Many investors know about the key metrics that provide clues about the U.S. economy's health, and where it's likely to head in the near term. Retail sales, housing starts, UPS (NYSE: UPS) and Fed Ex (NYSE: FDX) deliveries and, of course, those infamous corrugated box orders, all provide clues about demand at the retail and wholesale levels, and are positively correlated with increases in U.S. GDP.

What's another metric worth monitoring? Office rents -- and here, like the recent statistics reported for the above metrics, the numbers are not good.

Office rent charges per square foot in Midtown Manhattan, London's West End, and Central Tokyo fell in Q3 for the first time since 2002, a report from CB Richard Ellis indicated Tuesday (pdf).

In New York, Midtown Manhattan office rents fell 2.7% to $98.08 per square foot. Midtown Manhattan includes the headquarters of many of the world's multinational corporations and the Broadway theater district, but does not include the Wall Street financial district, which is located in Lower Manhattan.

In London's West End, occupancy costs declined 5.1% to $248.66 per square foot. In Central Tokyo, costs dropped 5.3% to $184.26 per square foot.

Economist Richard Felson told BloggingStocks Tuesday the fact that rents are declining the world's top three financial centers for the time since 2002 is not a good sign. "It's further evidence of the spread of the recession," Felson said. "Very rarely do you see rates in all three cities declining at the same time. Then again, very rarely do you see all three regional economies in recession at the same time, either, which shows you the fix we're in."