Earth Day: How much do you know about climate change? Take our quiz.

Carbon dioxide exists at an average concentration of about 390 parts per million in the Earth's atmosphere, according to monthly reports by National Oceanic and Atmospheric Administration. Prior to the Industrial Revolution, it existed in concentrations of 278 ppm.

Carbon dioxide exists at an average concentration of about 390 parts per million in the Earth's atmosphere, according to monthly reports by National Oceanic and Atmospheric Administration. Prior to the Industrial Revolution, it existed in concentrations of 278 ppm.

Carbon dioxide exists at an average concentration of about 390 parts per million in the Earth's atmosphere, according to monthly reports by National Oceanic and Atmospheric Administration. Prior to the Industrial Revolution, it existed in concentrations of 278 ppm.

Carbon dioxide exists at an average concentration of about 390 parts per million in the Earth's atmosphere, according to monthly reports by National Oceanic and Atmospheric Administration. Prior to the Industrial Revolution, it existed in concentrations of 278 ppm.

2. On average, what percentage of the Earth's atmosphere by volume consists of carbon dioxide?

A full moon appears through the blue glow of the Earth's atmosphere, in this photo taken aboard the Space Shuttle Discovery in 1999. (NASA/Reuters/File)

About 4 percent
About 0.4 percent
About 0.04 percent
About 0.004 percent

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