Resource depletion and The Limits to Growth

My reading of Britannia Unchained, on which I posted a few weeks ago,  has  nudged my recent studies on to the subject of resource depletion. The following caught my eye:-

"The Limits to Growth, written by a team of MIT researchers, published in 1972, argued that humanity's food and resources would soon run out. By 1992, the authors predicted, world supplies of zinc, gold, tin, copper and oil could be exhausted." (Britannia Unchained, Location 208 in the Kindle edition, in which page numbers don't show)

Actually, they didn't predict any such thing. 

They presented a table (Table 4, pp56-60) which showed how long known reserves would last given the then current rate of extraction and its assumed rate of growth. They then showed in the same table a figure based on five times known reserves, as a proxy for total world reserves. The remaining years for zinc, for instance, were 18 years (known reserves) and 50 years (five times known reserves). The accompanying text in The Limits to Growth reads:-

"Of course, the actual nonrenewable resource availability in the next few decades will be determined by factors much more complicated than can be expressed by either the simple static index [the years of remaining extraction assuming no growth] or the exponential reserve index [years of remaining extraction assuming exponential growth]."

I was curious to see what has actually happened to mineral reserves since 1970. Using 2010 data from the US Geological Survey and 2011 data from the International Lead and Zinc Study Group, I did the same calculation for zinc and got the following:-
 

	1970	2010
Extraction per year (million tonnes)	5.3	12
Growth rate	2.90%	3.80%
Known reserves (million tonnes)	123	250
Years to depletion	18	15
5 x known reserves (million tonnes)	615	1250
Years to depletion	50	42
Estimated world resource (million tonnes)	n/a	1900
Years to depletion	n/a	51

(1970 figures are from The Limits to Growth.)

Known reserves and the rate of extraction in 2010 were more than twice what they were in 1970, and growth rate of extraction was also higher. Actual world resources of zinc were clearly much higher than five times the known reserves. Even resources remaining in 2010, after 40 years of extraction, are estimated to be fifteen times the known reserves in 1970.

Although the authors of Britannia Unchained were rather sloppy in the way they described what was in The Limits to Growth, they had a point - at least in the case of zinc. My nerdiness doesn't extend to doing the same exercise for the other minerals they mention.

The lesson I draw from this is that, unless zinc is an exception, depletion of minerals generally may not be problematic in the next few years. That leaves energy, water, land and food as possible crunch issues as the world population continues to grow.