In a 2012 paper for the United Nations/DESA – Super-cycles of commodity prices since the mid-nineteenth century – Bilge Erten and José Antonio Ocampo review the literature on the theory of Commodity Super-Cycles and go on to suggest that the current cycle began in 1999. Here is an extract from their concluding remarks:-
The decomposition of real commodity prices based on the BP filtering technique provides evidence of four past super-cycles ranging between 30 to 40 years. For the total real non-fuel commodities, these cycles have occurred (1) from 1894 to 1932, peaking in 1917, (2) from 1932 to 1971, peaking in 1951, (3) from 1971 to 1999, peaking in 1973, and (4) the post-2000 episode that is still ongoing. These long cycles, which possess large amplitudes varying between 20 to 40 percent higher or lower than the long-run trend, are also a characteristic of sub-indices. Among the agricultural indices, the tropical agriculture exhibits super-cycles with much larger amplitude relative to non-tropical agriculture. The amplitudes of super-cycle components of real metal and crude oil prices are comparable to those of agricultural products in earlier parts of the twentieth century, but they become much more pronounced and strong in the latter parts of the century. The presence of co-movement among non-fuel commodity indices is supported by the correlation analysis across the entire sample, and a marked co-movement between oil and non-oil indices is present for the second half of the twentieth century.
Another important finding of the paper is that, for non-oil commodities, the mean of each supercycle has a tendency to be lower than that of the previous cycle, suggesting a step-wise deterioration over the entire period in support of the Prebisch-Singer hypothesis*. This finding applies especially to tropical and non-tropical agricultural prices, as well as metals in previous cycles.
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