When solar energy reaches the Earth, it is converted into heat, which then dissipates, especially at night. Greenhouse gases in the atmosphere absorb this heat and re-radiate it in all directions. The resulting cycle of absorbed heat between the Earth's surface and the atmosphere maintains the planet's temperature. Due to higher concentrations of greenhouse gases, particularly carbon dioxide from the burning of fossil fuels, global average temperatures have risen during the past century by approximately one degree Fahrenheit. According to climate models, temperatures are expected to rise an additional 4 to 10 percent during the next century due to continued human emissions of greenhouse gases.
The enhanced greenhouse effect produces several positive feedback loops in the environment that may accelerate the warming process in the future. While most human-caused greenhouse emissions are in the form of carbon dioxide, methane and nitrous oxide, the warming produced by these gases causes additional evaporation of water, resulting in higher atmospheric levels of water vapor. This extra water vapor causes the atmosphere to retain yet more heat. Similarly, as the climate warms quickly, arctic permafrost may melt, releasing significant amounts of stored methane. Global warming may also destroy wetlands, which hold large amounts of terrestrial carbon. As the wetlands become drier, this plant matter will decompose, with the carbon combining with oxygen to produce carbon dioxide.
Rapidly warming temperatures are likely to result in changes to ecosystems, as individual species become unable to survive in progressively warmer temperatures. While some species will be able to migrate to areas with suitable climates, many will simply die off, resulting in ecosystems with less biodiversity. Since ecosystems rely on food chains and other complex interspecies relationships, loss of biodiversity is a serious concern. Global warming can also encourage the proliferation of invasive species, which potentially degrade ecosystem integrity.
Rising global temperatures cause gradual melting of ice caps and glaciers, resulting in rising sea levels. During the past century, ocean levels rose by between 4 and 8 inches. During this century, climate models predict sea levels to rise by between 1 to 3 feet, to levels that could potentially flood many coastal cities. Even more worrisome is the potential collapse of the Greenland ice sheet or West Antarctic ice sheet, which could result in a sea level rise of 30 feet or more. Such a rise would submerge many islands and large continental areas along the coast, displacing millions of people.