USA referaat
aspx?TY=E&IN=03&YR=2009
http://www.fao.org/forestry/32032/en/
http://www.fao.org/forestry/country/19971/en/usa/
http://www.fao.org/forestry/country/32185/en/usa/
http://www.intracen.org/appli1/TradeCom/TP_EP_CI.aspx?RP=842&Yr=2009
http://www.intracen.org/appli1/TradeCom/TP_IP_CI.aspx?RP=842&YR=2009
http://www.intracen.org/appli1/TradeCom/TP_IP_IC.aspx?TY=I&IN=03&YR=2009
https://www.cia.gov/library/publications/the-world-factbook/geos/us.html
http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1755
http://instruct.uwo.ca/biology/320y/climveg.html
http://upload.wikimedia.org/wikipedia/commons/e/e4/Vegetation.png
https://www.cia.gov/library/publications/the-world-factbook/rankorder/2119rank.html?
countryCode=af&rankAnchorRow=#af
http://2010.census.gov/2010census/
http://www.photius.com/rankings/world2050_rank.html
http://www.photius.com/rankings/geography/population_density_persons_per_sq_km_2010_
0.html
http://www.nationmaster.com/time.php