1. World population clock – world meters". Worldometers.info. Retrieved April 12, (2016).
2. Population seven billion: UN sets out challenges". BBC. October 26, 2011. Retrieved October 27, (2011).
3. J. Coleman (October 31, 2011). "World's 'seven billionth baby’ is born". The Guardian (London). Retrieved October 31, (2011).
4. 7 billion people is a 'serious challenge'". UPI. October 31, 2011. Retrieved November 9, (2011).
5. Life cycle cost analysis of alternative vehicles and fuels in Thailand;Goedeckeb;Energy Pol,2007