Since CO2 is confined exclusively to the troposphere its higher concentration may act as a serious pollutant.
When there is an increase in CO₂ concentration, the thick layer of this gas prevents the heat from being radiated out.
This thick CO₂ layer, Thus functions like the glass panels of a green house, allowing the sunlight to filter through but preventing the heat from being re-radiated in outer space. This is the so called Green House Effect.
There are other gases also which contribute to green house effect.
There are SO2, NO2, CFC discharged by industries and agriculture.