>> He wants you to think he's comparing the last 100 years. In reality he's comparing two different time periods because U.S. acceleration ruins the wow factor.
> That's an amazingly pedantic and petulant response to what appeared like a perfectly valid comparison. The US economy became the world's largest one some time in the 1920's, which means at least 70 of the 100 years in the 20th century was when the US was the world's largest economy and growing
According to [1], in 1900 the US had a population of ~76M, China ~496M. And according to [2] the US reached 200M in 1967, 300 2006. In In 2012 China is estimated to have a population of ~1351M, the US: ~318M in 2014.
So lets say the numbers are for 200M people * 100 ~ 20000 "M person years" for the US, and 1300M * 3 ~ 3900 "M person years" for China. So that's 6.6/3.9 "megatons/M person-years" ~ 1.7 for China and 4.5/20 "megatons/M person-years" ~ 0.225 for the US. Now I'm not sure how the original article uses the sources[3], but being conservative, it lists production numbers for US cement in 1997 and 2001 as 75 mega ton and 85 mt respectively. Now, if we take 80 * 3 * 33 we get 7.4 gt, rather more than the 4.5 gt number -- so it would appear most of the US cement was produced in later years, and so multiplying by 100 years might not be correct (and 200 M might be too low wrt population).
All in all, I'd say they're interesting numbers, but rather meaningless without more data.
[1] https://en.wikipedia.org/wiki/List_of_countries_by_populatio...
[2] http://usatoday30.usatoday.com/news/nation/2006-10-17-popula...
[3] http://minerals.usgs.gov/minerals/pubs/mcs/