Solar energy reaches the Earth in a wide range of wavelengths. Most solar light energy is absorbed by the Earth, transformed into heat, and re-radiated back toward space. Greenhouse gas molecules absorb this heat, and again re-radiate it, creating a continuous process of heat cycling back and forth between the Earth's surface and the atmosphere. Without the greenhouse effect, the planet would be much colder, with average temperatures swinging more widely between day and night.
Human greenhouse gas emissions during the 20th century have significantly altered the composition of the atmosphere. During the last hundred years, atmospheric carbon dioxide has risen by approximately 35 percent, mainly as a result of burning fossil fuels and deforestation. Methane levels have also risen significantly due to natural gas drilling and the digestive processes of livestock, particularly cows. The result is an enhanced greenhouse effect which increases the amount of trapped heat in the atmosphere.
Global warming is the rise in average global temperatures that is occurring due to the enhanced greenhouse effect. Climate scientists estimate that average temperatures rose approximately one degree Fahrenheit during the past century. Various estimates for the next century forecast a rise of between four and ten degrees Fahrenheit. Rising temperatures place strains on ecosystems, as individual species are unable either to tolerate higher temperatures or to migrate to more suitable climates, resulting in loss of biodiversity. Global warming is also likely to result in a significant rise in sea levels as ice caps melt, threatening coastal cities and low-lying island communities.
Global warming results in a number of synergistic consequences that further enhance the greenhouse effect. For instance, water vapor is the most common greenhouse gas in the atmosphere. As global temperatures rise, more water evaporates from the oceans, and the atmosphere is able to hold greater concentrations of water vapor, creating a positive feedback loop. Similarly, large quantities of methane are locked in the permafrost of Arctic regions. The melting of permafrost causes this methane to be released into the atmosphere, further enhancing the greenhouse effect.