The Times today reports that the amount of commercial real estate on the market in Manhattan has doubled since last year, and that rents are down “20 to 30 percent from the going rents at the end of the summer — to around $75 to $80 a square foot annually in Midtown and around $45 a square foot downtown.”

Behind those numbers is a big (new) problem for the financial industry. During 2006 and the months surrounding it in the preceding and succeeding year, financing for office towers was done not on current rents — but on the expectation, often, of double-digit rent increases, year after year after year. Each of these expected double-digit annual rental increases, sometimes over a decade or more, was already priced into the value of the building by office-tower buyers and their financiers.

No, Manhattan didn’t see the same level of speculative building during the 2000s that it did in the Eighties. But back then, the immediate Enron-style valuation of future price increases into today’s price was nowhere near as prevalent as it was two and three years ago.

The financial hit from this invisible speculative building could be just as bad as the financial hit from glass-and-steel speculation was two decades ago.