1. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. The Carbon Cycle If the atmosphere is to maintain a constant amount of carbon dioxide, the rate of removal of atmospheric carbon dioxide must be balanced by the rate of return of the gas. The mechanism that maintains the level of carbon dioxide in the atmosphere is called the carbon cycle.

2. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. What processes produce carbon dioxide? 1. Respiration During respiration, carbon and hydrogen compounds in food are converted into carbon dioxide and water. C 6 H 12 O 6 (aq) + 6O 2 (g)  6CO 2 (g) + 6H 2 O(l) + energy 2. Combustion of fuels An example of such fuel is methane. CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(l) + energy

3. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. What processes remove carbon dioxide? 1. Photosynthesis During photosynthesis, green plants convert carbon dioxide and water into glucose and oxygen (Fig. 20.7) in the presence of sunlight. 6CO 2 (g) + 6H 2 O(l)  C 6 H 12 O 6 (aq) + 6O 2 (g)

4. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. The Greenhouse Effect Carbon dioxide is not the only greenhouse gas. Other gases, such as methane (CH 4 ) and nitrous oxide (N 2 O), are even stronger infrared absorbers. The natural greenhouse effect is crucial for maintaining the proper temperature needed to sustain life on Earth. Without these greenhouse gases, Earth’s surface temperature would be –40 °C and it would be permanently covered with ice.

5. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Human activities like the burning of fossil fuels and large scale cutting down of forests are causing some greenhouse gases, especially carbon dioxide, to build up rapidly in the atmosphere. This means that carbon dioxide is being added to the atmosphere at a higher rate than photosynthesis can remove the excess gas. The effect of carbon dioxide build-up is an increase in the Earth’s average temperature. Global Warming

6. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. a decrease in crop yields world-wide because the areas that are currently covered by vegetation may become deserts, the melting of large quantities of ice in the North Pole and South Pole. This will cause the levels of oceans to rise and flood low-lying countries such as the Netherlands. the rapid evaporation of water from the Earth’s surface. When this happens, carbon dioxide dissolved in the oceans will be driven out into the atmosphere. This adds further to the greenhouse effect. Consequences of Global Warming