Atmosphere and Climate

Atmosphere and Climate

Origin of the Atmosphere

Since the creation of the Earth the atmosphere underwent many changes in different geological ages. It became more or less stable since 580 million years ago (Cambrian period). Geological changes shaped the present lithosphere and hydrosphere.

At the beginning the atmosphere probably had water vapour (60–70%), carbon dioxide (10–15%) and nitrogen (8–10%) but no oxygen. As the Earth cooled down from its very hot state, there was massive condensation of water vapour which resulted in incessant rainfall for thousands of years. The rainwater filled in the cavities of Earth’s surface forming seas, oceans and lakes.

Life forms first appeared in the seas some 3.5 billion years ago as blue–green algae. The evolution of plant life on Earth transformed the atmosphere. Plants assimilated carbon dioxide from the atmosphere for photosynthesis and in the process released oxygen into the atmosphere. The present atmosphere consists of nitrogen (78.08%), oxygen (20.95%), argon (0.93%) and carbon dioxide (0.33%).

The process of evolution went on—the living world appeared in the sequence: plants, animals and man. Some 5 million years ago man appeared last of all only when the atmosphere built enough stock of oxygen and the optimum environment for life forms prevailed.

The Climate of the Earth

The earth is a complex system in which the oceans, atmosphere and living species (biosphere) interact with one another and this has ultimately protected life on earth for about 3 billion years.

The earth has a unique mechanism for stabilising and controlling the global climate:

(i) The plants and animals balance carbon dioxide level of the atmosphere which in turn acts as a global thermostat rewriting earth’s temperature within optimum limits.

(ii) The ocean plankton (plant and animal species) plays important role in regulating global climate.

(iii) Sudden shifts in ocean current, e.g. gulf stream can initiate mini ice ages which last for centuries.

(iv) Tiny changes in the earth’s orbit, favoured by changes in the proportions of atmospheric gases, control the advance and retreat of glaciers. The mechanism with which carbon is handled helps in the stability of the earth’s climate.

Carbon, the backbone of biological chemistry, is the component of rock and in the form

of carbon dioxide, keeps the earth warm. With rise in carbon dioxide level in the atmosphere, say by a volcanic explosion or burning of fossil fuel, there is a corresponding increase of uptake of carbon dioxide by green plants from air and also by the oceans where it is turned into rock at the ocean floor. Similarly, if there is decrease of carbon dioxide in the atmosphere, there is less evaporation of sea water and less rainfall and carbon in the ocean rock in the floor burns through volcanoes and makes up the carbon dioxide deficiency in the atmosphere.

The earth swings back and forth between hot and cold periods with rise and fall of carbon dioxide level in the atmosphere. Thus during the dinosaur era some 100 million years ago carbon dioxide content was 4–5 times higher than to-day’s and the earth was warmer by about 5° C. But during the Ice Age about 18000 years ago carbon dioxide dropped to 60 per cent of its present level and the dinosaurs were extinct for lack of vegetation, i.e. their food.

The oceans provide another mechanism for governing climate. Covering 70 per cent of the planet, oceans act as a huge fly wheel in the earth’s machinery. As a huge reservoir of carbon (fixed as HCO3 mineral deposit on the ocean floor), the oceans regulate the carbon dioxide level and hence global climate. Ocean currents also play major role in setting long term climate patterns.