For the record, I much preferred the idea of Cyrodiil being temperate forest. In my mind it better suits the racial and cultural theme of the Imperials.
That's because the Imperials have been greatly copied on Roman culture with Morrowind, which wasn't the case when the first Pocket Guide of the Empire was released. If I recall correctly, the First Pocket Guide mentioned that the Imperials were much closer to an indigenious race which fits perfectly with the jungle landscape. I couldn't imagine Romanesque Imperials living in a jungle, nor would I imagine indigenious Imperials living in the current Cyrodiil's climate.
I recall an interview done with one of the devs dating back from Arena, Daggerfall and Morrowind (Possibly Redguard as well) who mentioned that a disagreement occured between him and one of the other current Devs. He wanted the Imperials to be a unique, original race while the other Dev wanted the Imperials to be un-original and copied from the Romans culture, and he won. The interview was very interesting, showed some excellent points of views between the original devs and the new ones. The dev's name was Douglas Goodall.
The interview was posted on the Imperial Library and another Ex-dev asked it to be removed, alas!
Too bad we'll never see the Imperials as they were described in the First Pocket Guide. They've been simplified, their culture became much less interesting and the world they live in a boring landscape to explore.
As for what to believe in terms of Lore, we have no choice but to take what Beth has done with Oblivion. Unless that young Prince Attrebus from the books decides to change Cyrodiil to a jungle by spitting fire to burn the trees and throw in seeds on the soil.