WASHINGTON — When Nouriel Roubini talks, the world listens. Roubini is, of course, the once-obscure New York University economist whose dire warnings about a financial crisis proved depressingly prophetic. Last week, Roubini was shouting. Writing in The Financial Times, he warned that the Federal Reserve and other government central banks are fueling a massive new asset "bubble" that — while not in imminent danger of bursting — will someday do so with calamitous consequences.

Here's Roubini's argument. The Fed is holding short-term interest rates near zero. Investors and speculators borrow dollars cheaply and use them to buy various assets — stocks, bonds, gold, oil, minerals, foreign currencies. Prices rise. Huge profits can be made.

But this can't last, Roubini warns. The Fed will eventually raise interest rates. Or outside events (a confrontation with Iran, fear of a double-dip recession) will change market psychology. Then, investors will rush to lock in profits, and the sell-off will trigger a crash. Stock, bond and commodity prices will plunge. Losses will mount, confidence will fall and the real economy will suffer.

"The Fed and other policymakers seem unaware of the monster bubble they are creating," writes Roubini. "The longer they remain blind, the harder the markets will fall."

Haven't we seen this movie before? Well, maybe. Like home values a few years ago, asset prices have risen spectacularly. Since its March 9 low, the U.S. stock market has gained more than 50 percent. An index of stocks for 22 "emerging market" countries (including Brazil, China and India) has doubled from its recent low. Oil at about $80 a barrel has increased 150 percent from its recent low of $31.

Gold is near an all-time high around $1,090 an ounce. Meanwhile, the dollar has dropped against many currencies. Half of Roubini's story resonates.

But the other half is less convincing: that prices, driven by cheap loans, have reached speculative levels. Remember that the economy seemed in a free fall early this year. Terrified consumers and cautious companies hoarded cash, cut spending and dumped stocks. Since then, the mood and economic indicators have improved. Higher stock and commodity prices have mostly recovered the big losses of those panicky months. Today's prices are usually below previous peaks. Oil's peak was nearly $150 a barrel.