On Tuesday September 22 the Asian Development Bank said that the economies of developing Asia–that’s Asia excluding Japan, Australia, and New Zealand–would grow by 3.9% in 2009 and by 6.4% in 2010. That would be enough to lead the global economy out of the current slowdown, the bank opined.

The numbers are impressive. So impressive that they reversed Monday’s loss on U.S. markets and led to a resumption of the rally on Tuesday.

But it turns out that there’s a big “if” buried in the fine print of that prediction. And as is so often the case the fine print is about China.

Here’s what the Asian Development Bank said.

First a bit more detail on the headline numbers.

Asian economies excluding Japan, Australia, and New Zealand, will grow by 3.9% in 2009. That’s up from the bank’s call for 3.4% growth for 2009 issued earlier this year. In 2010 those economies will grow by 6.4%. That’s up from an earlier projection of 6% growth.

China will lead the charge, growing by 8.2% in 2009 and 8.9% in 2010.

Second, here’s the fine print.

China’s current growth rate–7.9% in the second quarter–will be sustained, and indeed pick up speed in 2010, if there’s a moderate recovery in the international economy and if China continues its stimulus spending into 2010.

All projections have to be based on some assumptions and there’s nothing wrong with using these.

But investors looking at a stock market thast has already climbed by more than 50% from the March lows and at commodity and materials stocks that in many cases have gained even more should pay attention to the contingent nature of thes projections.

The reason that money is pouring into Asian stock markets is because investors think these economies will grow faster than the U.S. economy for the rest of 2009 and into 2010. They doubt the willingness and the ability of UJ.S.consumers to resume their pre-bust spending spree.

Of course, the bulk of Asia’s fastest growing economies–China most particularly–are built on an export model so the question of when U.S. consumers will start sending again is critical to the ability of these economies to grow by 7% to 8% next year.

And, of course, there’s no guarantee that China will pump another $500 billion in stimulus into its economy next year. This year that money went mostly into infrastructure and investment in plants, real estate, and the stock market.

The package has revived China’s GDP growth but it has further unbalanced an economy already dangerously tilted toward investment and away from consumption. That’s led to potential bubbles in the prices of real estate and stocks. The government has taken these proto-bubbles seriously enough to rattle an interest rate saber at the banks. But the bubble in industrial capacity is perhaps more serious. Tens of billions from the stimulus have flowed into investments in new plant and equipment in industries already suffering from over capacity.

It’s certainly not a given that this investment will produce any positive returns or that the government will be willing to pour more stimulus money down these particular ratholes in 2010.

So, yes, “if” China continues its stimulus sending in 2010 and the global economy is strong enough to absorb some of the goods produced by the new capacity created by this stimulus, then the develoing economies of Asia will lead the global economy out of near recession.

“If” not? Well, then expect new lower projections from the Asian Development Bank come 2010.