The natural greenhouse effect has in the past maintained temperatures in a range suitable for life. Without the greenhouse effect, the Earth would alternate between extremely hot and very cold conditions. Most mainstream scientists agree that human activity during the past 100 years has led to a 25 percent increase in atmospheric carbon dioxide levels, leading to an enhanced greenhouse effect. Most of the increase has been due to the burning of fossil fuels, although deforestation, livestock and wetland changes have also contributed. Human industrial activity has also contributed to higher atmospheric concentrations of greenhouse gases other than carbon dioxide.
Oil, coal and natural gas are each formed through the decomposition of carbon-based organic matter. When these materials are used to produce energy, much of the carbon gets released into the atmosphere. Scientific measurements indicate that atmospheric carbon dioxide has risen from a historic average of approximately 260 to 280 ppm (parts per million) to around 380 ppm as of 2009. Currently the world satisfies approximately 85 percent of energy needs through fossil fuels. Given economic growth considerations and a lack of alternative large-scale energy sources, fossil fuel use is certain to continue into the foreseeable future, leading to yet higher concentrations of carbon dioxide in the atmosphere.
As forests contain large repositories of carbon, deforestation unlocks much of this carbon into the atmosphere. Livestock are one of the biggest contributors to greenhouse emissions. The process of raising livestock results in increases in carbon dioxide, methane and nitrous oxide, accounting for a larger percentage of greenhouse emissions than the transportation sector. Many natural processes also contribute greenhouse gases to the atmosphere, including volcanoes and natural organic decay.
Water vapor is the most common greenhouse gas in the atmosphere, and accounts for most of the greenhouse effect. As global warming increases temperatures, increasing amounts of water evaporate. Furthermore, the atmosphere's carrying capacity for water vapor increases as temperatures rise. Therefore, an increase in atmospheric water vapor occurs as a collateral consequence of increased human-caused emissions, resulting in a feedback loop that speeds the process of global warming.