Habitats for some human populations and many wild animal populations are significantly affected by logging in rain forests. Three major effects on rainforest habitats of extensive logging are destruction, degradation and fragmentation. Destruction is when habitats are rendered completely uninhabitable by a species. Degradation is when the species is placed under significant stress by inadequate breeding locations, inadequate food and inadequate shelter. Fragmentation is when habitats are sliced up and isolated into "islands" around which the margins are dangerous or uninhabitable.
Increased levels of carbon in the atmosphere are believed to be the most significant factor in climate change and global warming. According the Richard Houghton in "Science and Technology," rainforests worldwide constituted 13 percent of the earth's terrestrial surface in 1990. Within that 13 percent area is 40 percent of the world's sequestered carbon. According to a CNN report in 2007, the percentage of terrestrial surface covered by rainforests has shrunk to 6 percent. A rainforest holds an average of 600 metric tons per hectare of carbon, compared to 5 metric tons per hectare on the agricultural land that typically replaces it. In 1990, the loss of rainforests to logging was estimated to cause 35 percent of the world's total atmospheric carbon.
Extensive logging in rainforests removes the protective canopy between rainfall and the ground, it removes the forest mat and root systems that bind the soil, as well as attenuate the rate of runoff for rain water. Exposed topsoil is stripped away, erosion begins to cut into the topography of the ground, and the acceleration of runoff contributes to flooding that destroys more topsoil downstream. Large cleared areas quickly lose soil fertility in tropical areas, leading to abandonment and erosion.
Eroded soil is carried into watercourses, changing sediment levels, stressing aquatic species, and contaminating drinking water. As larger areas are cleared, the lack of evaporation formerly produced by the trees creates a significant reduction in rainfall, and may watercourses shrink or dry up altogether, except during flash floods. In parts of the Amazon rainforest, between 50 and 80 percent of the water in the ecosystem is internally cycled by the ecosystem. In Madagascar, where large areas that once resembled the Amazon were deforested, the land and climate has been converted into a desert by lack of rainfall.