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Did you know?

The Earth's Cycles

In addition to energy, organisms require a supply of materials to live. These chemical elements are found in the biosphere but must be constantly recycled in order to maintain their availability. Following is an account of some of these cycles.

1 What are the ''biogeochemical'' cycles?

Biogeochemical cycles (bio meaning 'life', geo for 'earth' and chemical for the changing of matter from one form to another) are natural processes that recycle elements in various chemical forms from the environment, to organisms, and then back to the environment. Water, carbon, oxygen, nitrogen, phosphorous, and other elements pass through these cycles, connecting the living and non-living components of the Earth. The element cycles are one of the basic life support systems of the biosphere.

2 What is the water cycle?

The water cycle (or hydrological cycle) is the circulation of the waters of the Earth between land, freshwater lakes and rivers, the salt seas and oceans, and the atmosphere. This cycle collects, purifies, and distributes the Earth's fixed supply of water. This cycle comprises several main steps:
  • Through evaporation, water on the land and in the oceans is converted by solar energy into water vapor. Winds then move masses of water vapor around the Earth.
  • Through condensation, water vapor is turned into water droplets, which form clouds or fog.
  • Through precipitation, water returns to the Earth in the form of dew, rain, hail, or snow.
  • Through transpiration, water is absorbed by the roots of plants, passed through their stems and other structures, and released from their leaves as water vapor.
  • Through run-off, water moves from the land to the sea, or else from the land into the ground where it is stored (from which it eventually returns to the surface or to lakes, streams, and oceans).

3 In what ways do human beings affect the water cycle?

We deplete groundwater supplies, causing water shortages and subsidence (sinking of land when groundwater is withdrawn). We clear vegetation from land areas. When vegetation is removed, water flows over the ground more quickly, allowing less time for it to sink into the soil. This results in groundwater depletion. The increased surface run-off also results in accelerated soil erosion.

4 What is the carbon cycle?

Carbon is an essential component of all living things. It exists mostly as carbon dioxide in the atmosphere, oceans, and in fossil fuels stored beneath the Earth's surface. The major steps of the carbon cycle are the following:
  • Carbon dioxide in the atmosphere is absorbed by plants and converted into sugar, by the process of photosynthesis.
  • Animals eat plants, breaking down the sugars and releasing carbon into the atmosphere, oceans or soil.
  • Other organisms break down dead plant and animal matter, returning carbon to the non-living environment.
  • Carbon is also exchanged between the oceans and the atmosphere. This occurs in both directions at the interface between the air and water. When the level of carbon dioxide in the oceans (atmosphere) exceeds a certain level, it will flow from the higher concentration in the oceans (atmosphere) to the lower concentration in the atmosphere (oceans), maintaining equilibrium.

5 What is the oxygen cycle?

Oxygen composes nearly 21 percent of the atmosphere. It combines chemically with a multitude of other elements to form important substances such as water, carbon dioxide, plant nutrients and organic substances. The steps of the oxygen cycle are the following:
  • Plants release oxygen into the atmosphere as a by-product of photosynthesis.
  • Animals take in oxygen and, by the process of respiration, use it to break down sugars obtained from food.
  • Carbon dioxide is released by animals and used by plants in photosynthesis.

Oxygen is also cycled between the oceans and the atmosphere, maintaining oxygen equilibrium.

Source: The Primer on Environmental Citizenship.

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Water currents. /Claudio Contreras
  Water currents. /Claudio Contreras