Carbon dioxide is necessary for plants. Why do we consider it as a pollutant?

Explanation

Increasing concentration (more than normal) of `CO_(2)` is harmful and considered as pollutant. Higher concentrations of `CO_(2)` is one of the causes of greenhouse effect/global warming. Greenhouse gases (e.gu, `CO_(2),CH_(4)`, CFCs etc.) are increasing in the atmosphere due to human and natural activities. These gases trap the heat (infra red radiations) reflected by the earth. This heats up the atmosphere (leads to an increase in earth.s temperature). This phenomenon is called global warming which occurs due to green house effect. It may result in melting of polar ice leading to rise in sea level. Coastal regions, thus, will be flooded or might even be submerged.