Explain carbon cycle.

Carbon constitutes $49$ per cent of dry weight of organisms and is next only to water.

If we look at the total quantity of global carbon, we find that $71$ per cent carbon is found dissolved in oceans.

This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere. The atmosphere only contains about $1$ per cent of total global carbon.

Fossil fuel also represents a reservoir of carbon.

- Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.

According to one estimate $4 \times 10^{13} \mathrm{~kg}$ of carbon is fixed in the biosphere through photosynthesis annually.

A considerable amount of carbon returns to the atmosphere as $\mathrm{CO}_{2}$ through respiratory activities of the producers and consumers.

Decomposes also contribute substantially to $\mathrm{CO}_{2}$ pool by their processing of waste materials and dead organic matter of land or oceans.

Some amount of the fixed carbon is lost to sediments and removed from circulation.

Burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity are additional sources for releasing $\mathrm{CO}_{2}$ in the atmosphere.