Global warming is the increase in the average temperature of the Earth's surface (Mastrandrea n. pag.). According to scientists, humans are the main cause of global warming. Scientists say that global carbon dioxide emissions should be cut in half over the next fifty years, otherwise the effects could be irreversible (Eilperin n. pag.). The two main human activities that contribute to global warming are the burning of fossil fuels and land clearing. Most fossil fuels come from combustion in automobiles, factories, and power plants that provide energy to homes and office buildings (McClure n. pag.). Continued global warming may harm plants and animals that live in the sea and force land plants and animals to move to new habitats. It could also change weather patterns, causing floods, droughts and an increase in devastating storms. Global warming could melt polar ice caps enough to raise sea levels, and in some parts of the world, human diseases could spread and crops could decline (Mastrandea n. pag.). Additionally, during photosynthesis, processing trees and other plants are used to make food; they remove carbon dioxide from the atmosphere. Land clearing contributes to an increase in carbon dioxide by decreasing the rate at which it is removed from the atmosphere during photosynthesis. Some scientists say that increasing greenhouse gases doesn't make much difference to temperature. Instead, they believe that natural processes may have caused global warming. These processes include the increase in energy emitted by the sun. But most climate scientists believe that the increase in solar energy has contributed little to global warming. (Mastrandea n. pag.). However, World War...... middle of paper ......ld Book, Inc.McClure, Robert and Lisa Stiffler. “Scientists agree: humans are causing global warming.” Seattle News, Sports, Events, Entertainment | Seattlepi.com - Seattle Post-Intelligencer. February 2, 2007. Web. November 21, 2010. .Shah, Anup. “Climate Change and Global Warming – Global Issues.” Global Issues: Social, political, economic and environmental issues that affect us all — Global Issues. September 27, 2010. The web. November 27, 2010. .United Nations Environment Program - World Meteorological Organization. "What human activities contribute to climate change? Based on common questions about climate change." What's new? 1997. Internet. November 21. 2010. .