Well, today they’re not, they seem to be plummeting. Still they have been rising rapidly for years. Paul Krugman surveys some views, click through to the Frankel post as well. Yes I do think high and rising commodity prices have been a bubble — but not just a bubble — and no I don’t think that low real interest rates are much of a factor. (Recall Cowen’s Third Law: "All propositions about real interest rates are wrong.")
My basic explanation for rising commodity prices is simple. Most commodities are produced under conditions of short-run rising costs, often quite steeply rising short-run costs. Furthermore many production processes cannot do without these commodities in the short run. Coal, copper, and the like are not always easily substitutable for a factory within the medium run. (Furthermore until you are sure that the price increase is permanent, why re-gear at all? Why switch from copper plumbing to plastic plumbing, when price of copper might fall again?)
Now China has become wealthy quite fast but the country didn’t become wealthy by producing more commodities. That’s Albert Hirschman’s "unbalanced growth." So demand for most commodities has outstripped the supply, production can’t make up the difference in the short run, and commodity prices can rise sharply. Don’t forget that logistics and transport are a big part of the production process and so infrastructure often constrains the flow of supply.
In the long run price will adjust (even if you believe we are near "peak oil" this is true for most commodities.) People will substitute or find new sources of the commodity or find new ways of producing the commodity more efficiently. Infrastructure improves. But yes those adjustments can take ten years or more. And in the meantime we have a commodity price boom and on top of that a bubble to make these items look even more expensive.
One final kicker: lots of commodities are produced by governments and/or their production is heavily controlled by governments, most of all oil. Then supply adjustments will be especially slow and cumbersome. Read this article about coal:
…94 percent of India’s coal mining is in the hands of government-owned companies. The biggest, Coal India, produces four-fifths of the country’s coal. Because the government is worried about social unrest, the prices for coal and electricity are kept low.
See the problem?
The bottom line: The best long-run bet is still that there is nothing special about risk-adjusted rates of return on commodities. That probably means falling real prices and falling real costs over time. The Chinese demand aberration is a temporary blip superimposed on very consistent longer-run trends.