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CO2 proportion is small, but change is still driving warming | Fact check

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USA TODAY

The claim: CO2 from humans represents 3.2% of total CO2 in the atmosphere and 0.12% of all greenhouse gases in the atmosphere

A June 14 Facebook post (direct link, archive link) uses pie charts to show the purported breakdown of greenhouse gases in the atmosphere.

Amid other claims, it asserts that CO2 released by human activity represents 3.2% of total CO2 in the atmosphere and 0.12% of greenhouse gases in the atmosphere.

The post was shared more than 2,000 times in three weeks.

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Our rating: Partly false

The calculations in the post are flawed. The percentage of CO2 in the atmosphere from human activity is much higher than 3.2%. And while there are theoretical environmental conditions in which CO2 from human activity could account for 0.12% of greenhouse gases, that proportion varies greatly since the amount of water vapor in the atmosphere fluctuates and differs between locales.

Post misrepresents human contribution to greenhouse gases in the atmosphere

Human activity, such as burning fossil fuels, is responsible for roughly a third of the CO2 in the atmosphere, not 3.2% as the post claims.

"Since the onset of industrial times in the 18th century, human activities have raised atmospheric CO2 by 50% (one-third) – meaning the amount of CO2 is now 150% of its value in 1750," NASA reports.

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The post's underestimation of the human contribution to atmospheric CO2 may be due to a misunderstanding of Earth's carbon cycle. In this cycle, CO2 is released by natural phenomena such as volcanos, fires and decaying organisms and reabsorbed by ecosystems such as forests, seagrass beds and peatlands.

The vast majority of the carbon moving through this cycle − around 95% − is not from human activity, according to the 2022 Global Carbon Budget. However, the human contribution adds more CO2 into the cycle than can be absorbed by Earth's natural systems, Gavin Schmidt, a NASA climate scientist, previously told USA TODAY.

This excess CO2 has been accumulating in the atmosphere for decades, pushing CO2 levels higher than they have been in hundreds of thousands of years, according to the National Oceanic and Atmospheric Administration.

The methane and nitrous oxide data in the post is also flawed. While a pie chart in the post asserts that nitrous oxide is nearly three times more abundant than methane, there is almost six times more methane than nitrous oxide in the atmosphere, according to NOAA.

Water vapor levels in the atmosphere fluctuate

The 0.12% figure in the post, which it says is the percentage of greenhouse gases that come from human-created CO2, is also flawed because that figure varies.

It changes based on the water vapor concentrations in the atmosphere, and those range hugely – from around 0 parts per million to 40,000 parts per million depending on area environmental conditions, Jacobs said.

This means that water vapor can represent anywhere from 0% to 99% of greenhouse gases in the atmosphere in a given location. And depending on how much water vapor there is, the proportion of CO2 could also range between roughly 1% and 99% of greenhouse gases.

Given that range, the proportion of CO2 from human activity would vary from around 0.003% to 0.33%.

Because of this, CO2 from human activity as a proportion of all greenhouse gases, would only equal the amount stated in the post − .12% − when water vapor levels were very low.

While water vapor is a powerful greenhouse gas and there is typically more of it in the atmosphere than CO2, it is not the driving force behind global warming, Mark Zelinka, an atmospheric scientist at Lawrence Livermore National Laboratory, previously told USA TODAY. This is because water vapor behaves much differently than other greenhouse gases in the atmosphere.

When emitted by humans, CO2, methane and nitrous oxide remain in a gas form under normal conditions found in Earth's atmosphere, where they accumulate and cause warming. But the amount of water vapor in the atmosphere is determined not by human emissions, but by the temperature of the atmosphere itself, Zelinka said.

A warmer atmosphere holds more water vapor and a cooler one holds less. Jacobs said that even in a hypothetical scenario where humans dramatically manipulated the amount of water vapor in the atmosphere, the effects would not be long-lasting.

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Small proportion of CO2 has a big impact

While the data in the post is flawed, it is accurate that the proportion of CO2 in the atmosphere due to human activity is relatively small. But in the context of global warming, the proportion of CO2 compared to other gases isn't very relevant, Schmidt previously told USA TODAY.

Instead, it's the total amount of CO2 in the atmosphere that's significant.

This is because CO2 molecules trap heat in the atmosphere when they intercept energy emitted from the Earth's surface. The molecules then re-emit energy, but some is released back toward the Earth − ultimately slowing the escape of heat into space.

The more CO2 molecules that are in the atmosphere, the stronger this effect.

Between 1850 and 2022, humans added more than 1 trillion metric tons of CO2 to the atmosphere through land use changes and fossil fuel combustion, according to the 2022 Global Carbon Budget.

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When asked about the source of the data in the post, the Facebook user referred USA TODAY to a blog about fossilized plants in West Virginia that states it was last updated in 2011. The site doesn't appear to be connected to any climate research agency and doesn't prove the figures in the post.

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