If the area is not reforested, there is a net buildup in carbon dioxide. Worldwide deforestation currently accounts for roughly one-fourth of the carbon dioxide released into the atmosphere. Although deforestation is primarily a problem in the tropics, industrial countries are still the primary source of carbon dioxide, due to the use of fossil fuels and greenhouse gases such as chlorofluorocarbons and nitrous oxide. Such a change would cause a major disruption of natural ecosystems and agriculture and result in the flooding of coastal areas. Reversing the deforestation of the earth can slow global warming as trees take carbon dioxide out of the atmosphere, thus serving as a sink for excess carbon dioxide.
Trees affect regional climates because of their part in the hydrological cycle.
In one study, researchers found that three quarters of the rain that fell in a forested region of the Amazon Basin was returned to the atmosphere through evaporation and transpiration from the trees, with only one-quarter of the rainfall leaving the site as runoff into the streams to return to the ocean. The moisture recycled into the atmosphere replenishes clouds, moving the precipitation further inland. In vast areas of the interior of Africa, shrinking tree cover has seemingly weakened this link in the hydrological cycle, resulting in reduced rainfall and more frequent drought. The increase in runoff from deforested slopes is a disaster for communities downstream. Without the trees to slow runoff, the rain carries away topsoil and can cause flooding.