China's economy will be bigger than the U.S. economy by 2016, says the IMF, using a special measure of economic size called "purchasing price parity."
Measured in dollars, the U.S. GDP will still be 70% bigger than China's by then, but the trend is unmistakable: At some point, barring a wreck of China's economic freight train, China will eclipse the U.S. to become the largest economy in the world.
What will this mean?
Well, psychologically, it will mean that Americans will have to get used to the idea that they're not the biggest kid on the block anymore. For most participants in the global economy, China will have become the most important trading partner and market, and China's economic policies, not America's, will dominate the global conversation.
Similarly, and perhaps more momentously, China's military will eventually be larger than the U.S.'s, which will mean the U.S. will no longer be the most powerful country in the world. It's easier to have influence over other countries when you can speak softly and carry a big stick. Once China vaults past the US economically, it will have the wherewithal to carry an even bigger stick--unless the US wants to continue to bankrupt itself by outspending everyone else despite its smaller GDP.
Adding to the change in fortunes is the fact that America's financial situation is vastly weaker than China's: We have a huge deficit and massive debts; China, meanwhile, funds our consumption (by lending us money) and runs a surplus. Over the next couple of decades, therefore, much of the US resources that might have gone into growth and infrastructure spending, will be used instead on interest payments, budget cuts, and social programs. This will likely make the US feel poorer, especially relative to the explosive growth in China.
Regardless of when the exact date comes, in other words, we're headed for a new world order--one in which China, not America, will be the center of the world.
See Also: 15 Facts About China That Will Blow Your Mind