Deforestation is caused by the growing demand for forest products and the conversion of forest to agriculture as the human population continues to expand. In 1750 cropland and pastureland occupied 6-7% of the global land surface; by 1990 cropland and pastureland occupied 35-39% of the global land surface. It is estimated that the world is currently losing over 9 million hectares per year which is an area the size of Portugal. Deforestation not only affects the climate by increasing the atmospheric level of carbon dioxide but also affects the environment by inhibiting water recycling, triggering severe flooding, aquifer depletion, soil degradation and the extinction of plant and animal species.
Trees remove carbon dioxide from the atmosphere through photosynthesis (see my posting entitled The Carbon Cycle). Cutting down forests will cause a decline in photosynthetic activity which results in the atmosphere retaining higher levels of carbon dioxide. Forests also store an enormous amount of organic carbon which is released into the atmosphere as carbon dioxide when forests are cleared by burning. Clearly, deforestation contributes to global warming and ocean acidification (see my two postings entitled Solar Activity, Greenhouse Gas Levels and Climate Change on Our Earth and Atmospheric Carbon Dioxide Levels and Ocean Acidification).
Water recycling is the movement of rain from the forest to land masses further inland. When rain falls on forests the water is intercepted by the forest canopy. Some of this intercepted water is returned to the atmosphere by evaporation and transpiration (release of water vapor into the atmosphere through stomata on tree leaves) while the rest is returned to the ocean as river runoff. In a healthy forest about three fourth of the intercepted water is returned to the atmosphere as moisture laden air masses which move inland, cool and are converted to rain. Land cleared by deforestation returns only about one fourth of the rain water to the atmosphere. This air mass has less moisture and delivers less rain further inland. Deforestation inhibits water recycling and converts inland forest to dry land and potential waste land.
Severe flooding is a result of deforestation because removal of the forest leaves little vegetative cover to hold heavy rains. The inability of land void of forest to hold heavy rain water will also trigger mudslides like the ones that have occurred in recent years in California, China and other parts of the world. Severe flooding and mudslides are extremely costly because they devastate homes and communities.
The excess water from land cleared of forest becomes runoff water and enters the ocean instead of seeping downward into the soil to recharge aquifers. Aquifer depletion is already becoming a serious problem in certain areas of the planet and as the human population continues to grow so will the demand for fresh water.
Deforestation also results in soil degradation. Forests store nutrients that are required for all plant life. In the tropics almost all nutrients are stored in the vegetation because tropical soil has little organic matter and almost no nutrient storage capacity. If tropical forests are cleared for cropland, the land will yield crops for only a few years and when the nutrients are depleted they become waste land.
Today deforestation, especially, in the tropics, decimates plant and animal life. Tropical rainforest contain about 7% of land surface but over half of the plant and animal species on the planet. If tropical rainforest deforestation continues at the current rate, it is estimated that by the first part of the 21st century about half of the remaining rainforest will be lost along with about 5 to 10% of all the species on the planet.
Global warming is not just about temperature. Global warming directly affects humans because everything about our life is tied to the climate such as food production, water supplies, health and well being, etc.