Carbon and Life on Earth


Simple Carbon Cycle

CO2 balance is being disturbed

Carbon Cycle and Human Activity

Carbon is a very important substance. It is an important part of the composition of all living things, including human beings. Carbon is in the oceans, the air, and in rocks.

In the atmosphere, carbon is bonded to oxygen in a gas called carbon dioxide (CO2).

Plants use carbon dioxide from the air and sunlight to make their food. The carbon becomes part of the plant. Plants that die and are buried for millions of years may turn into fossil fuels, and their carbon becomes coal and oil. When humans burn these fossil fuels, the carbon re-enters the atmosphere as carbon dioxide.

Carbon dioxide is a greenhouse gas. It traps heat in the atmosphere. Without carbon dioxide and other greenhouse gases the Earth would be a frozen world. But, if there is too much carbon dioxide in the atmosphere, the earth can become too hot to sustain life.

Venus is an example of a planet where life has become impossible. The CO2 concentration of the atmosphere is so high that, due to the greenhouse effect, air temperatures have risen to 740°C.

Simple Carbon Cycle

A useful reference:

Animals, including human animals, breathe in oxygen and breathe out carbon dioxide. Plants, including trees, breathe in carbon dioxide and breathe out oxygen.

“When a tree breathes, it inhales carbon dioxide and exhales oxygen – the exact opposite of humans. As a tree matures, it can consume 22kg of carbon dioxide per year and releases enough oxygen for a human to breathe for two years!”

Trees and green plants are essential to the survival of humans and other animals!

They maintain a healthy balance between the concentration of oxygen and CO2 in the atmosphere.


CO2 balance is being disturbed

Atmospheric carbon dioxide fluctuated between 190 and 290 parts per million (PPM) over the past 800,000 years. We can tell this be looking at CO2 trapped in Antarctic ice, which has been formed over thousands of years.

However, since the middle of the 19th century (1850) there has been a steep rise in the amount of carbon dioxide in the atmosphere. Today it is as high as 415 ppm, 75% higher than it has been for the past 800,000 years!

Clearly something has been going wrong and human beings are the ones to blame.


Carbon Cycle and Human Activity

Source: UCAR Center for Science Education

Humans and the carbon balance

Actions by the growing numbers of human beings are upsetting the carbon balance.

  1. Burning fossil fuels.
    There has been a huge growth in industry across the world.  Industrialisation uses huge amounts of energy. Most of this extra energy has been obtained by extracting carbon locked away in underground deposits of fossil fuels. When coal and oil is burned, carbon dioxide is released into the atmosphere.
  2. Cement Manufacture
    Cement is an important building material in the modern world. To make cement, limestone has to be heated to high temperatures using fossil fuels. In the process, huge amounts of CO2 are released into the atmosphere. The initial stage of cement making involves using coal or oil to heat Calcium Carbonate to temperatures over 825°C. This calcination process produces Calcium Oxide, also called quicklime, and releases huge amounts of Carbon Dioxide.

CaCO3 → plus heat → CaO + CO2

  • Deforestation.
    • The chopping down of trees for wood and charcoal burning releases carbon into the atmosphere.
    • Burning forests to clear land for farming adds carbon to the air.
    • Chopping down trees for timber for making furniture and other things, does not release carbon directly (the carbon is still locked in the wood), but it reduces the number of trees and forests for photosynthesis. This means that there are fewer trees to remove excess CO2 from the atmosphere.
  • Rising population.
    For every 1 person in the world in 1800, there are now 7.5 people. This is a huge increase in world population, which is now 7.5 billion people. People compete with other animals and plants for space and resources. We put pressure on the natural environment, cause pollution and upset the carbon balance of the atmosphere.


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