Carbon dioxide gas is essential for photosynthesis by plants. It is also a green -house gas (GHG). Upto 350 ppm (ppm = parts per million) concentration in the atmosphere `CO_(2)` is both a good raw material as well as essential for keeping the Earth wark. However, when concentration of `CO_(2)` rises (as presently it is 387 ppm), it becomes a ollutant because it results in global warming. The global warming is quite dangerous because it tends to melt polar ice and glaciers existing on mountains raise in the water level of oceans and submerge severals coastal areas and islands.
