19. What is the Carbon Cycle
    The cycling of carbon from the atmosphere to organic compounds and back again not only involves life but also involves the atmosphere. The carbon cycle describes the pathway that carbon takes..
    http: //education.arm.gov/studyhall/globalwarming/e...
    
    
20. Climate Change
    The Climate Change site focuses on the science and impacts of global warming or climate change, and on actions by governments, corporations, and individuals that help address global warming issues. The site also features climate change related news, events, publications, reports, presentations, and links to other sites.
    http: //epa.gov/climatechange/index.html
    
    
21. Global Warming: U.S. EPA Student Center
    The earth’s climate is predicted to change because human activities are altering the chemical composition of the atmosphere through the buildup of greenhouse gases—primarily carbon dioxide, methane, and nitrous oxide.
    http: //epa.gov/kids/
    
    
22. Plate Tectonics
    This site explains the theory of plate tectonics.
    http: //cotf.edu/ete/modules/msese/earthsysflr/...
    
    
23. Global Warming Unit
    This unit includes classroom activities to help students understand global warming and its possible effects on human beings. Middle-High school level
    http: //commtechlab.msu.edu/sites/letsnet/noframes/...
    
    
24. Global Warming -- Research Issues
    To what extent does the Sun's variability affect and/or cause global climate change?
    http: //solar-center.stanford.edu/sun-on-earth/glob...
    
    
25. Global Warming: Focus on the Future
    http: //envirolink.org/
    
    

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