Green plants use carbon dioxide for photosynthesis and return oxygen to the atmosphere, even then carbon dioxide is considered to be responsible for green house effect. Explain why?
Carbon dioxide is a natural constituent of atmosphere and is vital for all forms of plant life. It forms about $0.033 \%$ by volume of atmosphere. It helps to maintain the temperature of the earth required for living organisms.
A balance of $\mathrm{CO}_2$ is maintained in air because $\mathrm{CO}_2$ is produced from respiration, burning of fossil fuels and decomposition of lime stone but at the same time, it is consumed in photosynthesis by plants.
However, human activities have disturbed this balance and $\mathrm{CO}_2$ level in atmosphere is in increasing order. This has happened due to deforestation, burning of more fossil fuel and industrialisation. It has been estimated that $\mathrm{CO}_2$ concentration has risen about $25 \%$ in the past century.
During the past nearly 120 years, the average temperature of the planet has increased by somewhere between $0.4^{\circ} \mathrm{C}$ to $0.8^{\circ} \mathrm{C}$. Current estimated are that doubling the $\mathrm{CO}_2$ concentration will result in a temperature increase of between $1.0^{\circ} \mathrm{C}$ and $3.5^{\circ} \mathrm{C}$. In green house effect, contribution of $\mathrm{CO}_2$ is $50 \%$ and of other trace gases is also about $50 \%$.
Explain how does green house effect cause global warming?
Visible light from the sun reaches the earth and heats it up. However, when the earth cools, the energy is re-emitted from the earth's surface in the form of infrared radiation which have longer wavelength and heating effect.
These infrared radiation can be absorbed by $\mathrm{CO}_2$ and $\mathrm{H}_2 \mathrm{O}$ vapours. The heat thus absorbed is radiated back to the surface of the earth. In this way, warming of the earth occurs.
If the rate at which solar energy is arriving the earth remains constant but the amount of $\mathrm{CO}_2$ in the air increases, the heat radiated back to the earth will increase. Consequently, the temperature of the earth's surface will increase.
Hence, global warming depends on the concentration of those gases which are responsible for green house effect.
A farmer was using pesticides on his farm. He used the produce of his farm as food for rearing fishes. He was told that fishes were not fit for human consumption because large amount of pesticides had accumulated in the tissues of fishes. Explain how did this happen?
Pesticides from soil are transferred into the crops and from the crops these are transferred into rearing fish food. Pesticides entered[ into water through rearing fish food and finally entered into the bodies of the fishes.
Therefore, pesticides are transferred from lower trophic level to higher trophic level through food chain. Over the time, the concentration of pesticides in fishes reach a level which causes serious metabolic and physiological disorders.
For dry cleaning, in the place of tetrachloroethane, liquified carbon dioxide with suitable detergent is an alternative solvent. What type of harm to the environment will be prevented by stopping use of tetrachloroethane? Will use of liquified carbon dioxide with detergent be completely safe from the point of view of pollution? Explain.
Tetrachloroethane, $\mathrm{Cl}_2 \mathrm{CH}-\mathrm{CHI}_2$ is suspected to be carcinogenic and also contaminates the ground water. The harmful effect will be prevented by using liquified $\mathrm{CO}_2$ along with suitable detergent.
Use of liquified $\mathrm{CO}_2$ along with detergent will not be completely safe because most of the detergents are non-biodegradable and they cause water pollution. Moreover, liquified $\mathrm{CO}_2$ will ultimately enter into the atmosphere and contributed to the green house effect.