NYT columnist Paul Krugman explains why he thinks the U.S. housing bubble is deflating.

Some excerpts:

This is the way the bubble ends: not with a pop, but with a hiss.

Housing prices move much more slowly than stock prices. There are no Black Mondays, when prices fall 23 percent in a day. In fact, prices often keep rising for a while even after a housing boom goes bust.

So the news that the U.S. housing bubble is over won't come in the form of plunging prices; it will come in the form of falling sales and rising inventory, as sellers try to get prices that buyers are no longer willing to pay. And the process may already have started.

Of course, some people still deny that there's a housing bubble. Let me explain how we know that they're wrong.

One piece of evidence is the sense of frenzy about real estate, which irresistibly brings to mind the stock frenzy of 1999. Even some of the players are the same. The authors of the 1999 best seller "Dow 36,000" are now among the most vocal proponents of the view that there is no housing bubble. ...

Krugman broke the U.S. market out into two components: The flatland zone and the zoned zone.

In the flatlands, it's easy to sprawl, so it's hard to get a bubble going. But in a coastal city like San Diego, it's harder to build new homes to meet demand, so it drives all prices up. And if mortgage rates fall, then people are willing to spend even more, believing their property is constantly rising in value.

Bubbles end when people stop believing that big capital gains are a sure thing. That's what happened in San Diego at the end of its last housing bubble: after a rapid rise, house prices peaked in 1990. Soon there was a glut of houses on the market, and prices began falling. By 1996, they had declined about 25 percent after adjusting for inflation.

And that's what's happening in San Diego right now, after a rise in house prices that dwarfs the boom of the 1980's. The number of single-family houses and condos on the market has doubled over the past year. "Homes that a year or two ago sold virtually overnight - in many cases triggering bidding wars - are on the market for weeks," reports The Los Angeles Times. The same thing is happening in other formerly hot markets.

Meanwhile, the U.S. economy has become deeply dependent on the housing bubble. The economic recovery since 2001 has been disappointing in many ways, but it wouldn't have happened at all without soaring spending on residential construction, plus a surge in consumer spending largely based on mortgage refinancing. Did I mention that the personal savings rate has fallen to zero?

Now we're starting to hear a hissing sound, as the air begins to leak out of the bubble. And everyone - not just those who own Zoned Zone real estate - should be worried.