So the rise of computer technology poses a threat that previous generations of machines didn't: The old machines replaced human brawn but created jobs that required human brains. The new machines threaten both.

"Technological change is more encompassing and moving faster and making it harder and harder to find things that people have a comparative advantage in" versus machines, says David Autor, an economist at the Massachusetts Institute of Technology who has studied the loss of midpay jobs to technology.

Here are the three scenarios that economists and technologists offer about jobs in the future:

â¿¿THE ECONOMY RETURNS TO HEALTH AFTER A WRENCHING TRANSITION

It has always happened before. Europe and the United States endured repeated economic and social upheaval during the 19th and early 20th centuries as their agricultural economies transformed into industrial ones. Columbia's Stiglitz argues that such pressures led to the collapse of the world economy in 1929 â¿¿ the cataclysm we call the Great Depression.

The mechanization of farming caused agricultural production to soar worldwide in the 1920s â¿¿ and prices to plunge. In the U.S., crop and livestock prices fell by 50 percent between 1929 and 1932. American farmers, who accounted for a fifth of the U.S. workforce, lost purchasing power and also struggled to pay their mortgages and other loans. As their debts went bad, banks began to collapse, squeezing credit and spreading panic. The economy went into free-fall.

Only World War II â¿¿ and the massive rearmament program it required â¿¿ restored the U.S. economy to full health. The experience was traumatizing. And today only 2 percent of Americans work on farms.

"Economies don't make these transitions well," Stiglitz says. People in the dying parts of the economy can't afford to invest in the education or retraining they need to find different work. "So you get workers trapped in the wrong sectors or unemployed," he says.

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