Also how that statistic has changed over the past 12,000 years or so (by Paul Chefurka, with data from Vaclav Smil):

<http://peakoilbarrel.com/wp-content/uploads/2016/01/Terrestr...>

Details:

<http://peakoilbarrel.com/confessions-of-a-doomer/>

<https://www.greenpeace.org/international/story/17788/how-muc...>