How Trees Affect Global Warming
Carbon Dioxide, Global Warming, and Photosynthesis
- The principle man-made greenhouse gas driving global warming is carbon dioxide. While it is not the most potent of the heat-trapping gasses, much more carbon dioxide is released by human industrial activity than any other greenhouse gas. Most of this comes from the burning of fossil fuels. As it turns out, carbon dioxide is also one of the necessities for photosynthesis, or the process where plants convert sunlight into usable energy. Plants "breathe" carbon dioxide in, and "breathe" out oxygen, making them consumers of that greenhouse gas.
Forests as Carbon Sinks
- As the largest and densest of plants, trees are a major consumer of carbon dioxide. This has led forests to be called carbon sinks, or depositories of carbon dioxide. Studies have proven the effectiveness on forests in absorbing carbon dioxide from the atmosphere, and thereby cushioning the greenhouse effect. The last EPA study on the subject was in 2004, but that revealed that America's forests soaked up roughly one-tenth of the country's output of carbon dioxide. At the time, the United States was the world's leading emitter of greenhouse gases. The same study showed that urban trees soaked up a further 1.5 percent.
Carbon Offsets
- The effectiveness of forests as carbon sinks has led to the creation of carbon offset plans, where carbon dioxide emitters offset their pollution by contributing to forest expansion programs. The idea is that by planting trees, they create a sink for the carbon the emit, either reducing or balancing their net pollution. Replanting trees into quick-growing tropical regions is an especially effective way of creating carbon offsets. However, there is a limit to what can be achieved this way. For the United States to achieve its target of a 7 percent cut in emission from its 1990 level as mandated under the Kyoto Protocol solely through this kind of carbon offset, it would need to plant a forest the size of the state of Texas.
Deforestation
4. This forest was burned to create farmland.
Because trees are a carbon sink, deforestation produces a double whammy. First, many trees are cut down either to serve as fuel or for making paper products, and both intents result in the direct release of much of their stored carbon. Second, cutting down a tree eliminates it as a carbon sink for the future. It is believed that tropical deforestation actually releases 20 percent of the carbon dioxide going into the atmosphere annually, and that figure does not include the loss of carbon sinking capacity. Deforestation is generally slowing around the world, but in recent years some areas have seen massive losses. Nigeria, for example, lost 85 percent of its forests between 1990 and 2005, while Africa in general continues to cut down its trees at double the world's average rate.
