While it's hard to ascertain for sure if this would really happen, it's surely fun to speculate. And speculations over such disruption caused by over-populous China have been many. One movie even depicted an obese kid coming up with the fantastic idea of throwing the earth out of its orbit by asking the Chinese to jump for his school project! Even Calvin couldn't measure up against this one!
Now, Scientific American finally puts the speculations to rest with a report on an experiment conducted to predict what would really happen. The result was an anti-climax for anarchy enthusiasts.
In continuation of the spirit of what-if, here's my question for the 21st century. What if all the people of India and China jumped together? I predict at least a flattened Manhattan!