I am certainly old enough to remember a variety of doomsday scenarios, usually built around decreases in agriculture productivity or population growth (or both). There have also been some modern scholarly papers that posit that things are going to get very very difficult indeed really soon. See, for example, http://www.cnr.uidaho.edu/sei_501/pdf%20files/MA%20Stocking_2003.pdf, http://www.springerlink.com/content/j684651843716477/, and http://www.springerlink.com/content/u1l8x71325166654/.
Advocating that we are facing an unusual coalescence of factors is a recent column by Thomas Friedman. It makes for an interesting read, even if you find his thesis of questionable merit. See http://www.nytimes.com/2011/06/08/opinion/08friedman.html?_r=1&partner=rssnyt&emc=rss.
I have to admit that when I read such columns, I cannot help but to recall one of my favorite ditties from ages gone by. See http://www.youtube.com/watch?v=L8-BI89mb9A.