Greenhouse gases, including water vapor, carbon dioxide and methane, are gases that absorb and reradiate heat within the atmosphere. The natural greenhouse effect has in the past maintained global temperatures at a level suitable for human life. Since the age of industrialization, the burning of fossil fuels and deforestation have contributed to a rapid rise in atmospheric carbon dioxide level, leading to increasing retention of heat. Rising temperatures can produce natural feedback loops which release additional carbon into the atmosphere, such as the cycle that occurs during the melting of arctic tundra.
The rise in global temperatures increases the melting rate of glaciers, ice caps and ice sheets. During the past century, oceans have risen from 4 to 8 inches, with the rise expected to accelerate during the next century. Some estimates predict a rise of up to 3 feet during the next hundred years. People living on islands and river deltas or in coastal cities will be particularly vulnerable to any rise in sea level, which could displace millions of people.
Global warming poses a serious threat to global biodiversity. As temperatures rise, some species will not be able to adapt to the change or migrate to an appropriate climate zone. The loss of a significant number of species directly due to warming can cause breakdown in the ecological balance and food chain, resulting in collateral loss of further species. Some scientists theorize that past mass extinctions have occurred due to climate change, notably during the Triassic period 200 million years ago and the Permian mass extinction 251 million years ago.
Global warming likely will result in increased transmission of specific diseases, as disease vectors in warmer climates spread out in a larger geographic range. Varieties of mosquitoes that carry malaria and dengue fever require warmer temperatures to survive. According to the Union of Concerned Scientists, a rise in global temperatures of 3 degrees could increase annual malaria rates by as much as 50 million to 80 million. Other diseases that threaten to become more prevalent with rising temperatures include cholera, Lyme disease, parasitic infections, tuberculosis and yellow fever.