How much carbon dioxide is produced by bus each year?

Some buses are electric, and the CO2 emitted by their operation would come from coal fired power plants. West coast electric buses would receive hydrologic power, emitting very little CO2.

Most buses are diesel powered, and average about 6 mpg. Since automobiles get about 20 mpg, a bus needs to carry 4 passengers to break even. More than that and the bus gains in efficiency over the typical passenger automobile. One gallon of fuel emits close to 20 pounds of CO2, even though the fuel itself weighs only 6.25 pounds. This is because the carbon in the fuel combines with oxygen in the air--a LOT of it.

So 20 pounds per gallon times a gallon every six miles equals 3.3 lbs per mile. The US Federal Transit Authority estimates each bus averages 37,000 miles per year. So that amounts to 123,000 lbs (62 tons) of CO2 per bus per year.

For the next part of the question we need to know how many buses there are. There are 450 to 480 thousand school buses in the US (close to half a million). The US probably has fewer buses per capita than most nations on earth, which rely more heavily on public transportation.