 in the atmosphere, Gavin Schmidt, a NASA climate scientist and director of the Goddard Institute of Space Studies, previously told USA TODAY.

Greenhouse gas molecules slow the escape of heat into space by absorbing heat energy emitted by the Earth's warm surface. The molecules then release their own energy and some of this energy escapes upward into space. But some is also radiated sideways or back downward, further warming the lower atmosphere.

"The ability to trap infrared heat depends on the probability of a photon (energy) from the surface encountering a greenhouse gas molecule – so the more molecules, the more trapping," Schmidt said. "Thus, it is the total amount of CO2 that matters, not so much its concentration. On Mars, for instance, the air is 95% CO2, but there is much less air – 150 times less than the amount on Earth – so the greenhouse effect is much less."

Through fossil fuel combustion and land use changes, humans contributed more than 1 trillion metric tons of CO2 to the atmosphere between 1850 and 2022, according to the 2022 Global Carbon Budget. This CO2 has trapped more than 1 quadrillion watts of energy in Earth's climate systems, Dargan Frierson, an associate professor of atmospheric sciences at the University of Washington, previously told USA TODAY.

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While the proportion of atmospheric CO2 contributed by humans is relatively small, it is more than the amount shown in the post.

The proportion of CO2 in the atmosphere is about .0423%, but humans have contributed roughly one-third of that, according to NASA. This means that the proportion of atmospheric CO2 from human activity is around .014%, not 0.0016%.

Atmospheric CO2 concentrations before the Industrial Revolution were at roughly .028%, according to NASA. The increase to the current level of .0423% is all a result of human activity, Schmidt previously told USA TODAY.

Exhaustive research shows greenhouse gases are causing modern global warming

Researchers established that greenhouse gases emitted by human activity are responsible for modern global warming through a thorough, step-wise process that included:

Fact check: How we know humans are causing warming: A brief history of climate science

CO2 released by humans is the only explanation of modern warming that accommodates all of the evidence, Willis previously told USA TODAY in a separate interview. He emphasized that researchers have examined other possible explanations for modern warming.

"Scientists have looked for other sources of heat, cycles of the sun, volcanos on the sea floor and pretty much everything else you can think of," he said. "Nothing besides humans burning fossil fuels can explain all of these things."

The Facebook user who shared the post did not provide evidence that CO2 from humans makes up only 0.0016% of the atmosphere.

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This article originally appeared on USA TODAY: Humans responsible for one third of CO2 in atmosphere | Fact check