Fact check: Human-generated CO2, not water vapor, drives climate change

Kate S. Petersen, USA TODAY
·7 min read

The claim: Human-generated CO2 is an insignificant contributor to climate change

The latest United Nations Intergovernmental Panel on Climate Change report was released in August, documenting dangerous, human-driven changes to Earth’s climate systems and predicting still more to come.

Though the report was described as a “code red for humanity” by UN Secretary-General António Guterres, a years-old claim soon resurfaced on social media claiming that humanity’s CO2 emissions are too minute to impact climate.

A version of the claim in the form of a graphic titled “How climate alarmists use numbers to deceive” was posted to Facebook on Sept. 19 and received hundreds of shares. The graphic also accuses the Environmental Protection Agency of publishing a misleading pie chart that “exaggerates CO2’s role as a greenhouse gas.”

“SCIENTIFIC FACT: Man-made CO2 constitutes only 0.1 to 0.2% of atmospheric greenhouse gases. It is an insignificant contributor to global temperature and climate," the post concludes.

This claim, which has been repeatedly debunked, is wrong. The human-generated CO2 in the atmosphere is causing significant changes in Earth’s climate, scientists say.

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USA TODAY reached out to the social media users for comment.

Human produced CO2 is changing Earth’s climate

Josh Willis, a climate scientist at NASA's Jet Propulsion Laboratory, told USA TODAY it's a logical fallacy to assume a proportionally small amount of a substance cannot have a large impact on a system.

“It's a commonly used technique to confuse people,” he said, pointing out that, for example, minute amounts of arsenic can kill an adult.

Willis said human-generated CO2 is indeed changing Earth's climate.

Carbon dioxide is a natural part of atmospheric and biological cycles on Earth. It is not just produced by burning fossil fuels. It is also expelled by animals and volcanic activity, for instance.

Once expelled, carbon dioxide gets absorbed or sequestered again, in forests, grasslands, bogs, the ocean and other “carbon sinks,” completing the cycle.

While in the atmosphere, carbon dioxide and other greenhouse gases trap heat, creating a habitable planet with stable, warm temperatures. This phenomenon is commonly referred to as the "greenhouse effect."

The problem is that humans are now facilitating the release of more carbon dioxide into the atmosphere than was previously cycling, while at the same time destroying sinks, such as forests. This is causing carbon dioxide to accumulate in the atmosphere in amounts unprecedented for hundreds of thousands of years.

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The amount of carbon dioxide in the atmosphere is proportionally smaller than that of some other gases, such as water vapor. And the amount generated by humans is even smaller than that.

But carbon dioxide also behaves differently than other gases. And multiple lines of evidence show the amount that is accumulating is sufficient to cause the changes documented by climate scientists.

Water vapor and CO2 behave differently in the atmosphere

The social media post correctly indicates that there can be more water vapor than CO2 in the atmosphere, but it is wrong to offer that as evidence CO2 is insignificant.

This "is an old myth," said Willis. "This one has been debunked many times."

The fact that there is typically more water vapor than CO2 in the atmosphere does not change the impact of CO2 on the atmosphere.

CO2 and water vapor both contribute to the "greenhouse effect" that warms the Earth and makes it habitable. However, they play very different roles. Increased CO2 levels cause climate change, whereas increased water vapor levels are caused by climate change.

At a given average temperature, average water vapor levels in the atmosphere remain relatively constant. Fluctuations are intrinsically short-lived, because water condenses and rains out of the atmosphere, Susan Solomon, an atmospheric chemist and professor at Massachusetts Institute of Technology, told USA TODAY in an email.

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In a warmer atmosphere, more water accumulates before it rains out. Therefore a warming atmosphere will see an increase in stable water vapor, which will, in turn, exacerbate warming.

However, for this to occur in the first place, the Earth's atmosphere would already have to be warming up. That is what's happening, and it's because of heat trapped by elevated human CO2 emissions.

"When the planet heats up, we get more water vapor and the planet heats up a bit more," said Willis. Unlike CO2, "humans don’t control the amount of water in the atmosphere – it’s a reaction to how warm the planet is."

Post misrepresents EPA pie chart

The post wrongly concludes CO2 is insignificant after claiming an EPA chart on the topic is misleading.

The unlabeled pie chart shows percentages for carbon dioxide, methane, nitrous oxide and fluorinated gases.

“No mention is made that this chart includes only 'man-made' greenhouse gases,” reads the accompanying text.

The text and image in the post are dated 2014, and USA TODAY was unable to find the exact pie chart pictured in the Facebook post. But the style and content match a pie chart found on the EPA’s Inventory of U.S. Greenhouse Gas Emissions and Sinks report webpage.

The current version of this chart is labeled, "Overview of U.S. Greenhouse Gas Emissions." It doesn't explicitly state the chart shows only emissions caused by humans, but elsewhere on the page the report notes that the topic is "total greenhouse gas emissions for all man-made sources in the United States."

The post also states that "by excluding water vapor, this chart is very deceptive and greatly exaggerates CO2's role as a greenhouse gas."

However, the chart only shows the relative percentages of the various man-made greenhouse gases that are emitted in the U.S. Nowhere on the page does it state that the chart is intended to depict the role or the importance of each type of gas in the greenhouse effect or climate change.

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USA TODAY has previously debunked a claim that a similar waterline shown in photos of a beach taken years apart prove that climate change isn’t real. USA TODAY also found that a video purporting to show climate activist Greta Thunberg denying the existence of a climate crisis was altered.

Our rating: False

Based on our research, we rate FALSE the claim that human-made CO2 is an insignificant contributor to climate change. Though human-generated CO2 typically comprises a smaller proportion of atmospheric greenhouse gases than water vapor, research has shown unequivocally that it has a large impact and is changing Earth’s climate.

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This article originally appeared on USA TODAY: Fact check: Human CO2, not water vapor, drives climate change

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