Global warming is an increase in global average atmospheric temperature that results in corresponding climate changes, possibly a result of the greenhouse effect. The greenhouse effect is a result of increased levels of greenhouse gases as a consequence of human industry and agriculture.
There have been numerous global warming periods in Earth's history. The long-term effects expected from the global warming in the 21st century include heat waves, drought, flooding, sea levels rising, melting of glaciers and polar ice caps, precipitation and temperature fluctuations, forest fires, and stronger La Niña and El Niño weather patterns. According to the National Resources Defense Council, climate changes will directly affect water supply sustainability throughout the United States in future decades.