Forests consume greenhouse gases and emit oxygen. Deforestation is one component of climate change. And it’s the product, by-and-large, of overpopulation.
The other obvious result of overpopulation around the globe is desertification. I grew up on land that was covered in grass in the 19th century. We have photos to prove it. By the time I walked there, however, only a few tufts of grass remained between the mesquite, sagebrush and creosote bushes. I’m an eye-witness to desertification. My predecessors moved on to the land with domesticated cattle, sheep, horses and goats. They subdivided it, homesteaded it, fenced it and then tried to make a living on it. There aren’t many animals there today but the grass isn’t coming back very fast. The United States Geological Survey cites the Rio Puerco basin in my own home state of New Mexico as a prime example of desertification. The process is not mysterious. Semi-arid grasslands sustain themselves through droughts by maintaining a dense mat of roots mixed with dormant seeds at the surface of the soil. In natural conditions, a grassland can lie dormant for years. When it is overgrazed, though, the mouths and hooves of the hungry foragers destroy the grass roots. Dormant seeds provide a little sustenance for desperate animals. When the rains come back, there are no roots to hold the soil in place or to regenerate the grassland. There are no dormant seeds to bring forth new life. The rain washes soil away. The land around the Rio Puerco is grotesquely eroded. The river itself is full of silt.
Desertification became well known in the 1930s, when parts of the Great Plains in the United States turned into the "Dust Bowl" as a result of drought, overgrazing and bad agricultural practices. We’ve learned to manage the land better and we reversed the desertification of the plains, but elsewhere the desert marches on, especially where people have no other option than to push their herds to the next patch of grass. The famine that periodically afflicts sub-Saharan Africa is, primarily, the result of desertification, which is a result of overpopulation, which in turn aggravates the severity of the famine. By 1973, the drought that began five years earlier in the Sahel of West Africa and the land-use practices there had caused the deaths of more than 100,000 people and 12 million cattle.
Droughts do not cause desertification. Droughts are common in arid and semiarid lands. Well-managed lands with intact root systems and dormant seed cover will revive when the rains return. Continued land abuse during droughts, however, prevents that recovery.
 United States Geological Survey. Desertification. http://pubs.usgs.gov/gip/deserts/desertification/