Human Population Growth and the Mass of the Earth

Albert A. Bartlett, late Professor Emeritus of Physics at the University of Colorado, spent the latter half of his life alerting both the community of physicists and the wider society to implications of unchecked exponential growth. Among the areas he addressed were the effects of exponential growth in human papulations and, as an illustration, calculated when the mass of human beings on Earth would equal that of the Earth itself. In the present work we refine this analysis to take into account the fact that human beings are made of Earth material and that total human mass grows at the expense of Earth mass. Assuming continued exponential population growth we find the time for this equality of masses to be substantially shorter than that calculated Bartlett's method [1,2]. Other growth scenarios such as a linear projection tangent to the exponential curve are also discussed. We hope that, with Bartlett's passing in 2013, a new generation of physicists will continue this educational effort.\\ \\ $[1]$ A. A Bartlett, Am. J. Phys. \underline{46}, 887 (1978).\\ $[2]$ D. W. Kraft, \underline {http://asee-ne.org/proceedings/2014/index.htm}.