According to reports, there is indeed "Green Lantern 2" and it is currently in the works. It even has an IMDB page already which says it is going to be released in 2013.
The writers of the Green Lantern sequel were said to have been hired a year ago and the story seems to be in place already since Ryan Reynolds, the Green Lantern lead star, was able to share the statement below to MTV:
"The inevitable war between Hal and Sinestro is something that will have to be dealt with in the second movie for sure, but there's always subplots and a lot of other interesting things going on that are setting up a possible third film," Reynolds said.
The Green Lantern movie is based on a DC Comics Superhero character with the same name. Aside from Reynolds, the film also stars Blake Lively ("Gossip Girl"), Peter Sarsgaard ("An Education"), Mark Strong ("The Eagle"), etc.
Check out below the "Green Lantern" (Part 1) movie trailer: