How is commercial root beer carbonated?

Yeast-carbonated root beer contains a small amount of alcohol. ... which contain the same sort of artificial colors and flavors as commercial root beers.

Since this is not my area of expertise, I searched Google under the key words "Root beer carbonation" to get these possible sources:

http://www.leeners.com/rootbeer.html
(Broken Link Removed)

Adding "commercial" to the key words gave me these sources:

http://hbd.org/brewery/library/RootB.html
http://www.jagaimo.com/bistro/rootbeerfaq.html
http://teachers.net/lessons/posts/1229.html

I hope this helps a little more. Thanks for asking.