One of your classmates says that the pollution from his car doesn't harm the environment because the exhaust is warm and warm air will rise out of the atmosphere since it is less dense. Is your friend correct? Why or why not?

Unfortunately no, my friend is not correct. The CO2 pollutants leaving his exhaust are warm, yes, but that is why our atmosphere is creating global warming which is affecting climate in all regions. When there is more people that means more cars and more pollutants being dispersed into our atmosphere.