Scientists are concerned about the rising levels of carbon dioxide (CO 2). What trait about this gas makes it so concerning to scientists?

It is a greenhouse gas i.e. Its ability to contribute to Global Warming.

Explanation:

Carbondioxide (CO2) is one of the world's colorless, odorless and tasteless gases in air. It is the major product of cellular respiration by living organisms. The increase in level of CO2 in the atmosphere is a major concern to scientisits because they have the ability to trap heat, hence, called a GREENHOUSE GAS.

CO2, in conjunction with other gases like methane, are responsible for keeping the earth warm as they help trap heat energy from the sun. However, an increase in the level of these greenhouse gases is contributing to what is called Global Warming, which is the gradual increase in the temperature of the earth's atmosphere.