At a hearing to review implementation of the bipartisan Infrastructure Investment and Jobs Act, House members queried four transportation experts.

Rep. Doug LaMalfa, R-Calif., puzzled them with an esoteric question about carbon dioxide.

"What percent of our atmosphere is CO2?" he asked.

The experts, appearing to guess, offered responses ranging from 5% to 8%. 

"I ask a lot of people that because all we hear is climate change, climate change; CO2, CO2…" LaMalfa said. 

"The answer is 0.04%." 

A video clip of that portion of the March 28 hearing of the House Transportation and Infrastructure Committee went viral. An April 12 Facebook post sharing the clip included the incorrect title: "Clean Energy ‘Experts’ Can’t Answer Simple Question."

See Figure 1 on PolitiFact.com

LaMalfa is correct that carbon dioxide makes up only about 0.04% of the Earth’s atmosphere, but that ignores the threat CO2 poses to the climate.

"It does not take much CO2 to warm the Earth by a lot, just like it does not take much poison to hurt or kill a person," said Aiguo Dai, an atmospheric and environmental sciences professor at the University at Albany.

Carbon dioxide accounts for 79% of U.S. greenhouse gas emissions, according to the U.S. Environmental Protection Agency. 

Most of the world’s scientists agree that greenhouse gases released into the atmosphere from human activity are driving climate change. 

Greenhouse gases are produced when fossil fuels such as coal and oil are burned for energy. 

And in the U.S., transportation is the largest source of greenhouse gas emissions.

When these gases accumulate in the atmosphere, they trap heat and raise surface temperatures, leading to changes in climate.

Environmental experts have addressed the question about carbon dioxide being a small percentage of the atmosphere.

"The key to carbon dioxide's strong influence on climate is its ability to absorb heat emitted from our planet's surface, keeping it from escaping out to space," University of North Carolina environmental sciences professor Jason West wrote. "Today the level of carbon dioxide is higher than at any time in human history."

Yochanan Kushnir, a climate physics scientist at Columbia University, wrote that because we are increasing the amount of atmospheric carbon dioxide by burning fossil fuels. Even in relatively small amounts compared with the entire mass of the atmosphere, this leads to global warming, he said. 

Robert Brecha, a University of Dayton sustainability professor, said the current rate is 0.042%, slightly higher than what LaMalfa claimed. Based on monitoring by the University of California, San Diego, Brecha said, the rate had been 0.028% about a century ago. 

That's a 50% increase.

"There has been a 50% change in what is an important part of controlling the planet's temperature, and that had been constant for thousands of years," he said.

Brecha said it is nonsensical to argue that the percentage of CO2 being small means that it’s not important. 

"Zero point zero four percent of my body weight would correspond to a cancerous tumor of about 1.5 in diameter," he said. "I would be pretty worried if that appeared somewhere on my body."

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