Final answer:
The concentration of atmospheric carbon dioxide in 2013 was approximately 395 parts per million (ppm). This increase is mainly caused by human activities such as the burning of fossil fuels and deforestation.
Step-by-step explanation:
The concentration of atmospheric carbon dioxide in 2013 was approximately 395 parts per million. This concentration, which is measured in parts per million (ppm), represents the number of carbon dioxide molecules for every one million molecules of air.
The increase in atmospheric carbon dioxide is mainly caused by human activities such as the burning of fossil fuels and deforestation.