Human-caused carbon dioxide emissions are the single greatest contributor to global warming. The concentration of atmospheric carbon dioxide has risen by 35 percent during the past 150 years, caused primarily by the burning of fossil fuels and deforestation. Fossil fuels, including oil, coal and natural gas, are formed through the decomposition of buried plant material over millions of years. Burning the fuels releases the stored carbon into the atmosphere and accounts for two thirds of human-caused carbon dioxide emissions.
One third of human-caused carbon dioxide comes from deforestation. As with fossil fuels, deforestation releases carbon that has been stored in plant matter, which combines with oxygen to form carbon dioxide. Deforestation is occurring at a rapid pace in tropical regions, including Brazil, Indonesia and many countries in Africa, mainly due to pressure from logging, agriculture and ranching. As of 2011, the world has lost approximately half of its old-growth forests, with approximately 30 million acres more cut down every year.
Most human-caused methane emissions result come from livestock and natural gas drilling. Livestock, particularly cows, produce methane during digestion, accounting for approximately 40 percent of human-caused methane emissions. The decomposition of livestock manure also contributes significant amounts of nitrous oxide to the atmosphere. Methane, which is the main component of natural gas, is also released to the atmosphere during the drilling process. Large quantities of methane are trapped in the the arctic permafrost. Melting permafrost due to global warming is likely to contribute large amounts of atmospheric methane in the future.
Water vapor is the gas most responsible for retaining heat in the atmosphere, since it has the highest concentration of the greenhouse gases. As temperatures rise due to higher levels of other greenhouse gases, water evaporates at a higher rate from the ocean, and the atmosphere can hold a higher volume of water vapor. Therefore, global warming produces a positive feedback loop, with increased evaporation enhancing the greenhouse effect.