first of all, all plane travel in the world wasn't stopped on 9/11. Only the plane travel in a few countries. And even if it was, it wouldn't make the earth's temperature drop a few degrees. An instand few degree drop in temperature would be catastrophic. There would be huge storms across the world, ocean currents would get messed up, the climate wouldn't recover for hundreds of years. In the past hundred years, the earth's temperature has only risen a few degrees.
Secondly, more and more research is showing that the single biggest thing humans have done to advance global warming was start farming 8000 years ago. Three papers published in the Journal Nature in the past two years and two papers published in the Scientific American this year support this theory. After the beginning of agriculture, humans have been very slowly increasing global warming (obviously more and more in recent years) compared to the damage done by churning up the earth and .
Now, this is only a theory, but it is becoming an increasingly popular theory in the scientific community about the roots of global warming.