This is confusing. People still need to get to the resort. If not by bus, it will be by car or van. The emissions aren't going away, they are just being traded to a different vehicle. One could argue this move would cause more emissions at times as one bus is unlikely to produce as much emissions as 20 cars would.
Just read an article on the topic, and...in short, the math is a bit complicated.
1. It depends how many people are in each car. Mostly in the US, cars operate with only the driver. They usually operate at like 1. 10- 1.15 people per car range. I assume MCO-WDW traffic would average closer to 3-4 people per car, but we'd need to know to calculate. Do business travelers/convention goers use ME?
Also- this perhaps also opens up the possibility more people will come to WDW via Sanford Airport, TPA and other FL airports.
2. We'd also need to know how many people are on the bus on average per mile of transit. To beat cars, the bus has to be carrying a fair number of passengers, and maintain that average for
the duration of the trip. If every ME bus stops at 3 hotels, and only 2 people are on the last leg - that decreases the efficiency benefit of the bus quite a bit. Right now, ME runs 24 hours a day. How full are the late night/early morning buses?
3. What type of fuel are the buses using? Are they regular diesel buses, or electric, natural gas? What about the MPG of the rental cars? at home, people tend to drive far more trucks/SUVs than when they rent.
The rough calculation (DOT 2010) is : U.S. bus transit, with 28% occupancy, emits an estimated 33% lower greenhouse gas emissions per passenger mile than the average U.S. single occupancy vehicle - but that assumes the low 1.10 people per car AND that individual drivers are driving a high ratio of SUV's and trucks.
You've think it would be easy to say buses are better, and generally they are- but right now- buses at half capacity really drives that math down. Mears buses can normally hold up to 55 people (but they make multiple stops, so not full the whole way) If MCO to WDW cars hold an average of 3 people per car; you'd get about 18 cars per bus. But buses aren't always full and we'd need to know the average # or people per car.