Four "grey swans" threaten the global economy, WPP Group CEO Martin Sorrell said, noting they are grey because they are known. They are the eurozone crisis, political concerns in the Middle East, the U.S. economy and China's economic uncertainty.

The “real issue” is the fiscal cliff, the large tax increases and spending cuts scheduled to go into effect next year in the United States, he told CNBC. Resolution of the fiscal cliff would improve the outlook, but Congress might remain deadlocked after the election no matter who wins.

The grey swans prompted WPP, a U.K.-based marketing and communications firm, to cut its revenue outlook for the second time this year. The company's third-quarter revenue growth slowed, as those economic difficulties continued to squeeze customer budgets, according to CNBC.

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The company, which offers advertising and other media services, reduced its full-year revenue growth forecast from 3.5 percent to 2.5 to 3 percent.

Most quarterly earnings reports for the third quarter have missed forecasts, Sorrell noted, predicting the downward trend won't end anytime soon.

"Overall we're more cautious than we have been," he told CNBC, saying the cautious atmosphere will continue into 2013. "At the moment, there's a tremendous degree of caution."

Monetary easing by central banks makes the markets feel better, but is little help to those in "the real world," he told CNBC.

Nevertheless, there are economic bright spots, he pointed out. "Asia, Africa and the Middle East continue to be better than expected, and the global economic slowdown hasn’t impacted Asia and Latin America as much as we thought it would."

Many economists warn that the United States will fall into a recession if it goes over the fiscal cliff, but Sen. Johnny Isakson, R-Ga., predicts Congress will reach a last-minute deal before the end of the year to extend the Bush tax cuts and postpone at least some of the spending cuts, according to Bloomberg.

“I don’t think we’re going off that cliff,” Isakson told Bloomberg. “When it’s 48 hours from going home for Christmas, we can do about anything.”

Editor's Note: Economist Unapologetically Calls Out Bernanke, Obama for Mishandling Economy. See What They Did