By Laura Mandaro

SAN FRANCISCO (Dow Jones) -- A global economy in tatters, massive U.S. job losses and a plunge in commodity prices have combined to spark an unusually high rate of warnings about corporations' fourth-quarter earnings, data show.

According to Thomson Reuters, the ratio of companies in the bellwether S&P 500 (SPX) index issuing negative outlooks on earnings as opposed to positive ones for the last three months of 2008 reached 4.1 -- double the historical average.

For all North American stocks, the ratio is 3.7, well ahead of the long-term average of 2.2, the provider of financial data said.

The most recent spate of warnings have helped clip a late-2008 rebound in stock prices.

"Pre-announcement activity is starting to cause concern among investors," said Tobias Levkovich, Citigroup's chief U.S. equity strategist, in a note Monday.

"Bellwether companies in retailing, metals and technology are highlighting difficulties and thus weighing on what had been some budding investor enthusiasm," he wrote.

The S&P 500 fell 1.7% in recent action and has lost more than 3% for the year, on top of last year's 38% collapse.

The fourth-quarter earnings season gets its unofficial start later Monday, when Alcoa Inc. (AA) reports financial results after the bell.

The aluminum producer's shares fell more than 7% Monday after Deutsche Bank cuts its rating to sell, citing recently announced production cuts and weaker aluminum prices.

The steady drumbeat of gloomier updates -- from companies ranging from semiconductor maker Intel Corp. (INTC), low-cost retailer Wal-Mart Stores Inc. (WMT) and oil producer Chevron Corp. (CVX) to media conglomerate Time Warner (TWX) and drugstore chain CVS Caremark Corp. (CVS) -- has soured Wall Street's forecasts for the quarter.

Analysts now expect S&P 500 earnings overall dropped more than 15% from the prior year -- a marked reversal from a sharp 46% rebound in profits on Oct. 1, says Thomson Reuters.

With profits in the year-ago quarter hit hard by banking losses, analysts had been expecting that easy comparisons would translate into healthy earnings growth.

Since early October, however, a confluence of negative factors -- more evidence of the severity of the U.S. recession and the scope of a global downturn, mounting domestic job losses and the toughest Christmas retail season since at least 1970 -- emerged to dampen investors' spirits regarding corporate profits.

And oil prices have skidded even further, wiping out more energy profits.

Profits from energy companies, which analysts polled on Oct. 1 thought would rise 22% from the year-ago quarter, are now expected to fall 18%.

Consumer discretionary companies, a sector that includes department-store chains and automakers, are likely to show a profit drop of 56%. At the start of the fourth quarter, analysts forecast a 12% gain for these companies' profits.

"Economic activity and revenue has fallen so quickly that companies are almost in reactionary mode to recalibrate their forecasts for 2009," said Marshall Kaplan, senior equity strategist for Smith Barney, Citigroup's retail brokerage.

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